Elmwood Visual Resource Centre

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					Elmwood Visual
Resource Centre

 Glossary of Terms
Related to Vision and
 Vision Impairment
     in Children

                    Page 1
This Glossary has been adapted from a
        number of sources by:

  Elmwood Visual Resource Centre
     Elmwood Normal School
         Aikmans Road

       Phone/Fax: 03 358 8141

                                        Page 2
What Does Vision Impairment Mean?

Generally speaking, a child who can demonstrate both of the following:

1.    A documented condition of the visual system which reduces acuities down
      to or below 6/18 in the better eye (wearing glasses if prescribed).

2.    Difficulty in visually accessing learning materials in the classroom, close or
      at a distance, because of 1. above.

Children with a vision impairment range from those who are completely blind to
those who are really borderline according to the above criteria.

Children with a vision impairment may have other, additional, conditions which
have an impact on their learning.

The Visual Resource Centre works with children, their families and their teachers
from birth through to the end of high school. It does not work with tertiary students.

                                Snellen type eye chart – 3 metre

                                From about here up, consider for enrolment

                         This eye chart is good for younger children -
                         they don’t need to be able to read.

Referrals come from parents, teachers at all levels (especially preschools),
hospitals, ophthalmologists and other eye specialists, GSE, the RNZFB and

There is a well-documented referral and enrolment procedure for Elmwood Visual
Resource Centre.

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What do those funny numbers mean?

             When children are tested on a Snellen or similar eye chart, testing is
             normally done at 6 metres distance.

             If the child is very young, a special 3-metre chart may be used
             instead. This may have pictures instead of letters for obvious

             The results are reported using a number code that reflects the
             smallest line the child was able to read at that distance.

Less than 6/60           Cannot read any of the chart at 6 metres.

6/60                     Can read only the top (biggest print) line

The 6/60 thing is a ratio: typically, the child has to be within 6 metres to read a
letter or sign that a normally-sighted person should be able to read at 60 metres.
Similarly, the following:

6/36                     Can read the top two lines only.

6/24                     Can read the top three lines only.

6/18                     Can read the top four lines only.

6/12                     Can read all but the bottom (smallest) line.

6/6                      Normal distance vision.

If the child cannot read any of the chart from 6 metres, the assessor may bring the
child closer to the chart.
They might record the child’s results as 3/60, which simply means that the child
had to be within 3 metres to read the top line.
If the assessor was using a 3-metre chart with a youngster, then the results would
always be recorded as 3/ something.
Americans use the same charts at the same distances, but record in feet instead
of metres. Hence the term 20/20 vision (6/6 to us).

  How well the child sees close up at desktop level is frequently more important
 than 6-metre acuity. Teachers use a functional vision assessment to assess
                                  close vision.

                                                                               Page 4
    What Area does Elmwood Cover?

    Elmwood VRC is responsible for providing services over about half the South

                                 Lewis Pass Summit                         Kaikoura
    Punakaiki                                                  Hanmer

    Greymouth           Arthurs Pass


                  Mount Cook
                        Twizel                       Fairlie            Timaru

    Haast                                                                  Waimate

    Children may be located at:

•           Home
•           Kindergartens, etc
•           Special Preschool facilities, e.g. CCS, Champion Centre, Conductive Ed.
•           Primary Schools
•           Intermediate Schools
•           High Schools, Area Schools
•           Special Units, attached or unattached
•           In hospital undergoing long-term treatment (e.g. brain tumour).

    About 70% of the roll is located within the greater Christchurch district.

                                                                                      Page 5
Absorption Spectrum
The relationship between the wavelength of light striking a pigment and how
strongly the light is absorbed.

Accessory Oculomotor Nucleus
Receives input from the pretectal area, innervates the ciliary ganglion. Mediates
pupillary light reflexes.

Accessory Optic System (AOS)
Region of the vertebrate midbrain to which some optic nerve fibres project. Cells
respond to large slowly moving textured patterns and are selective for both
direction and speed of motion suggesting they are involved in the computation of
global motion. Possibly used for the detection of retinal slip and used in image
stabilization. Consists of two sets of retina ganglion cell fibres and three target
nuclei in the anterior portion of the midbrain, the dorsal, medial and lateral nuclei.

                                                                                Page 6
Accidental View
A special view of an object in which small perturbations in lighting or viewing
direction drastically change its appearance. For example, imagine looking at a
pencil straight on from the tip. (As opposed to Generic View).

The adjustment of the shape of the crystalline lens by the ciliary muscles to allow
the eye to focus at different distances.

Accommodative insufficiency:
loss of or less than age expected normal amounts of accommodation

Persons who have achromatopsia do not have normal "cone vision." In normal
eyes there are 6 million cone photoreceptors, located mostly at the centre of the
retina. Persons with complete achromatopsia must rely on their "rod vision." In the
normal eye there are 100 million rod photoreceptors. Rods are located mostly at
the periphery of the retina. Rods "saturate" at higher levels of illumination. Rods
do not provide colour vision or good detail vision. Therefore, persons with
achromatopsia are either totally colour-blind or almost totally colour-blind, and they
have poor visual acuity. Without normal cone vision, their eyes are not able to
adapt normally to higher levels of illumination. There are many variations in the
severity of these symptoms among individual achromats. There are complete rod
monochromats, incomplete rod monochromats, and blue cone monochromats.
Complete rod monochromats have the most severely impaired vision of all
achromats. See Rod monochromatic, Cone monochromatic

Action Spectra
A photoreceptor's relative spectral sensitivity.

You can see that in spite of their names (“Red”, “Green”, and “Blue”), each class
of cone shows an absorption, or action, spectrum that extends over a range of
wavelengths, with the result being considerable overlap in the three action
spectra. In addition, the names we assign to the three classes of cones are a bit
of an unfortunate misnomer: with the possible exception of green cones, the

                                                                               Page 7
names do not correspond well with the colour we perceive when viewing
monochromatic light with a wavelength corresponding to the λmax for the cone in
question. For example, our perception of what we call “Blue” light is not a
response to light that preferentially activates only so-called blue cones. Instead,
light with a wavelength of light to which we assign the colour “Blue” (from 450 to
490 nm, give or take) actually stimulates each of the three cone types with roughly
comparable effectiveness. Likewise we perceive yellow, orange, or red,
depending on the relative activation of green and red cones.
The overlapping action spectra, coupled with the complexity of the neural
pathways involved in processing the information provided by the cones (see the
links below for more on this fascinating topic), allow us to perceive (= discriminate)
a huge number of different colours, depending on the wavelength(s) of the
photons impinging on the cones. This is because the colour we perceive when
viewing light of a given wavelength depends on how much the cones of each class
are stimulated by that wavelength, which in turn is determined by how efficiently
the cones of each colour class absorb the photons with that wavelength. The
perceived colours for various wavelength ranges are indicated across the top of
the graph.

See Visual Acuity.

The ability of the eyes to adapt to a change in lighting conditions. Visual
adaptation to changes in luminance level is affected in several diffuse retinal
degenerations. The speed of cone cell adaptation may become longer years
before the child has real night blindness. Adapting to light from dim or dark usually
takes a few seconds. Adapting to dim or dark from light takes from 10 to 40

the accessory structures of the eye, including the eyelids, lacrimal apparatus, etc.

Heading towards. A system's afferent signals are those entering the system from
elsewhere. (As opposed to Efferent)

A possibility for action afforded to a perceiver by an object. The affordances of an
object depend upon the perceiver as well as upon the characteristics of the object.
For example, a stream affords such actions as jumping and paddling to a person,
but to an frog it affords swimming.

Aggregate field
The area within which the receptive fields of cells in a single hypercolumn of the
visual cortex fall.

A long lasting change in perceived brightness that occurs after prolonged viewing
of a given luminance that remains after the luminance has been changed.
Presumably due to adaptation in the exposed region.

                                                                               Page 8
Aicardi Syndrome
                                      Aicardi syndrome is a rare genetic disorder identified
                                     by the French Neurologist, Dr. Jean Aicardi in 1965.
                                     This syndrome is defined as a "rare disorder",
                                     meaning that there are less than 20,000 cases
                                     worldwide. The number of identified cases of girls
                                     with Aicardi syndrome is approximately 300 - 500
                                     Recently, Dr. Aicardi has noted that due to advanced
                                     technology in the form of Magnetic Resonance
                                     Imaging (MRI), absence of the corpus callosum is
                                     less of a marker than it has been in the past.

Aicardi syndrome is characterized by the following "markers":

•     Absence of the corpus callosum, either partial or complete (the corpus callosum is
      the part of the brain which sits between the right and left sides of the brain and
      allows the right side to communicate with the left.)
•     Infantile spasms (an infantile form of seizures)
•     Mental retardation
•     Affects females only, or in very rare cases, males with Klinefelter Syndrome (X-
•     Lesions or "lacunae" of the retina of the eye that are very specific to this disorder
•     Other types of defects of the brain such as microcephaly, (small brain); enlarged
      ventricles; or porencephalic cysts (a gap in the brain where there should be healthy
      brain tissue)

Children are most commonly identified with Aicardi Syndrome between the ages of three
and five months. A significant number of these girls are products of normal births and
seem to be developing normally until around the age of three months, when they begin to
have infantile spasms. The onset of infantile spasms at this age is due to closure of the
final neural synapses in the brain, a stage of normal brain development. The known age
range of affected children is from birth to the mid 20’s.
Treatment of Aicardi syndrome primarily involves management of seizures and
early/continuing intervention programs for developmental delays. Prognosis for these
children varies. Published medical information in professional journals is somewhat limited
for a syndrome that is over thirty years old and these articles are written by and for
specialists. A limited bibliography of these journal articles can be found in this web site.
Genetic research is ongoing into the cause of this disorder.

                                                                                     Page 9
                             A hereditary deficiency of pigmentation, which may involve the
                             entire body (complete albinism) or a part of the body (incomplete
                             albinism); believed to be caused by an enzyme deficiency
                             involving the metabolism of melanin during prenatal
                             development; inherited as an autosomal dominant or recessive
                             trait; in the X-linked type, ocular albinism is only visible
                             ophthalmologically in the female carrier; in complete albinism,
                             there is usually lack of
                             pigmentation in skin and
                             hair, as well as in retinal
                             & iris tissue; in
                             incomplete albinism,
                             skin and hair may vary
                             from pale to normal; in
                             ocular albinism, function
may vary from normal to impaired. Impairments
may involve the retina (especially the macula) and
iris; photophobia, nystagmus, and refractive
errors are typical. If acuity is decreased, it
commonly ranges between 6/18 and 6/60. Visual
fields are variable; colour vision is usually normal.
Prognosis: non-progressive.
                                                                     Magazine cover - albinism
A depth percept can cause a shift in perceived form, namely change in the
perceived distance between patterns in a configuration. For example that a pattern
AB C is viewed in one eye and the pattern A BC is viewed through the other, the
letter B is seen in depth at a position halfway between A and C.

Alternating Squint
Where a squinting right eye and straight left eye alternates with a squinting left eye
and a straight right eye. See also strabismus.

Amacrine Cell
A type of neuron seen in the retina.

Ambient Vision
The role of vision involved in orienting an animal in space and guiding its larger
movements. Sensitive to motion and dependent on peripheral vision. (As opposed
to Focal Vision).

                                                                                      Page 10
                                       (also known as "amblyopia ex anopsia,"
                                       which is dimness of vision from disuse). In
                                       the absence of organic eye disease, reduced
                                       visual acuity in one eye (uncorrectable with
                                       lenses) due to cortical suppression;
                                       commonly caused by strabismus or by
                                       unequal refractive errors, but may also be
                                       caused by opacities of the lens or cornea. In
                                       strabismus, the image from the deviating eye
                                       is suppressed; fusion is lost, as is depth
                                       perception. If treatment is not instituted
                                       early, vision fails to develop in the deviating
                                       eye, and cannot be regained. In older
children (over about 8 years of age), amblyopia may be untreatable. (see also

Amblyopia ex Anopsia
Poor vision due to disuse of the eye. The most common causes are strabismus or
a marked refractive error in one eye.

Perceptually present, but not having a real phenomenological presence. (As
opposed to Modal).

Amodal Completion
The creation of a subjective contour that is without local sensory attributes e.g.
contrast difference. It is neither a cognitive inference nor a projection but a direct
perception. Seen most often in displays that have partially occluded objects. (See
also Modal Completion).

Amsler grid
Test card. Grid (black lines on white background or white lines on black
background) used for detecting central visual field distortions or defects, such as
in macular degeneration.

Normal Vision                                With macular degeneration

                                                                               Page 11

                          Device for creating 3-D images using red and blue filters
                          for the two eyes. Frequently used in topographical
                          mapping and archaeology, such as this photo of the
                          ancient town of Qumran near the site of the Dead Sea
                          Scrolls discovery.

In any scene, the object which is the lightest is perceived as being white, or is
'anchored' to white.

                 Rare, congenital absence or partial absence of the iris; genetically caused
                 by an autosomal dominant or recessive hereditary pattern. Often, the iris is
                 vestigal (little more than a margin is present) and the eye appears to have
                 no colour (only a larger than normal pupil). Other deformities of the
                 anterior chamber are also often present (e.g., cataract), and glaucoma
                 frequently develops before adolescence. There is usually decreased acuity
(circa 6/60), photophobia, possible nystagmus, cataracts, displaced lens, and
underdeveloped retina; visual fields are usually normal, unless glaucoma develops.

A condition in which each eye has a different refractive error. For example, one
eye is mildly myopic while the other is very myopic. When associated with far-
sightness it is a common cause of amblyopia in children.

Congenital absence of the eyeball(s). The term may also be used to describe
surgical removal of the eye, or enucleation.

Anterior Chamber
The space in front of the eye, bounded by the cornea in front and the iris behind.
It is filled with aqueous.

Anterior ocular segment:
the part of the eye anterior to the crystalline lens, including the cornea, anterior
chamber, iris and ciliary body

Absence of the crystalline lens of the eye. Post-cataract aphakia describes an eye
after removal of a cataract affected lens. An eye with a lens removed is called

                                                                                    Page 12
Aqueous (or aqueous humour)
A clear, watery fluid which fills both the anterior and posterior chambers in the
front of the eye.

Type of ultrasound, radar-like device that emits very high frequency waves that are
reflected by the ocular structures and converted into electrical impulses. Used for
differentiating normal and abnormal eye tissue or for measuring length of eyeball.

Vague eye discomfort arising from use of the eyes; may consist of eyestrain,
headache, and/or brow ache. May be related to uncorrected refractive error or
poor fusional amplitudes.

                                        A common refractive
                                        error caused by an
                                        irregular curve of the
                                        cornea and/or the
                                        crystalline lens. It can
                                        usually be corrected by
                                        glasses or contact lenses.

                                        Distorted magazine cover - astigmatism

One of the commonly used cycloplegic drugs used to dilate the pupil.

Automated lamellar keratoplasty (ALK).
Excision of the outer corneal layers (lamellae) with a computer controlled
keratome (knife), usually as a part of a refractive keratoplasty procedure.

Automated perimeter:
A computer-driven device used to plot defects in the visual
field. Usually this is a large hemisphere shell into which the
patient's head is placed. Various points of lights, sometimes
of different sizes, intensities and colours are projected onto
the screen. The patient then indicates whether the light is
seen and the response is recorded. The computer then plots
the effective visual thresholds within the targeted visual field.
                                               Field plot for one eye.

                                                                                  Page 13
                 Eyeglasses that incorporate two different powers in each lens,
                 usually for near and distance corrections.

Literally, two sides. In vision terms, involving both eyes.

Binocular Disparity
The difference in position of two retinal images of an object that do not fall on
exactly corresponding retinal positions.

Binocular Rivalry
Occurs when the two eyes are presented with different stimuli. Instead of seeing a
summation of the two images, our perception switches from one image to the

Binocular Vision
The co-ordinated use of the two eyes to fuse the resultant images and produce a
perception of depth. See fusion.

Photoreceptors, most notably rods, can be driven to saturation by bright visual
stimuli and become insensitive to light changes in this region, then they are said to
be 'bleached'.

                 A common, chronic, bilateral
                 inflammation of the lid margins;
                 may be staphylococcal (ulcerative)
                 or seborrheic (non-ulcerative), or a
                 combination of the two; may run a
chronic course over a period of months or years if
not treated adequately. The seborrheic type is
associated with dandruff. Symptoms are itching,
burning, irritation, and scaly appearance of the lid margins. Conjunctivitis, mild
keratitis, chalazions and hordeola may be complications.

See vision impairment and legal blindness.

A phenomenon reported in individuals suffering from cortical blindness (i.e.
damage to the primary visual cortex resulting in blindness). Individuals with
blindsight report that they are unable to see, yet under forced choice conditions
are able to indicate the presence and location of visually presented objects.

                                                                              Page 14
Blind Spot
The area on the retina where the optic nerve enters the
eyeball. This area has no photoreceptors and therefore
no visual input. The cortex appears to fill-in this missing
information so we are not conscious of the blind spot.
This small area can be measured and in glaucoma, as
the nerve fibres die, the blind spot tends to enlarge and
elongate. This is one of the diagnostic hallmarks of

                              Behaviours sometimes seen in children with vision
                              impairment, including eye- pressing and poking, light
                              gazing and head waving. They are thought to be a
                              form of self-stimulation.

                              Eye poking

Ultrasound device that provides a cross-section view of tissues that cannot be
seen directly. High frequency waves are reflected by eye tissues and orbital
structures and converted into electrical pulses, which are displayed on a printout.

                      Infantile glaucoma, caused by abnormal development of the angle
                      formed by cornea and iris; Schlemm's Canal is usually collapsed;
                      onset at birth or before age 3 (over 80% of cases are evident by 3
                      months of age); usually an autosomal recessive trait. Symptoms are
                      excessive tearing, photophobia, increased intraocular pressure, and
cupping of the optic disk. The eyes usually appear abnormally large; corneal haze is not
uncommon. In untreated cases, blindness occurs early. The earlier the defect appears, the
less favorable the prognosis. Long-term visual prognosis is fair. Depending on when
treatment is instituted, there may be lowered acuity or restricted visual fields. Must be
treated surgically (goniotomy, trabeculotomy, or trabeculectomy).

Canal of Schlemm
A circular canal at the junction of the sclera and the cornea through which the
aqueous passes. It maintains the pressure in the front of the eye and any
disruption to its function can lead to glaucoma.

The membrane which encloses the crystalline lens.

                                                                                 Page 15
A clouding or opacity of the lens, believed to be caused by
chemical changes in the lenticular structure/material.
Etiology includes: hereditary, congenital anomalies
associated with disease or syndrome; infection, severe
malnutrition, or drugs during pregnancy; systemic disease

                                                               (e.g., diabetes); trauma (e.g.,
                                                               head injury or puncture
                                                               wound); normal
                                                               manifestation of old age.
                                                               May be congenital, senile, or
                                                               traumatic. Symptoms include
                                                               whitish appearance of the
                                                               pupil and blurred
                                                               vision/decreased acuity
(especially at distance). The congenital type may also include nystagmus, squint,
photophobia; traumatic cataract symptoms include general redness and irritation of the
eye, and may be complicated by infection, uveitis, retinal detachment, and glaucoma.

Cataract extraction.
Removal of a cloudy lens from the eye. An extracapsular cataract extraction leaves the
rear lens capsule intact; with an intracapsular extraction there is complete removal of lens
with its capsule, usually by cryoextraction.

Central retinal artery.
First branch of the ophthalmic artery; supplies nutrition to the inner two-thirds of the retina.

Central retinal vein.
Blood vessel that collects retinal venous blood drainage; exits the eye through the optic

Central Visual Acuity
An eye's best vision. Results from stimulation of the fovea and the macular area.

Central and peripheral vision
The sizes of structures inside the eye are measured in distance units called degrees. A
straight line through the centre of the cornea to the centre of the retina is designated as
the 0° point. (Review: ocular structure) From that reference point, imagine any angle with
its vertex at the centre of the eye, with one of its rays on the 0° point. The distance
spanned between the two rays of the angle is indicated by the measure of that angle.
From the centre of the eye, a 1° radius circle is an area known as the foveola, where visual
acuity is clearest. The foveola is centred on a 5° radius circle called the fovea where visual
acuity is also very clear. As we move farther and farther in any direction from the fovea,
towards the periphery, visual acuity drops as a function of the decrease in density of

                                                                                       Page 16
The mechanism by which the external energies of light are transformed into neural activity
is called transduction. The process of transduction is a chemical reaction in nature, as
opposed to mechanical. The two types of photoreceptors found on the retinal surface are
rods and cones. The cones are used mainly for photopic, or day-time vision; and the rods
are used mainly for scotopic, or night-time vision.

          Photopic                          Mesopic                          Scotopic
daytime vision, uses             dusk or dawn vision, uses
                                                           nightime vision, uses rods
cones                            cones and rods

The photoreceptor cell is composed of a tube-like section and the traditional cell-like
section. Inside the tubes are pigments that bleach from a purple colour when struck by
light. This change in chemical composition indicates to the brain that a certain
photoreceptor, in a known location, has been activated. These tubes are long and narrow
in order to facilitate directional sensitivity. In other words, the ambient light inside the eye is
not registered as much, and only light coming from a certain location can activate a certain
receptor cell. Without directional sensitivity, any light source would activate so many of the
photoreceptors that we would not be able to tell where the light was coming from.

Rods are 500 times more sensitive to light intensity than cones are, and there are 120
million of them in each eye. We require this degree of sensitivity for our scotopic vision
because there is not as much light present in the environment to begin with. The density of
rods drops as we approach the periphery, and the fovea. Both the fovea and the blind spot
have absolutely no rods on them. The fovea, as we'll see very shortly, is reserved for the
cones. Interestingly, rods are the most sensitive to the short wavelength end of the
spectrum, or bluish lights, but they only convey intensity information to the brain, not
wavelength. (Review: light) Therefore, in scotopic vision, circles of constant intensity but

                                                                                         Page 17
variable frequency (wavelength) will APPEAR to be brighter and brighter as the colour
becomes more bluish.

There are three types of cones, each one corresponding to maximal sensitivity to bluish,
greenish and reddish light. As opposed to rods, there are only 6 million cones in each eye,
and they are mostly centred in the fovea. The density of cones also drop, but much sharply
than that of rods, as we approach the periphery. The ultra-high density of cones in the
fovea is the reason why our vision is so clear in the centre. In a bright environment, all of
the rod pigments are completely bleached out, and thus the brain ignores the activity of
rod cells in photopic vision.

Dark Adaptation
The sensitivities of both rods and cones change constantly as we move around in areas of
varying lightness. The cones take about 10 minutes to adapt to darkness, while the rods
take around 20-30 minutes. After the 20-30 minutes however, the rods are much more
sensitive to light than the cones, and thus scotopic vision is in control. This concept will be
covered in greater detail later on, however we can cite the fact that our vision is clearest in
the centre as part of evidence that humans are designed for the daytime. Now consider
this interesting question that still remains unanswered. Before artificial lighting existed, our
change from photopic to scotopic vision was strictly dictated by the rise and set of the sun.
Now, humans control when their eyes will be using photopic or scotopic visual systems.
Has the advent of artificial lighting modified the sensitivities of cones or rods at all, with
respect to the predisposed sensitivities given to us through evolution?

Inflamed lump in a meibomian gland (in the eyelid). Inflammation usually subsides, but
may need surgical removal. Sometimes called an internal hordeolum.

                                                                                       Page 18
                                   C.H.A.R.G.E. Association
                                   A group of associated developmental defects often
                                   occurring together. C - coloboma, H - heart defect, A -
                                   atresia, R - retarded growth or development, G - genital
                                   defect, E - ear defect or deafness.
                                   Other findings: mogrognathia; cleft lip; facial palsy; feeding/
                                   sucking problems as a result of velopharyngeal
                                   incompetence; renal abnormalities; omphalocele;
                                   tracheoesophageal fistula; rib anomalies; ptosis.
                                   Caused by: altered morphogenesis during 2nd month of
                                   Percent of children who have this characteristic as part of the
                                   CHARGE Association syndrome.

80%        Coloboma - ranging from isolated iris coloboma, without visual impairment, to clinical
           anophthalmos; retinal coloboma most common.
80%        Heart defect - tetralogy of Fallot, patent ductus arteriosus, double outlet right ventricle with an
           atrioventricular canal, ventricular septal/atrial septal defect, right-sided aortic arch.
58%        Atresia choanae - passageway from posterior nasal cavity into the nasopharynx closed,
           membranous and/or bony.
87%        Retarded physical growth and/or mental development (94%) (usually normal birth weight, but
           delayed growth during the first 6 months postnatally); CNS deficiency.

88%        Genital hypoplasia - hypogonadism, especially in males.
88%        Ear anomalies and/or deafness - ranging from small ears without malformation, to cup-shaped
           lop ears; either sensorineural or mixed sensorineural and conductive deafness (ranging from mild
           to profound).

See optic chiasma.

                                 A type of posterior uveitis, almost always affecting the
                                retina; usually follows an active microbial invasion of the
                                tissues by a causative organism which is rarely recovered
                                (definite etiological diagnosis is seldom possible); generally
                                classified as granulomatous. The onset may be in utero
                                when caused by the Toxoplasma gondii, probably the most
                                common cause (see Toxoplasmosis) . If granulomatous
                                uveitis is acquired, the onset is insidious: vision gradually
becomes blurred, pain is minimal, mild photophobia is present, and the pupil is often
constricted and/or irregular in shape. Fresh lesions seen through the ophthalmoscope
appear as yellowish-white patches through a hazy vitreous. As healing occurs, the vitreous
clears and pigmentation appears at the edges of the lesions. In the healed stage, there is
considerable pigmentation (i.e., "scars") and scotomas occur where the lesions are
located; these healed areas usually do not result in significant visual loss. If the macula
has not been involved, recovery of central vision is complete. The disease can last months
to years, sometimes with remissions and exacerbations, and is capable of causing
permanent damage with marked visual loss.

                                                                                                       Page 19
A membrane between the sclera and the retina, lining most of the eyeball. It is
rich in blood vessels and provides nourishment to the retina, vitreous and
crystalline lens.

                                   Inflammation of the choroid.

                                   Choroiditis (central vision loss)

The optics of the eye are such that short wavelengths are refracted slightly more
than long wavelengths creating a positional disparity on the retina. When viewed
by two eyes a stereoscopic disparity is obtained. This is termed Chromostereopsis
or Chromatic Stereopsis

Ciliary Body
A structure composed of muscle and blood vessels and situated behind the iris. It
is responsible for focussing (accommodation) and for the manufacture of aqueous.

Ciliary Muscle
The muscle of the ciliary body which allows the crystalline lens to focus.

                                         Congenital cleft in some part of the eye
                                         (commonly the iris, but may also occur in
                                         the lid(s) or pigment epithelium and
                                         choroid); caused by faulty closure during
                                         prenatal development; usually hereditary;
                                         secondary complication: cataracts.
                                         Associated conditions are:
                                         microphthalmia, polydactyly and mental
                                         retardation. Depending on the extent and
                                         location of the coloboma, there may be
                                         decreased visual acuity, nystagmus,
                                         strabismus, photophobia, and a loss of
visual fields. See also C.H.A.R.G.E. Association.

                                                                             Page 20
Colour Blindness
Colour blindness is not a form of blindness at all, but a deficiency
in the way you see colour. With this vision problem, you will have
difficulty distinguishing certain colours, such as red and green or
blue and yellow. Red-green colour deficiency is the most common
form of colour blindness; a less common form is blue-yellow colour
deficiency. See Colour Vision Problems

Colour Deficiency
A defect of the cones which affects colour detection; called "achromatopsia" in its most
extreme form; X-linked genetic defect occurring in 8% of men and 0.4% of women; may
also be acquired as a result of retinal disease (specifically when it affects the macula) or
poisoning. Type depends on which cones are affected:
Cone Monochromats have only one type of cone and may be red-green, red-blue, or
green-blue blind; occurs one in a million.
Dichromats have two types of cones; this group is further divided into: Protanopes (red-
blind; see blue and green), Deuteranopes (green- blind; confuse shades of red, green and
yellow), and Tritanopes (blue- blind; see red and green).
Anomalous Trichromats make up the largest group and are similar to the Dichromatic
group except in intensity (Protans and Protanopes, Deutrans and Deuteranopes, Tritans
and Tritanopes ... similar but milder defects).
Rod Monochromatism is very rare; there is complete lack of cone function and
accompanying photophobia, nystagmus, and poor visual acuity; visual fields are normal.
The photophobia and nystagmus reduce with age.

Colour vision testing is designed for assessing children and adults who have congenital
colour vision deficiencies. Often, the deviations from normal colour vision are so mild that
they do not have any practical consequences, especially in childhood if the child's difficulty
is understood by the teacher and parents. Adults with colour vision defects should also be
aware of this characteristic in their vision because some jobs require good, or even
perfect, colour vision.
                              In congenital colour vision defects, the abnormality is usually
                              in the structure and function of a cone pigment (rarely in
                              more than one). Red-green colour vision defects are X-
                              chromosomally inherited, thus more common in males (8%)
                              than in females (0.4%).
                              A normally sighted person sees all colours of the spectrum
                              (Figure 2A) whereas a person with a red-green defect
                              (Figure 2B) has a grayish confusion area (In brackets in
                              Figure 2B) within which he or she does not see the difference
                              between some shades of red and green and misses them.
Additive Colour Mixes
                                                               See also Ishihara Colour Test

                                                                                      Page 21
                        Figure 2A                         Figure 2B
Figure 2.

   A. Coloured surfaces in this figure represent all spectral colours, saturated at the outer
      end of the spokes.

   B. This illustration depicts how the picture in Figure 2A is seen by a person with a
      deutan defect. Green tones and the opponent tones of purple-red are seen as dull
      and therefore easily confused with each other. Since these colours are on the
      opposite sides of the colour circle, there is an "axis" of deficiency across the colour
      circle. The colour space of this person is blue-yellow with confusion of colours in the
      red-green axis. Individual variations in confusions of hue are great.

                                LEA 16-colour screening test

                                                                                    Page 22
                                Colour Vision Variations

The cause of the disproportionately high numbers of males affected compared to females is
that colour blindness is a X chromosome sex linked recessive disorder. This means that the loci
for the most frequent types of colour blindness are on the X chromosome. The normal ability
to see colours depends on several genes, X linked and autosomal (Rothwell 1993). Being a
recessive trait, the presence of an other X chromosome that is not carrying the recessive i.e.
the dominant allele for colour vision, will mask the effect of the recessive. The presence of the
second X chromosome is the normal homozygous genotype of females therefore explaining the
lower incidence, as two recessives must be present for the phenotype to be exhibited.

                                                       Normal colour Vision

                                                       Monochromate Vision

                                                 Protanomalous Vision (Anomalous
                                                Trichromatic) Red system defective

                                                 Triteranomalie Vision (Anomalous
                                               Trichromatic) Green system defective

                                               Deuteranomalous Vision (Anomalous
                                               Trichromatic) Green system defective

                                               Protanopia Vision entire defectivity of
                                                         the Red system

                                              Deuteranopia Vision entire defectivity of
                                                        the Green system

                                               Tritanopia Vision entire defectivity of
                                                          the Blue system

                                                                                          Page 23
Concave Lens
Also called minus lens. Used to correct myopia.

Congenital Oculomotor Apraxia (Cogan's Oculomotor Apraxia)
An inability to make voluntary eye movements, leading to compensatory head

Cone Dystrophy
                                     Poor functioning of the cones (colour
                                     perceptors) leading to washed out
                                     monochrome image with poor central vision.

                                     See also: Cones, Cone Monochromatism, Rod

                                     Magazine cover – cone dytrophy

Cone Monochromatism
A rare condition where there is a disturbance of cone function resulting in colour
blindness, photophobia and reduced visual acuity. Functional vision in photobic,
or bright light is impaired, but in scotobic, or dim light is unimpaired. See also
achromatopsia, rod monochromatic, and colour deficiency

One of the two types of light sensitive cells in the retina. They function in bright
light (photobic) and are essential for colour vision and good visual acuity. They
are at greatest concentration in the macula. There are three types:
•   L Cones
    Long wavelength sensitive cones
    (red). Are most sensitive to a
    wavelength of approximately
•   M Cones
    Medium wavelength sensitive
    cones (green). Are most sensitive
    to a wavelength of approximately
•   S Cones
    Short wavelength sensitive cones
    (blue). Are most sensitive to a
    wavelength of approximately 419nm.
See also Action Spectrum, Absorption Spectrum

                                                                               Page 24
Present at birth.

Congenital Glaucoma
See Glaucoma

Congenital Nystagmus
Nystagmus which is not secondary to other conditions which cause vision
impairment. See Nystagmus.

A mucous membrane covering the undersurface of the eyelid and the outer
surface of the eyeball, excluding the cornea.

A contagious infection of the conjunctiva causing redness, grittiness and discharge. May
be caused by:

• Purulent (Pussy) discharge. Lids often stick together
• Swelling of the conjunctiva
• Meaty Redness
• Tearing
• Irritation, pain and/or a gritty feeling
• Usually affects only one eye, often spreads to fellow eye.

• Watery discharge
• Burning
• Irritation
• Glossy looking redness
• Infection usually begins with one eye, can spread.
• Often follows upper respiratory tract infection.

• Usually affects both eyes
• Itching
• Tearing
• Swollen eyelids
• Watery or stringy discharge

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Contact Lens
Small transparent discs made of hard or soft plastic which
are worn on the eye to provide correction.

Contrast Sensitivity
Contrast sensitivity refers to the ability of the visual system to distinguish between
an object and its background. Poor contrast sensitivity results in a washed-out,
poorly defined image.

                                                                     Poor contrast

The co-ordinated inward movement of both eyes to allow fixation on a near object.
See also divergence.

Convergent Squint
Where the squinting eye turns inwards (towards the nose). See also strabismus
and esotropia.

Convex Lens
Also called plus lens. Used to correct hypermetropia.

The curved, transparent part at the front of the eye through which light passes.

Corneal Graft
An operation to replace a damaged or opaque cornea (or part of a cornea) with a
donor cornea.

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Corneal Opacity
Partial or complete cloudiness of the cornea which
prevents light entering the eye, causing vision
impairment. It may be congenital, or as a result of other
conditions such as herpes simplex infection or glaucoma,
or be caused by trauma.

Corneal Dystrophy
A number of conditions resulting in corneal opacities.
The hereditary dystrophies usually become evident in the
early years of life.
                                                  Cornea – side-on
Corneal Scarring
May be caused by injury to the cornea (abrasion, laceration,
burns, or disease); depending on the degree of scarring, vision
can range from a blur to total blindness. Surface abrasions,
although extremely painful, heal transparently (do not leave scars). Deeper abrasions and
ulcerations/lacerations result in a loss of corneal tissue, which is replaced by scar tissue.
Scars left from burns depend on the type and depth of burn: boiling water or a curling iron
leave superficial scarring; acids or alkalies cause deeper damage unless neutralized
immediately. Scarring from disease (usually an inflammation) is usually the result of a
proliferation of new blood vessels into the clear cornea, to assist in the healing process.
Diseases which cause vascularization include herpes simplex, syphilis, and keratitis.

Corneal Ulcer
 An ulcer on the cornea causes pain and
discomfort in the eye and usually makes the
white of the eye turn pink or red. In addition
you can see the ulcer as a whitish patch in the
centre or periphery of the cornea (as above).
The vision in that eye is misted over or
impaired. This condition needs immediate
attention as the bacterial infection can spread
extremely rapidly and damage the eye
permanently even causing blindness in some
cases. These ulcers are more common with
extended wear contact lenses.

Cortical Vision Impairment
A term used to describe an apparent lack of visual functioning, in spite of anatomically and
structurally intact eyes. The cause is assumed to be a lack of cortical functioning (i.e., the
visual cortex of the brain is non-functional). Children with "cortical blindness" do not exhibit
nystagmus, however. (Nystagmus may be the way the nervous system responds to bad
vision, since it occurs simultaneously with many visual impairments.) Neither a CAT scan
nor a VEP can confirm cortical function. In the absence of other abnormalities (e.g., optic
atrophy, microcephaly, frequent seizuring), the prognosis is good for regaining some
degree of visual functioning in children with "cortical blindness."

Corrected Acuity
Acuity measured whilst wearing prescribed glasses or contact lenses.

                                                                                       Page 27
When referring to glasses or contact lenses, the prescribed refraction required to
allow eyes with myopia, hypermetropia, or astigmatism to have clear, normal

                                    Crouzons Disease
                                    Misshapen skull/head/facial features, resulting in
                                    optic atrophy and poor vision.

Cross-eyes. See esotropia.

Cycloplegic refraction.
Assessment of an eye's refractive error after lens accommodation has been paralysed with
cycloplegic eye drops (to eliminate variability in optical power caused by a contracting

Surgical use of intense cold to destroy unwanted tissue, such as retinal scar tissue
or to repair holes in the retina.

Crystalline Lens
The natural lens of the eye which is transparent and which helps light rays to focus
on the retina.

Cycolpegic Drugs
Eye drops or cream which temporarily paralyses the iris and ciliary body, causing
the pupil to enlarge and near vision to blur. Used for refraction in ophthalmological
examination. Effects last from two days to two weeks.

Cyctomegalovirus (C.M.V.)
A virus which can be transmitted from a mother to her unborn child and which can
cause vision impairment as well as other impairments.

Delayed Visual Maturation
Slower than normal development of vision in infants.

De Morsier's Syndrome
See Optic nerve hypoplasia. Usually in both eyes, causes vision impairment.
Other impairments are also present.

                                                                                 Page 28
Depth Perception
See fusion, stereopsis

Diabetes mellitus:
a chronic metabolic disorder characterized by a lack of insulin secretion and/or
increased cellular resistance to insulin, resulting in elevated blood levels of simple
sugars (glucose) and including complications involving damage to the eyes,
kidneys, nervous system and vascular system

Diabetes type I: IDDM
insulin dependent, resulting from destruction of the insulin producing pancreatic
islet cells

Diabetes type II: NIDDM
non-insulin dependent, resulting from tissue resistance to insulin
Diabetic retinopathy
Spectrum of retinal changes accompanying
long-standing diabetes mellitus. Early stage is
background retinopathy. May advance to
proliferative retinopathy, which includes the
growth of abnormal new blood vessels
(neovascularization) and fibrous tissue.

                             Diabetic retinopathy

                         Dilated pupil.
                         Enlarged pupil, resulting from contraction of the dilator muscle or
                         relaxation of the iris sphincter. Occurs normally in dim
                         illumination, or may be produced by certain drugs (mydriatics,
                         cycloplegics) or result from blunt trauma.

Dioptre (D)
The unit of measurement of the power of refraction of a lens. A unit of curvature
used to specify the power of lenses and mirrors. One diopter equals one inverse
meter (m-1). Prescriptions for convex lenses to correct hypermetropia are prefixed
by plus (e.g. +2D) and for concave lenses to correct myopia by minus (e.g. -2D).

                                                                                    Page 29

                            Normal                      with diplopia

Double vision. The perception of two images from a single object. In children,
usually the result of a squint, or strabismus.

Dislocated Lens
                                Also called subluxation. Lens dislocation may be partial or
                                complete, and may be hereditary or result from trauma. If
                                hereditary, it is usually bilateral and associated with other
                                disorders (e.g., aniridia, Marfan's syndrome). It may be
                                complicated by cataract formation. Vision is blurred if the
                                lens is dislocated out of the line of vision. If dislocation is
                                partial and the lens is clear, visual prognosis is good.
                                Traumatic lens dislocation can follow a blow to the eye. If
                                the dislocation is partial, the eye may be asymptomatic, but
                                if the lens has become totally detached and is floating in the
                                vitreous, there is blurred vision. A quivering iris may be a
                                symptom of lens dislocation, due to the lack of lens
attachment and support. Iritis and glaucoma are common complications.

Distance Vision
Visual acuity tested at 6 metres.

The co-ordinated outward movement of both eyes to allow fixation on a distant
object. See also convergence.

Divergent Squint
Where the squinting eye turns outwards (away from the nose). See also
strabismus and exotropia

Dry Eye Syndrome
Tears function to keep the eyes lubricated and
comfortable. Dry eyes are caused when the tear
glands do not produce enough tears and/or the tear
ducts drain too much tears off the eye surface. Dry
eyes can also be produced by a poor tear quality,

                                                                                      Page 30
and not necessarily a volume problem. Either situation causes the eyes to feel
irritated, scratchy, burning, red and uncomfortable.

Eccentric Fixation
Maintaining a direct gaze with part of the retina other than the fovea. See also

German word for 'intrinsic gray'. When both entire retinal images are stabilized,
the observer does not perceive black but a non-descript gray fog.

Electro-Magnetic Spectrum

The electro-magnetic spectrum refers to the different types (or wavelengths) of
radiation arriving from the sun. Fortunately, the earth’s atmosphere reflects nearly
all (harmful) wavelengths back into space, with the exception of visible light and
radio waves which do no harm. The depletion of the ozone layer of the atom-
sphere is a worry because it then allows the entry of invisible but harmful ultra-
violet rays.

Electro-Oculogram (EOG)
A test to assess the function of the retina. Sedation is required for young children.

Electroretinogram (ERG)
A test to assess the function of the retina. Sedation is required for young children.

The normal condition of the eye in respect to accommodation and refraction, i.e.
neither myopic nor hypermetropic.

                                                                             Page 31
                Inward turning of upper or lower eyelid so that the lid margin rests against
               and rubs the eyeball.

The surgical removal of the eyeball from its orbit. Indications for removal include:
1.    When trauma is so extensive that the form of the eyeball cannot be preserved;
2.    Prevention or treatment of sympathetic ophthalmitis;
3.    Severe pain in a blind eye;
4.    Iridocyclitis, phthisis bulbi, and glaucoma when accompanied by severe pain or
      inflammatory symptoms;
5.    Malignant tumors;
6.    Anterior staphyloma, if the eye is blind, troublesome and disfiguring;
7.    Early panophthalmitis;
8.    Intra-ocular foreign bodies which cannot be removed and which cause irritation;
9.    Cosmetic improvement in blind and disfigured eyes;
10. Unilateral retinoblastoma

Epicapsular Stars
Remnants left on the eye of the hyaloid canal, from development in the womb.

A muscle condition in which when both eyes are open each eye points accurately
at the target. However, upon covering one eye the covered eye turns inwards.
Also known as over-convergence

Eye misalignment in which one eye deviates inward (toward nose) while the other
fixates normally.


Excimer laser
Class of ultraviolet lasers that removes tissue accurately without heating it. In refractive
corneal surgery, controlled by computer to make precise pre-programmed shavings of eye
tissue to produce a given optical correction. Used for photorefractive keratectomy (PRK);
combined with automated lamellar keratoplasty (ALK) to produce LASIK (laser in situ

A muscle condition in which when both eyes are open each eye points accurately
at the target. However, upon covering one eye the covered eye turns outwards.
Also known as under-convergence.

                                                                                    Page 32
                                          A muscle condition in which when both eyes
                                          are open one eye is turned outwards. The eye
                                          turn may occur 100% of the time and is called
                                          constant or some of the time and called

Extraocular Muscles
Also called extrinsic muscles. The six muscles which move the eyeball.

Examination Under Anaesthetic (EUA)

Eye Contact
Looking at another person's eyes. It is important as an indicator of functional
vision in young children and as a non-verbal form of communication.

Eye Dominance
The tendency of one eye to assume the major function of seeing, assisted by the
other eye. Usually on same side of body as dominant hand.

Eye lids. Structures covering the front of the eye, which protect it, limit the amount of light
entering the pupil, and distribute tear film over the exposed corneal surface.

                               Eye Pressing
                               A form of self-stimulation often seen in children with
                               damage to the retina, such as retinopathy of
                               prematurity, Leber’s congenital amaurosis.

Any condition with a traceable family history.

Farsighted, long sighted
See hypermetropia.

Fechner's Paradox
The fact that a monocular view may look brighter than a binocular view.

Field Loss
An area in the visual field where there is blindness or reduced awareness.

Field of Vision
Normally between 180 and 200 degrees. See visual field.

                                                                                       Page 33
The act of maintaining a direct gaze on an object with the fovea of each eye.
Eccentric fixation refers to fixation with other areas of the retina.

Floaters are tiny clumps of gel or cells inside the
vitreous, the clear jelly-like fluid that fills the inside of
your eye. Eye floaters and spots are usually
harmless, but flashes of light may indicate a problem.

Fluorescein angiography
Technique used for visualizing and recording location and size of blood vessels and any
eye problems affecting them; fluorescein dye is injected into an arm vein, then rapid,
sequential photographs are taken of the eye as the dye circulates.

1. Accommodation.
2. The point to which light rays converge after passing through a lens.

Fovea (Fovea Centralis)
Central part of the macula which produces the best visual acuity. It contains the
greatest concentration of cones and no blood vessels. Humans move their eyes
so that images of interest are projected onto their foveas.

Functional Vision
The use that is made of one's vision to interpret the environment. Two people with
a similar vision impairment may have very different functional vision as a result of
a variety of factors.

The interior surface of the eyeball including the retina, optic disc and macula. Can
be seen with an ophthalmoscope.

The blending of the images from the two
retinas to form a single image in the brain.
See also binocular vision.

                                                                                 Page 34
Ganzfeld Phenomenon
A uniformly illuminated surface of a uniform field (a ganzfeld) quickly disappears
from visual perception and one experiences a grey fog.

Gaze Palsy
An inability of the eyes to move together in a specific direction, e.g. left gaze palsy.

Generic View
A view of an object that is not special. In other words, the view won't change
drastically with small perturbations in lighting or viewing direction. (As opposed to
Accidental View).

Gestalt Theory – Perceiving Wholes
The field of Gestalt psychology covers how we come to perceive whole objects or
forms from being presented with only parts. For example, in a large lecture hall, if
a speaker was standing behind a podium, we would not see their legs but when
we close our eyes and conjure up a perception of the speaker, we would not see
just see a floating torso and head. Gestalt psychology maintains that the whole is
greater than the sum of all its parts, and that the parts are transposable. Although
the parts may be changed, the same whole form will still be perceived. Also, every
note of a song may be taken an octave higher, but the same song would be
perceived. The relationships between the parts do not change.

A condition of the eye characterized by intraocular pressure (i.e., the production of
aqueous fluid by the ciliary body exceeds the drainage rate through the trabecular system
and Canal of Schlemm). Classification includes three basic types: primary glaucoma (both
open-angle or simple glaucoma, the most common type, and closed-angle or
                                                       acute/chronic type), congenital
                                                       glaucoma (buphthalmos or
                                                       hydrophthalmos and juvenile types
                                                       associated with congenital
                                                       anomalies), and secondary
                                                       glaucoma (due to changes in the
                                                       lens or uveal tract, trauma,
                                                       rubeosis, surgical procedures, or
                                                       topical corticosteroids). Heredity
                                                       seems to predispose individuals to
                                                       glaucoma, although it may also be
                                                       related to medications and/or
                                                       surgical procedures in other parts of
                                                       the eye. If untreated, the effects can
cause damage to the optic disk, optic nerve, restricted visual fields, and corneal edema;
cataracts also often develop. Complete blindness can result. When treated early, the
condition can be successfully managed medically. Because its onset is so gradual, all
adults should be checked regularly for glaucoma.

                                                                                    Page 35
A form of tumour. See optic nerve glioma.

Examination of the anterior chamber angle through a goniolens (special type of
contact lens).

Grave’s Disease
Graves' disease is a term used to describe the
commonest variety of hyperthyroidism, which is
regarded as having an autoimmune
basis. Autoimmune disease may be understood
as a process by which the body sees some part of
itself as being foreign and reacts to it much the
same way that it would with any bacteria or virus.
In the case of Graves' disease, the body sees the thyroid gland as the foreign
object and produces antibodies that attack the thyroid gland. This will often (but
not in all cases) cause the thyroid gland to become over active. Graves' eye
disease is currently believed to be due to a similar autoimmune reaction.
However, in the case of Graves' eye disease (Infiltrative Thyroid Ophthalmopathy),
different antibodies attack the muscles associated with eye and eyelid movement.
Although the thyroid gland and the eye may be under attack by the same immune
system, it is felt that both conditions remain independent of one another. The
antibodies that attack the eye can cause
inflammation and swelling of the muscles around the
eye, which is what can eventually cause protrusion of
the eyes, double vision and retraction of the eyelids
(see above photo's). In some cases the muscles
may be enlarged up to eight times their normal size
and may mimic an orbital tumour.

See Preferential Looking

                                                                          Page 36
                                       Defect in one half of the visual field. The
                                       most common type is the homonymous
                                       hemianopia, where the same half, left or
                                       right, is lost in each eye.
                                       The most common defect, right
                                       homonymous hemianopia, occurs in
                                       corresponding halves of the right field of
                                       vision. It can also occur in corresponding
                                       halves of the left field of vision (left
homonymous hemianopia), in the upper half of the field (superior hemianopia), the
lower half (inferior hemianopia), or both outer halves of the field (bitemporal
hemianopia). In hemianopia, half of the field is blanked out on both eyes.

Literally, "half vision;" a condition resulting from malfunction or damage to one side of the
optic tract (see diagram below). Images from only one half of each eye reach the brain;
thus, there is only reception of half-fields for each eye.

A condition following damage to the parietal stream of one hemisphere in which
individuals are unaware of stimuli from the hemifield that projects to that

Herpes Simplex
Herpes Simplex keratitis (HSK) is a viral
infection that if left untreated can have
devastating ocular consequences.
Approximately 90% of the population over the
age of 15 has been exposed to the type I
herpes virus. This herpes simplex virus is
contagious and can be transmitted by skin
contact with others and from one part of the
body to another (i.e. if one touches a cold sore
then the eye, one can infect the eye).
Interesting enough, the corneal infection or
"keratitis" form of the disease is not the bodies first contact with the virus. The
initial infection (acute primary herpes simplex) most often occurs in childhood.
The primary exposure to the virus may cause multiple whitish blisters around the

                                                                                      Page 37
eye termed viral eczema. Infrequently, a mild eye infection may be present. The
acute primary herpetic infection is generally self limiting, meaning that within a
given period of time, the infection resolves without any treatment or medical
intervention. The primary exposure to the virus may be so subtitle that it may not
even be noticed.
It is the secondary herpetic infection, acute secondary herpes keratitis, which is
more troubling for the eye. After the primary infection in childhood, the virus tends
to remain dormant within the nerves of the body, especially the trigeminal nerve.
The virus may become reactivated during emotional or physical stress,
overexposure to ultraviolet light (i.e. tanning) and/or in conditions of immune-
compromise (i.e. chronic diseases such as cancer, aids etc.). There are
approximately 700,000 new cases of herpes simplex epithelial keratitis in the
United States each year.

Rhythmic variations in pupil size, independent of light intensity.

Disease of the choroid; caused by an invasion of a fungal organism; transmitted
by airborne spores found in dried animal excrement; the peripheral fundus has
"punched-out" spots similar to healed chorioretinal lesions, but smaller and less
pigmented. Macular involvement may occur later (believed to be a result of earlier
choroidal sensitization and subsequent reinfection); these macular lesions may
progress to hemorrhagic detachments. There is no vitreous haze. There is a
positive reaction to a skin test for the disease.

Homonymous Hemianopia
See hemianopia.

A common staphyloccal infection of the lid glands; essentially an abscess, with pus
formation; symptoms include swelling, redness, and pain. Two types are classified: internal
hordeolum (relatively large, affecting the meibomian glands; may point toward the skin or
toward the conjunctiva) and external hordeolum (also known as a "sty;" smaller and more
superficial; an infection of the glands of Moll or Zeiss; painful; always points toward the
skin side of the lid margin). (see Sty)

The perception of colour. (See also Saturation).

                                                                                  Page 38
Hypermetropia, Hyperopia
Long sight. Distance vision is better than
near vision.

                                         Upward deviation of the eye is usually due to
                                         a paresis of one of the muscles that either
                                         elevate or depress the eye.

Abnormally wide-set eyes.

Clinical sign. Blood in the anterior chamber, such as following blunt trauma to the eyeball.
intraocular pressure. Fluid pressure inside the eye.

Abnormally narrow-set eyes.

                                                                                    Page 39
Refers to a relatively uncommon group of skin disorders characterized by the presence of
excessive amounts of dry surface scales. It is regarded as a disorder of keratinization or
cornification, and it is due to abnormal epidermal differentiation or metabolism.
The ichthyosiform dermatoses may be classified according to clinical manifestations,
genetic presentation, and histologic findings. Inherited and acquired forms of ichthyosis
have been described, and ocular alterations may occur in specific subtypes. These include
corneal flaking, corneal scarring and corneal ulcers.

Iconic Memory
Short-lived visual sensory memory which lasts about 500 msec. Disrupted by
masking (i.e. by a bright light). Tied to anatomical coordinates.

Intra-Ocular Pressure
The pressure within the eye. The fluid pressure within the eye created by the
continual production and drainage of aqueous fluid in the anterior chamber.

Intrinsic Images
A proposal put forward by Barrow and Tenebaum (1978) in which the visual
system maintains several representations of the visual scene including lightness,
illumination, depth etc. and the user can access each individual representation
separate from the others.

IOL (intraocular lens).
Plastic lens that may be surgically implanted to replace the eye's natural lens.

The coloured part of the eye. A circular muscular diaphragm, it regulates the
amount of light entering the eye by changing the size of the pupil.

IRITIS, a form of Anterior Uveitis is a term for an
inflammatory disorder of the coloured part of the eye (iris). In
the majority of cases there is no specific cause.
Occasionally, iritis is just one symptom of a disease that
affects other organ systems. These are called connective
tissue diseases and include: rheumatoid arthritis , sarcoid ,
lupus , scleroderma , Behcet's disease , anklylosing spondylitis, Reiter's disease,
Crohn's disease, ulcerative colitis, and B-27 disease. Sometimes, it is necessary
to establish whether iritis/uveitis is a manifestation of one of these or some other
underlying systemic disease.
The symptoms of iritis include light sensitivity, red eye, blurred vision, tearing,
pain, and sometimes floaters. The pupil may appear small in the affected eye
when compared to the normal pupil. Frequently iritis is a recurrent problem; after a
few episodes patients become very astute at early diagnosis. Iritis is sometimes

                                                                                  Page 40
confused with conjunctivitis, a much less serious disorder of the clear outer lining
of the eye.

Ishihara Colour Test
Familiar to most people, the Ishihara Colour Test uses a series of colour-dot
numerals to determine if a child has deficiencies in colour resolution. The most
common type of deficiency is the red-green difficulty in males, which accounts for
about 99% of all colour vision problems.

Colour blindness (colour vision deficiency) is a condition in which certain colours
cannot be distinguished, and is most commonly due to an inherited condition.
Red/Green colour blindness is by far the most common form, about 99%, and
causes problems in distinguishing reds and greens.
Another colour deficiency Blue/Yellow also exists, but is rare and there is no
commonly available test for it.
Depending on just which figures you believe, colour blindness seems to occur in
about 8% - 12% of males of European origin and about one-half of 1% of females.
Total colour blindness (seeing in only shades of gray) is extremely rare.
There is no treatment for colour blindness, nor is it usually the cause of any
significant disability. However, it can be very frustrating for individuals affected by
it. Those who are not colour blind seem to have the misconception that colour
blindness means that a colour blind person sees only in black and white or shades
of gray. While this sort of condition is possible, it is extremely rare. Being colour
blind does keep one from performing certain jobs and makes others difficult.

Kay Test
A test of visual acuity using pictures, suitable for children 18 months and older.

A term used to define a wide variety of corneal infections, irritations, and inflammations;
since each type of condition is unique, medical diagnosis and treatment is essential.
Corneal ulcers are commonly caused by bacterial or fungal invasions following superficial
corneal abrasions; among the common infectious agents are: staphyloccus, streptococcus,
herpes (both simplex and zoster), adenovirus, rubeola, rubella, mumps, trachoma,
infectious mononucleosis, and pneumococcus; also at fault may be Vitamin A deficiency or
broad spectrum antibiotic drug reactions. Corneal ulcers may also follow trauma, may be

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associated with other eye infections (e.g., conjunctivitis), may be related to other corneal
disorders (e.g., degenerative conditions, or ptosis, which may cause a "dry eye"), or may
arise from a variety of systemic disorders (especially those of autoimmune origin).
Symptoms of corneal infection include extreme pain and photophobia.

                                  A rare, bilateral, degenerative disease; inherited
                                  as an autosomal trait; affects all races; appears
                                  in the second decade of life, and progresses
                                  slowly between the ages of 20 and 60;
                                  associated with a number of other diseases,
                                  including Down's syndrome, atopic dermatitis,
                                  retinitis pigmentosa, aniridia, Marfan's syndrome.
                                  Keratoconus is literally an increasing conical
                                  shape to the cornea; the central cornea thins and
                                  may rupture in advanced stages. Blurred vision is
                                  the only symptom, however, examination shows
a distorted corneal reflection and an inability to see the fundi.

Obtaining corneal curvature measurements with a keratometer.

Lacrimal Gland
Almond-shaped structure that produces tears. Located at the upper outer region of
the orbit, above the eyeball.

The production of tears.

Acronym: Light Amplification by Stimulated Emission of Radiation. High energy light
source that uses light emitted by the natural vibrations of atoms (of a gas or solid material)
to cut, burn or dissolve tissues for various clinical purposes: in the retina, to treat diabetic
retinopathy and macular degeneration, to destroy leaking and new blood vessels
(neovascularization); on the iris or trabecular meshwork, to decrease pressure in
glaucoma; after extracapsular cataract extraction, to open the posterior lens capsule.

Acronym: LAser in SItu Keratomileusis. Type of refractive surgery in which the cornea is
reshaped to change its optical power. A disc of cornea is raised as a flap, then an excimer
laser is used to reshape the intrastromal bed, producing surgical flattening of the cornea.
Used for correcting myopia, hyperopia, and astigmatism.

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                         Laurence-Moon-Biedl Syndrome
                         Hereditary disorder characterized by retinitis pigmentosa,
                         intellectual impairment, obesity, extra fingers and toes and
                         underdeveloped sex organs.

Lazy Eye See amblyopia

Leber's Congenital Amaurosis
An abnormality of the retina occurring in infancy or early childhood, causing
blindness or near-blindness.

Leber's Optic Atrophy
An hereditary optic atrophy with rapid loss of vision in late teenage or early adult
years. Initially may be total loss of vision with subsequent restoration of peripheral

Legal Blindness
A measurement of a person's vision impairment for legal purposes, such as
eligibility for a pension. Based on visual acuity, visual fields or a combination of
these. People who are legally blind may have some usable vision. In New
Zealand, legal blindness refers to less than 6/60 in the best eye with correction, or
fields less that 20 degrees.

1. The natural, crystalline lens of the eye.
2. Any piece of glass or other transparent material which bends light rays
   predictably. See also convex lens, concave lens, contact lens.

L-H Test
A test of visual acuity using picture symbols, suitable for children 18 months and

Light Adaptation
See adaptation.

Light Perception (L.P.)
The ability to distinguish light from dark.

See hypermetropia.

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Low Vision
Low vision is a general term that refers to a permanent functional vision loss that
cannot be corrected by medication, surgery, or glasses. Low vision patients may
experience a wide array of diseases, field defects, and degrees of vision loss.
However, in general, low vision can be defined as a bilateral decrement to visual
acuity or visual field resulting from a disruption of the visual system. This
decreased acuity or field results in an impaired ability to perform work, leisure, or
daily living activities. See vision impairment.

Low Vision Aids
A variety of devices used by children and adults with vision impairment to improve
visual functioning. Examples are the CCTV and hand-held magnifiers.

Amount of light coming off of a surface. Subjectively, the unit of brightness
measured in candela per meter squared (cd/m2).

Abnormally large eyeball. See also buphthalmos, microphthalmus.

The small spot on the retina surrounding the fovea. Has the greatest
concentration of cones and is responsible for central visual acuity.
Macular Degeneration
                                            Hereditary and untreatable group of diseases
                                            which affect the macular area of the retina.
                                            When the disease affects the choriocappilaris,
                                            as in central areolar choroidal sclerosis, there is
                                            gradual loss of central vision in middle life.
                                            Genetically, this type of macular degeneration is
                                            autosomal dominant or recessive. Other types of
                                            macular degeneration affect the pigment
                                            epithelium. Stargardt-Behr disease is
                                            transmitted genetically as an autosomal
                                            recessive factor; it results in gradual
                                            deterioration of the pigment epithelium, which
                                            begins at different ages and progresses at
                                            different rates; rate and degree of visual loss
                                            varies from family to family. In Best's vitelliform
                                            macular degeneration (an autosomal dominant
                                            genetic pattern), changes are confined to the
                                            macula. Initially, it resembles a poached egg
"sunny side up;" later, the "yolk" appears scrambled, and central vision is seriously
decreased. (see also Scotomas)

Macular edema:
the collection of fluid in and under the macular portion of the retina

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Marfan's Syndrome
The Marfan syndrome is a heritable condition that affects the
connective tissue. The primary purpose of connective tissue is
to hold the body together and provide a framework for growth
and development. In the Marfan syndrome, the connective
tissue is defective and does not act as it should. Because
connective tissue is found throughout the body, the Marfan
syndrome can affect many body systems, including the
skeleton, eyes, heart and blood vessels, nervous system, skin and lungs. One or
more of the following conditions are common among persons with the Marfan
syndrome: high refractive error, flattened corneal curvature, dislocated lenses,
early onset cataracts, glaucoma, retinal detachment and strabismus.

Black pigment in the pigment epithelium cells that absorbs light not captured by
the retina preventing it from being reflected off the back of the eye.

Meningioma (Optic Nerve Sheath)
Meningiomas are believed to arise from arachnoid cap cells, and they usually are attached
to the dura. These tumors may arise from any location where meninges exist (eg, nasal
cavity, paranasal sinuses, middle ear, mediastinum).
In children, the more common locations of meningiomas include the orbit, the temporal
region, the foramen magnum, the tentorial region, the subfrontal base, the sellar region,
and the ethmoidal air sinus. Such locations for meningiomas are rare in adults.
Histologically, the meningothelial type is seen most frequently. As compared to adults, the
tumors in children tend to be more aggressive in terms of growth rate, tumor size,
propensity to undergo malignant changes, and recurrence rate.

                         Abnormal smallness of one or both eyes; congenital, and almost
                         always hereditary (usually recessive, but may also be dominant) .
                         Other ocular abnormalities also occur, including cataract,
                         glaucoma, aniridia, and coloboma. Systemic and anatomic
                         abnormalities also often occur; these include: polydactyly,
                         syndactyly, clubfoot, polycystic kidneys, cystic liver, cleft palate,
                         and meningoencephalocele. Deficient vision is the rule.

Minus lens.
Lens that is thicker at the edges than in the centre, increasing divergence of
incoming light rays. Corrects nearsightedness (myopia).

pupillary constriction

An individual with only one spectral channel (i.e. retina receptors have the same
spectral sensitivity). See Action Spectra, Rod Monochromatic

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Monocular Depth Cues
Relative size, texture gradients, perspective, shadow, height of retina image,
interposition, motion parallax.

Monocular Vision
Vision in one eye only.

pupillary dilation

Short sight. Near vision is better than
distance vision.


Near Vision
Visual acuity tested at approximately 40 cms, corresponding to normal reading

Abnormal formation of new blood vessels, usually in or under the retina or on the iris
surface. May develop in diabetic retinopathy, blockage of the central retinal vein, or
macular degeneration.

A progressive disease best known for its heterogeneity and variability. NF-1 can
manifest in many different ways in many different tissues within the same person
and among family members. Neurofibromatosis is a disorder of the
neuroectodermal system that results in benign hamartomatous tumours of any

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organ or system (most notably the skin, the eyes, and the nervous system) that
increase in number and size throughout life. These tumours are of tissues derived
from neural crest, particularly sensory nerves, Schwann cells, and melanocytes.
Initial manifestations most often occur in childhood.

Concerning the nervous system.

Night Blindness

Poor vision at night or in dim
illumination. Usually indicates a
defect in the retinal rods.

The picture shows normal night
vision (left and right) and the
effect of night blindness (middle

Null Point
Position of gaze where the eye movements of nystagmus are minimized or

Involuntary, rhythmical, repeated oscillations of one or both eyes, in any or all fields of
gaze; may be pendular (with undulating movements of equal speed, amplitude, and
duration, in each direction) or jerky (with slower movements in one direction, followed by a
faster return to the original position). Movements may be horizontal, vertical, oblique,
rotary, circular, or any combination of these. Generally, the faster the rate, the smaller the
amplitude (and vice versa). The defect is classified according to the position of the eyes
when it occurs. Grade I occurs only when the eyes are directed toward the fast
component; grade II occurs when the eyes are also in their primary position; grade III
occurs even when the eyes are directed toward the slow component. The cause of
nystagmus is unknown. Reduced acuity is caused by the inability to maintain steady
fixation. Head-tilting may decrease the nystagmus and is usually involuntary (toward the
fast component in jerky nystagmus, or in such a position to minimize pendular nystagmus).
Head nodding often accompanies congenital nystagmus. Dizziness or vertigo may be
experienced if oscillopsia (illusory movements of objects) occurs. Nystagmus may be
induced with an optokinetic drum or through the stimulation of the semicircular canals.
Congenital nystagmus of the pendular type usually accompanies congenital visual
impairment (e.g., corneal opacity, cataract, albinism, aniridia, optic atrophy, chorioretinitis).
Nystagmus may also accompany a number of neurological disorders, and may be a
reaction to certain drugs (including barbiturates).

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Ocular hypertension:
Elevated fluid pressure. The normal pressure is about 10 to 20mmHg, with the
majority of people falling between 13 and 19. Over 20 is considered "suspicious"
over 24 "cautiously concerned" and warranting immediate investigation, and over
30 is considered to be "urgent" and a potential emergency situation.

A medical doctor specialising in the diagnosis and treatment of eye diseases.
Performs surgery and may prescribe glasses and contact lenses.

An instrument for examining the interior of the eye. There are two
direct: Directs a fine beam of light into the eye and enables the
          examiner to see the lighted area.
indirect: Provides a magnified image of the inside of the eye by use of a bright light
          worn on the examiner's head and a hand held lens.

Optic Atrophy
Dysfunction of the optic nerve; may be congenital or acquired. If congenital, it is usually
hereditary. The milder form is autosomal dominant and has a gradual onset of
deterioration in childhood but little progression thereafter; the more severe form is
autosomal recessive and is present at birth or within 2 years; this form is accompanied by
nystagmus. Leber's Disease has an unclear mode of inheritance but is suspected of being
X-linked, since it only rarely occurs in women; optic neuropathy occurs more commonly in
20-30 year old males; some vision is retained but there are varying degrees of impairment.
The acquired type of optic atrophy may be due to vascular disturbances (occlusions of the
central retinal vein or artery or arteriosclerotic changes within the optic nerve itself), may
be secondary to degenerative retinal disease (e.g., papilledema or optic neuritis), may be
a result of pressure against the optic nerve, or may be related to metabolic diseases (e.g.,
diabetes), trauma, glaucoma, or toxicity (to alcohol, tobacco, or other poisons). Loss of
vision is the only symptom. Pale optic disk and loss of pupillary reaction are usually
proportionate to the visual loss.
Enhancing visual function may require high levels of illumination and enlarged print with
high contrast; magnification may be useful in some cases. Colour perception may be
Since it is impossible to identify exactly which fibers of the optic nerve are involved, it
should not be assumed that the optic nerve is not functioning at all; a better assumption
might be that the visual stimulus is experiencing difficulty in getting to the brain; if and
when it arrives, it may be in distorted or incomplete form. Thus, perceptual function may be
greatly affected. Moreover, day-to-day classroom performance of a student with optic
atrophy may fluctuate with no apparent reason The child may not be aware of these
fluctuations; the classroom teacher should be alerted to their possibility.

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Optic Chiasm
Location where the optic nerve axons from the two retinae join and are
reorganized into two separate groups (the optic tracts) that encode information
about the right and left visual fields. (See Visual Pathways)

Optic Disc, optic nerve head
              The head of the optic nerve in the eyeball, the appearance (colour,
              size) of which is important in diagnosing eye disease. Due to the
              lack of light-receptive cells, this point is sometimes called the “blind

Optic Nerve Glioma
Tumour of one optic nerve, causing vision impairment.

Optic Nerve Hypoplasia
Congenital under-development of the optic nerve. The optic disc is smaller than usual.
Produces varying degrees of vision impairment. Should not be confused with optic
atrophy. Optic nerve hypoplasia is an undeveloped optic nerve due to a neurological insult
early in the prenatal developmental period; the optic nerve has started to develop, but
regresses. Ophthalmologically, the nerve head appears unusually small, and is
surrounded by a white "halo" of scleral tissue showing through. The anomaly appears to
be related to chronic alcohol and drug abuse by the mother during the prenatal period.
CAT scans of these children reveal defects in the septum pelucidum and posterior corpus
callosum; those who lack the septum pellucidum have spatial orientation problems. At
least one third of these children had low APGAR scores and end up to have endocrine
problems. Most have some type of neurological problems Visual acuity ranges from
normal to severely impaired. See also Septo-Optic Hypoplasia.

Optical Power (of a lens)
Measure of how strongly a lens bends incoming rays. Measured in diopters and
equal to the reciprocal of focal length. Human eye has an optical power of about
58.8 diopters.

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                 Professional who makes and adjusts optical aids, e.g., eyeglass lenses,
                 from refraction prescriptions supplied by an ophthalmologist or

Optokinetic Reflex
Elicited by coherent movement of a large region of the visual field. Helps stabilize
the visual scene on the retina. (See Optokinetic Nystagmus).

Optokinetic Nystagmus
Eye movements characterized by a series of smooth movements in the same
direction of a moving visual scene interspersed with rapid movements in the
opposite direction. (See also Optokinetic Reflex).

Non-medical professional trained to examine eyes, prescribe glasses and contact
lenses and detect eye disease.

a non-surgical procedure using contact lenses to alter the shape of the cornea to
effect a change in the refractive error

A muscle condition in which when both eyes are open or one eye is covered, both
eyes point accurately towards the target.

Discipline dealing with the diagnosis and treatment of defective eye coordination,
binocular vision, and functional amblyopia by non-medical and non-surgical
methods, e.g., glasses, prisms, exercises.

An allied health professional who tests vision and who works to correct strabismus
and to improve binocular vision.

(Literally 'seeing again') is a form of visual preservation. It refers to reappearance of an
image after some time when the original external stimulus is no longer available. Neural
basis of palinopsia is unknown but many palinopsia patients have some right-hemisphere

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non-inflammatory swelling/elevation of the optic nerve often due to increased intracranial
pressure or space-occupying tumour.

(also known as optic neuritis) A general term implying inflammation of the optic nerve, but
also includes degeneration or demyelinization of the optic nerve; included retrobulbar
neuritis, a condition affecting the optic nerve behind the optic disk (i.e., there are no visible
changes of the optic nerve head unless optic atrophy has occurred). Causes include
demyelinating diseases (e.g., multiple sclerosis, postinfectious encephalomyelitis),
systemic infections (viral: polio, flu, mumps, measles; bacterial: pneumonia et al),
nutritional and metabolic diseases (diabetes, pernicious anemia, hyperthyroidism), Leber's
Disease, secondary complications of inflammatory diseases (e.g., sinusitis, meningitis,
tuberculosis, syphilis, chorioretinitis, orbital inflammation), toxic reactions (to tobacco,
methanol, quinine, arsenic, salicylates, lead), and trauma. Papillitis is differentiated from
papilledema; it is unilateral instead of bilateral, shows less elevation of the nerve head,
and sluggish pupillary response. In optic neuritis, there is usually a severe but temporary
loss of vision for several days and pain in the eye when moved.

Parallel Pathways
Anatomical and physiological evidence exists for parallel pathways in the visual system
e.g. Ventral and Dorsal streams for spatial and object processing. Parvo and Magno
pathways from LGN to V2, V2 and V4. Hubel and Livingstone (1987) supported a few for
strong segregation of the M and P pathways. Recent evidence (see Merrigan and
Maunsell, 1994 for a review) strongly disputes this claim and the general viewpoint today
is that much inter-stream communication occurs and that strict segregation of paths is

Partial Sight
See vision impairment.

Patching. Covering an amblyopic patient's preferred eye,
to improve vision in the other, neglected eye.

Perceptual Constancy
The phenomenon that the perception of an object remains constant despite changes in the
its size, lighting conditions and orientation.

Test. Method of charting extent of a stationary eye's field of vision with test objects
of various sizes and light intensities. Aids in detection of damage to sensory visual

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Peripheral Vision
Vision using parts of the retina other than the macula, that is, outside the direct
line of vision. It is less distinct than central vision but is important for mobility and
safety. (See Central and Peripheral Vision)

Persistent Hyperplastic Primary Vitreous (P.H.P.V.)
A developmental abnormality consisting of an opaque mass behind the crystalline
lens. Usually affects one eye only. Cataract may be present, the eye may be
small (microphthalmic) and vision is usually poor.

Surgical procedure. Use of ultrasonic vibration to shatter and break up a cataract, making
it easier to remove.

Surgical procedure which uses a laser beam to halt the spread of abnormal blood
vessels in the retina, for example, in retinopathy of prematurity.

A condition of abnormal sensitivity to light (i.e., the amount of light entering the eye);
usually, the iris is unable to constrict enough to reduce the light entering the eye. This
condition is normally a symptom of associated disorders or disease (e.g., corneal
inflammation, aphakia, iritis, or ocular albinism) Some drugs and/or poisons also can
cause photophobia by causing pupil dilation (notably, amphetamines and antihistimines,
cannabis and cocaine, atropine, scopolamine, mydriotics and cycloplegics and strychnine).

Bright light conditions where only the cones are functional.

The transformation of incoming light to a neural impulse

Pigment Epithelium
Cells lining the back of the eye containing melanin which resynthesize photopigment for

Yellowish-brown subconjunctival elevation composed of
degenerated elastic tissue; may occur on either side of
the cornea. Benign.

"Pink eye." See conjunctivitis.

Plus lens.
Lens that is thicker in the centre than at the edges, adding optical power to incoming light
rays. Corrects farsightedness (hyperopia).

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A measure of optical blur produced by natural imperfections of the lens of the eye. The
response to a point of light produces a certain amount of blur, the amount of this blurring is
known as the pointspread.

Target items in a visual search that have features which allow them to be detected quickly
regardless of the number of distracters presents are said to "pop-out".

Posterior Chamber
The space between the back of the iris and the front of the crystalline lens, filled
with aqueous.

Preferential Looking
One method of determining whether an infant can distinguish between two displays is by
observing their preference for looking. A specialized video camera can be set up on their
heads that point to the same thing they are looking at. Since infants have no convergence
ability, and must move their entire heads to focus on anything, this method works well.
When presented with two displays that a baby can tell apart from each other, the baby will
choose to look at one over the other, and usually it is one that seems interesting, or novel
to them. This is called preferential looking.
When a baby looks at the same display over and over again, they become familiarized
with it, and become bored. Therefore, the image that was formerly new becomes less and
less interesting to them as time goes on, and their attention span for that display
decreases. This is known as habituation. A test can be set up so that a baby is in a
darkened room (on a lap of a parent to prevent crying), and images can be flashed on the
screen in front of them for any amount of time. The first time such an image pops up, the
baby looks up, and the time the baby keeps looking at the image is measured. Once the
baby stops looking, the screen becomes dark again, and after a short interval, the same
image is shown again. The looking time is measured again, and the process is repeated to
give a graph similar to the one shown below.

After habituation to an image has been determined, a new image can be displayed on the
wall, and if the baby can tell that this is indeed a new image, the looking time will go up.
This is known as dishabituation. If not however, the looking time continues the pattern of
habituation upon the presentation of a new image, then it can be safely concluded that the
baby was not able to tell the difference between the two images. This kind of testing was
used to measure many of the sensory abilities of infants presented earlier.
When an infant of at least 3 months of age is presented with an image containing motion
and depth information, known as dynamic occlusion, the infant will preferentially reach and
try to grab for what seems to be the closer object. Dynamic occlusion is a truly

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foundational informational cue of depth, delivered by motion. When a surface moves in
front of another surface, the leading edge of the moving surface continually occludes, or
hides the surface below it. At the trailing edge of the moving surface, the background
surface is continually being exposed. These two absolute visual cues are processed by the
brain to give knowledge of depth.
Eye movement
It is possible to track where on an image a baby tends to look. Young infants will tend to
focus on one feature of an image, while an older infant will look at the whole figure. This is
probably because younger babies do not have convergence abilities, and thus in order for
them to see the whole figure; they would have to move their heads around to trace the
lines. Older infants have convergence abilities, and thus when they look at a figure, they
can rotate their eyes around to look at the whole image.

                                       A condition in which the focusing ability of a
                                       person's eyes has decreased to the point where
                                       vision at his reading distance becomes blurred
                                       and difficult.

PRK (photorefractive keratectomy).
Surgical procedure. Use of high intensity laser light (e.g., an excimer laser) to reshape the
corneal curvature; for correcting refractive errors. Includes laser sculpting, LASIK.

Progressive addition lens.
Type of near-vision eyeglass lens designed so that power for near increases gradually
from zero (in centre) to maximum add (lower portion) with no telltale bifocal demarcation

Proliterative retinopathy. See diabetic retinopathy.

A loss of the ability to identify members of complex categories such as faces

An artificial substitute for a missing body part, such as an artificial eye to replace
an enucleated eye.

Abnormal wedge-shaped growth on the bulbar conjunctiva. May
gradually advance onto the cornea and require surgical removal.
Probably related to sun irritation.

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A drooping of the upper eyelid when the eyes are open; may occur in one or both eyes;
may be constant or intermittent. If the condition is
congenital, it is usually a failure of the levator muscle to
develop; it may be hereditary (dominant). If the condition is
acquired, it is usually the result of a mechanical factor (the
lid is simply too heavy for the levator muscle to lift it),
associated with disease (commonly muscular dystrophy or
myasthenia gravis), or paralytic (related to the
malfunctioning of the 3rd cranial nerve). If the lid droops
enough to partially cover the pupil, the person attempts to
compensate by raising the eyebrow and/or by tilting the head back. If the drooping lid
obscures the pupil completely, amblyopia can develop. Disease-caused ptosis progresses
gradually; neurally associated ptosis has a range of developmental patterns and
behaviors, depending on the type of cranial nerve involvement.

The black circular opening in the centre of the iris which regulates the amount of
light entering the eye.

Radial keratotomy (RK)
Surgical procedure. Series of spoke-like (radial) cuts made in the corneal periphery to
allow the central cornea to flatten, reducing its optical power and thereby correcting

                               1. The bending of light rays as they pass from a
                               clear medium of one density to another of a different
                               density, e.g. light rays entering the eye and falling on
                               the retina.

                               2. The determination of an eye's refractive error and
                               of the best correction to be prescribed.

                               Light refracted through a glass

Refractive Error
In a normal eye, parallel rays of light focus exactly on the fovea (the central area of the
macula), when the eye is in a state of rest (i.e., the lens does not have to accommodate).
In a farsighted (hyperopic) eye, under the same conditions, the eyeball is too short, and
the light rays focus (theoretically) behind the fovea. In a nearsighted (myopic) eye, under
the at-rest conditions, the eyeball is too long, and the light rays come to a focus before
they reach the fovea.
Astigmatism is an irregular curvature of the cornea in one or more of its meridians. The
lens may also contribute to astigmatism (as in old age, when it may become somewhat
irregular in shape because of cataractous changes). Astigmatism may be simple (i.e., not
                                                                                    Page 55
combined with hyperopia or myopia), or compound; when an eye has both myopia and
astigmatism or hyperopia and astigmatism, it is a compound defect. Astigmatism may also
be "mixed" (when myopia is combined with hyperopic astigmatism, or when hyperopia is
combined with myopic astigmatism).
In middle age (beginning any time past age 40), the lens becomes less flexible and less
able to accommodate for nearpoint viewing; this condition is called "presbyopia" and is
described as "when arms aren't long enough." In addition to simple myopia or hyperopia
and the variations of astigmatism, and because the human organism has two eyes which
must have coordinated visual reception for good vision to occur, a multitude of refractive
variations are possible. Anisometropia refers to different refractive errors in each eye, and
aniseikonia denotes a difference in the image size in the two eyes.
Refractive errors tend to be inherited, but there is no pattern of inheritance. Size of the
eyeball, shape of the cornea, shape of the lens, and depth of the anterior chamber are all
variables in refractive errors These variables increase the possible ocular combinations for
refractive errors.
Symptoms of myopia include squinting and frowning; hyperopia may cause a lack of
interest in reading, rubbing of the eyes, or even headache, dizziness, or nausea.
Astigmatism may cause visual fatigue, headaches, frowning, and squinting.
Degenerative myopia (sometimes also called progressive myopia) is similar to simple
myopia except that the degenerative changes occur in the optic disk, choroid and retina,
sclera, and vitreous, and are not related to the degree of myopia (i.e., the myopia does not
increase; the structure of eye parts changes in such a way that visual function is negatively
affected). Loss of central vision, retinal detachment, and vitreous opacities are typical;
cataracts and secondary glaucoma may be additional complications. Progressive myopia
is genetically determined as a recessive trait.
Myopia and hyperopia are treated by the use of spherical concave and convex lenses,
respectively. Astigmatic corrections are cylindrical and are added to any prescription for
myopia or hyperopia. Presbyopia necessitates the use of bifocals or trifocals. In the
absence of disease or other ocular abnormalities, glasses or contact lenses are the only
treatment needed for refractive errors.

Refractive Errors – Implications
The wearing of glasses does not "strengthen" or weaken" eyes, or affect the degree and
progress of myopia. Eye exercises are of no benefit in improving refractive errors, since
they cannot alter the size of the eyeball or the refractive power of the lens.
Current interest in radial keratotomy (the surgical changing of the shape of the cornea)
suggests possible "cures" for high myopia and astigmatism, but the surgical techniques
are still in experimental stages. Long-term effects have yet to be observed, analyzed, and
Contact lenses have revolutionized the corrective lens industry, however, both hard and
soft contact lenses continue to be somewhat controversial. As with surgical solutions to
refractive errors, there are precautions and problems yet to be solved. Contact lenses are
preferable to glasses in some kinds of corneal anomalies and in unilateral aphakia, but
their general use still assumes user integrity. Severe ocular injury can occur if contact
lenses are used carelessly. Moreover, certain occupations (e.g., beauticians) should avoid
the use of the soft lenses, since they tend to absorb chemicals that could be harmful to the
eye. The decision to wear contact lenses must be made with care; they are not "for

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About 80% of children are hyperopic at birth because of the shortness of the eye;
approximately 5% are myopic. During the years between ages 2 and 25, there is a gradual
decrease in hyperopia; myopia usually increases somewhat during the teen years, and
levels off at around age 25, regardless of lighting, rest, amount of close work, or vitamins
taken. Most astigmatism remains fairly constant throughout life (in the absence of
Educational implications for refractive errors focus primarily on identification through early
and continual, adequate vision screening, referral, and follow-up. Most refractive errors
found in school children can be corrected with lenses. The major exception is the child with
high myopia, who may be unable to be fully corrected. Magnification, higher levels of
illumination, and/or telescopic aids may be indicated. Certainly, a low vision evaluation
should be pursued for these children.
In the case of degenerative myopia, the associated problems should be
addressed (e.g., central vision loss). Educational adjustments may need constant
re-evaluation as the ocular condition progresses.

Refractive Power
The strength of prescribed glasses or contact lenses. See also dioptre.

The inner lining of the eye containing cones and rods which
convert light signals into electrical impulses which pass
along the optic nerve into the brain.

Retinal Degeneration
Retinal tissue may degenerate for a number of reasons. Among them are: artery or vein
occlusion, diabetic retinopathy, R.L.F./R.O.P. or disease (usually hereditary). Retinitis
pigmentosa, retinoschisis, lattic degeneration, and macular degeneration are characterized
by progressive types of retinal degeneration. (see also Macular Degeneration and Retinitis

Retinal Detachment
                                       The retina is only firmly attached at the optic nerve
                                       head and at the ora serrata; elsewhere, the vitreous
                                       and general structure of the eye hold the retina in
                                       place. If, through disease, trauma, or a puncture
                                       wound, the retina is thinned or tears, vitreous fluid
                                       can leak behind the retina and cause it to pull away
                                       from its normal position. If the peripheral retina pulls
                                       away it can usually be reattached with little loss of
                                       function. If,
                                       the macular
                                       area is
                                       pulled away
                                       and is
separated from its capillary nourishing
connections, severe visual impairment (even total
blindness) can result. Among the predisposing
conditions for retinal detachment are: high myopia,

                                                                                      Page 57
aphakia, vitreous abnormalities (including diabetic retinopathy), retinal degeneration, and
trauma. Symptoms may range from none, to "flashing lights" and/or "floaters," to loss of
visual function. Immediate medical attention is essential.

Retinal Dysplasia
Abnormal development of the retina. Sometimes familial and frequently
associated with other developmental defects.

Retinal pigment epithelium
Pigment cell layer just outside the retina that nourishes retinal visual cells. Firmly attached
to underlying choroid and overlying retinal visual cells.

Retinitis Pigmentosa

A group of diseases which result in the degeneration of the retina; cause is unknown, but
suspected to be an enzyme in the retina; most types are hereditary, but the pattern of
inheritance varies. Rods are destroyed, beginning in the mid-periphery, and gradually
advancing inward toward the macula; "tunnel vision" is retained in the most common
types, but central acuity is also diminished in other types; many are myopic. Many develop
cataracts but are not as likely to have glaucoma or detached retinas. In Usher's syndrome
(5% prevalence among R.P.), central acuity is retained but there is accompanying hearing
loss; this type emerges during the teen years. In the "centro-peripheral" type of R.P. (17%
of cases), onset is around 6-15 years of age, there is no nystagmus, and both central and
peripheral vision are affected. In Leber's Disease (26% of R.P. cases), both central and
peripheral vision are affected, nystagmus is present, and cataracts do not usually develop;
it is congenital. The most common type (52%) also appears during the teen years, is
typified by severe and progressive peripheral loss, but good central acuity (no worse than
20/50) is maintained until around age 60. Varieties of the common type are genetically
different, and have characteristic loss of central acuity at different ages (e.g., 40-60). A few
syndromes have R.P. as a component (e.g., Laurence-Moon-Biedl). Night blindness is the
initial symptom and occurs early; deterioration of peripheral vision follows.

A malignant and life-threatening intraocular tumor which appears in children; 2/3rds
appear before age 3, however, rare cases have been reported at almost every age. About
30% are bilateral. It usually is unnoticed until it has progressed to a point of producing a
white pupil (unless it has caused a strabismus and is diagnosed earlier, since blind eyes in
children will often turn inward). Generally, the earlier the tumor is discovered, the better the
chance to treat it and prevent its spread through the optic nerve and orbital tissues. About

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94% of these tumors are due to mutations, but survivors will pass the mutated gene on
about 50% of the time.

Disease of the retina due to various causes.

Retinopathy of Prematurity (R.O.P.)
Formerly called retrolental fibroplasia (R.L.F.). One of the most devastating, baffling, and
controversial ocular conditions involving young children; first reported in the 1940's and
theorized to be caused by excess oxygen administered to premature infants; current
theories suggest that the cause may not yet be known, since it has also begun to occur in
full-term infants who did not receive oxygen. Among the causative theories are: too much
light, too soon, in hospital neonatal units; Vitamin E deficiency; multiple factors associated
with prematurity. Ocular involvement follows a pattern of initially constricted ocular blood
vessels, followed by vascular dilation and fibrovascular proliferation into the vitreous (thus,
the term "retrolental fibroplasia" -- literally, fibrous retinal vessel tissue behind the lens). In
some cases, there is partial or complete regression of the condition, with little ocular
damage. In other cases, the retina is pulled away and blindness ensues. Occasionally, the
condition is unilateral. Myopia and strabismus are common among children who retain
useful vision, and there is a possibility of retinal detachment in later life. Glaucoma, uveitis,
cataract, or phthisis bulbi (shrunken blind eye) may also occur months or years later.
o Stage 0: This is the mildest form of ROP. It is immature retinal vasculature. No clear
   demarcation of vascularized and nonvascularized retina is present. Only a suggestion
   of the border is noted on examination.
o In zone 1, this may appear as a vitreous haze, with the optic nerve as the only
   landmark. Weekly examinations should be performed.
o In zone 2, bimonthly examinations should be performed.
o In zone 3, examination every 3-4 weeks should be sufficient.
o Stage 1: A fine, thin demarcation line between the vascular and avascular region is
  present. This line has no height and no thickness.
o In zone 1, this should appear as a flat, thin line (usually nasally first). No elevation from
  the avascular retina should be present. The retinal vessels should be smooth, thin, and
  supple. Weekly examinations should be performed.
o In zone 2, bimonthly examinations should be performed.
o In zone 3, examination every 3-4 weeks should be sufficient.
o Stage 2: A broad, thick ridge clearly separates the vascular from the avascular retina.
o In zone 1, if there is any hint of pink or red in the ridge, this is an ominous sign. If there
  is any vessel engorgement, the disease should be considered threshold and treatment
  commenced within 72 hours.
o In zone 2, if there are no vascular changes and the ridge has no engorgement, the eye
  should be examined within 2 weeks. Prethreshold is defined as stage 2 with plus
o In zone 3, examination every 3-4 weeks should be sufficient, unless of course there is
  any vascular tortuosity or straightening of the vascular arcades.
o Stage 3: The extraretinal fibrovascular proliferation (neovascularization) may be
  present on the ridge, on the posterior surface of the ridge or anteriorly toward the
  vitreous cavity. The neovascularization gives the ridge a velvety appearance, a ragged
o In zone 1, if there is any neovascularization, it is serious and requires treatment.

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o In zone 2, prethreshold is defined as stage 3 without plus disease, or stage 3 with less
  than 5 contiguous or 8 noncontiguous hours. Threshold is stage 3 with at least 5
  contiguous or 8 noncontiguous hours and plus disease.
o In zone 3, examination every 2-3 weeks should be sufficient, unless there is any
  vascular tortuosity or straightening of the vascular arcades.
o   Stage 4: This stage is a subtotal retinal detachment beginning at the ridge. The retina
    is pulled anteriorly into the vitreous by the fibrovascular ridge.
o   Stage 4A does not involve the fovea.
o   Stage 4B involves the fovea.
o   Stage 5: This stage is a total retinal detachment in the shape of a funnel.
o   Stage 5A is an open funnel.
o   Stage 5B is a closed funnel.
Plus disease is defined as dilation and tortuosity of the peripheral retinal vessels, iris
vascular engorgement, pupillary rigidity, and vitreous haze, which are part of the
subclassification given to the above stages. The presence of plus disease is an ominous

A hand-held device for measuring an eye's refractive error. It does not require a
verbal response from the patient.

A term meaning a splitting of the retina into two layers; the separation is due to peripheral
cystoid degeneration. The juvenile type occurs before 10 years of age, and macular
involvement is frequent. The senile type occurs from age 20 on, commonly after age 40,
and there is usually no significant visual loss. If no retinal holes develop, there are rarely
serious consequences. If the retina does tear, and subsequently detaches, it may be
treated with photocoagulation, cryotherapy, diathermy, or scleral buckling. If the
detachment threatens the retina, it must be treated if vision is to be retained. Retinoschisis
appears to be an X-linked trait

Retrolental Fibroplasia (R.L.F.)
See retinopathy of prematurity.

Binocular Rivalry
When two different images are presented to the two eyes simultaneously, a
mosaic-like percept can be obtained, in which different regions of the image
correspond to left and right eye images. The brain will quickly suppress one
image, or will alternate between the two images as a coping mechanism.

One of the two types of light sensitive cells in the retina. They function in dim
lighting conditions (scotopic) and are responsible for night vision. Achromatic,
lower acuity and temporal resolution than cones, outnumber cones 20 to 1. Rod
system is convergent (many rods target one bipolar cell).

Rod Monochromatic
An absence of functioning cones (colour receptors) on the retina. Resulting image
lacks colour, and central vision is poor as the fovea (responsible for fine central
vision) consists mostly of cones in normally sighted children.
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Rubella (German Measles)
Maternal rubella (German measles) in the first trimester of pregnancy is generally
responsible for a triad of defects in the fetus: heart defects, hearing problems and eye
problems; mental retardation also often accompanies these defects. Ocular involvement is
typically cataracts (bilateral in 75% of the cases) but also may include uveal colobomas,
searching nystagmus, microphthalmus, strabismus, retinopathy, and infantile glaucoma.

The richness of hue. Indicates how much a colour has been diluted by greyness.

Using the eyes to search the environment. An important step in visual
development and which may be absent in children with severe vision impairment.

Schlemm's Canal
Circular channel deep in corneoscleral junction (limbus) that carries aqueous fluid from the
anterior chamber of the eye to the bloodstream.

The white of the eye. The tough outer covering of the eyeball in which is set the
clear cornea.

Scleral Buckle
A surgical procedure used in repair of a retinal detachment, where the sclera is
indented or "buckled" inward.

                                         Portion(s) of the retinal field that are non-
                                         functional (i.e., blind areas). Scotomas may be
                                         central, if caused by macular or optic nerve
                                         disease, or peripheral if the result of chorioretinal
                                         lesions or retinal holes. Field testing, if carefully
                                         done, can identify the areas affected.

                                         Donut Scotoma

Scotopic Vision
Vision under dim lighting conditions, using mainly the rod receptors. See also
photopic vision.

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(1) The reciprocal of the minimum stimulus strength required for the stimulus to be
detected reliably (1/threshold) May be related to noise and other mechanisms.
(2) Physiologically measured sensitivity averaged over many stimulus/response
cycles. A measure of gain. Noise free.

Sensitivity Control
Adjustment of eye sensitivity to compensate for a change in illumination.

Sensory Fusion
The combining of drawing together of two images which fall on different points in
the two retinas without changing the vergence of the two eyes.

                       Septo-Optic Dysplasia
                       A rare disorder characterized by visual impairment,
                       pituitary deficiencies, and specific brain abnormalities
                       (absence of the septum pellucidum and corpus callosum).
                       SOD may be apparent in early infancy, during childhood,
                       or, in some cases, as late as adolescence. In some cases,
                       individuals with SOD will have brain abnormalities that
                       affect just the structure of the brain, while others will have
                       abnormalities that affect the function of the brain. Typically,
                       however, a child will have both types, resulting in problems
                       ranging from mild to very severe.

Sheridan-Gardiner (STYCAR) Test
A test of visual acuity using letters which can be named or matched.

See myopia.

Size Constancy
The fact that the perception of the sizes of objects remains remarkably constant at
varying distance despite the fact that the image projected on the retina changes.

Slit Lamp
An instrument which allows layer-by-layer examination of the clear parts of the
eye, such as the cornea, crystalline lens and iris.

Snellen Chart
Eye chart using letters of decreasing size to test
visual acuity. Not suitable for young children. Other
tests using pictures and symbols can be used with
young children. See Kay Test, L-H Test,
Sheridan-Gardiner Test.

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Spectral imaging
Images created using different parts of the electro-magnetic spectrum.
This can be dramatically illustrated with views of astronomical bodies viewed
through telescopes (some on space platforms) equipped with different multi-
spectral sensing devices. Below are four views of the nearby Crab Nebula, which
is now in a state of chaotic expansion after a supernova explosion first sighted in
1054 A.D. by Chinese astronomers. The upper left illustration shows the Nebula in
the high energy x-ray region; the upper right is an image in visible light; the lower
left was acquired from the infrared region; and the lower right is a long wavelength
radio telescope image.

                                                                             Page 63
See strabismus.

  Medical term       Definition of term Common term

Esotropia                                Cross-eyed
                    Eye turned in
(convergent squint)                      Boss-eyed

                     Eye turned out      Wall eyes
(divergent squint)

                     Eye turned up

                     Eye turned down

Stabilized Retinal Images
Technique for examining perception without eye movement accompanied by loss
of colour and contour perception. Demonstrates that movement of an image
across the retina is vital for perception.

                                 Stargardt's Disease
                                 Hereditary degeneration of the macula occurring
                                 between the ages of 6 and 20. Causes central
                                 vision loss.
                                 The effects of beginning Stargardts on print

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Stereopsis (Stereoscopic vision, depth perception)
The visual blending of the images perceived by each eye to produce the visual
perception of solidity and depth. See Fusion.

Stiles-Crawford Effect
Light entering the eye through the centre of the pupil is more effective than light
entering at a peripheral point near the edge of the pupil. This is largely to the
directionally sensitive photoreceptors which are oriented toward the centre of the
pupil. This helps to minimize the effect of stray light.

Strabismus (Squint, turned or crossed eye)
                                 The failure of the two eyes to simultaneously
                                 direct their gaze at the same object and to
                                 work in unison. See also alternating squint,
                                 convergent squint, divergent squint, diplopia.

                                     Magazine cover - strabismus

Stroop Effect
When subjects are asked to name the
colours of the ink in which the words
are printed, their performance is
considerably slowed if the words
themselves are the names of other

                An infection in an eyelash follicle of the eyelid. Produces pain,
                swelling and pus.

See dislocated lens.

Superior Colliculus
A nucleus in the midbrain that coordinate visual, somatic and auditory information
adjusting movements of the head and eyes towards a stimulus. contains four
types of maps, a visual map, a map of the body surface, a map for sound in space
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and a motor map. Receives inputs from the retina (via the optic tract), the visual
cortex, the somatosensory system and auditory systems. Mediates saccadic eye
movements. Is the source of two major descending tracts, the tectospinal tract and
the tectopontine tract in addition to regions of the brain stem controlling eye

Where the image from one eye is ignored by the brain. Occurs in the presence of
a strabismus or where there is a marked difference in the refractive error of the
two eyes. Can lead to amblyopia.

Sympathetic Ophthalmia
A rare, bilateral granulomatous uveitis, associated with either a perforating eye injury in the
region of the ciliary body or a retained foreign body in the eye. The exact cause is
unknown, but it is believed to be related to sensitivity (i.e., a reaction) to uveal pigment. It
has been known to follow uncomplicated intraocular surgery for cataracts or glaucoma.
The injured eye becomes inflamed first, and the other eye follows (i.e., "sympathetically").
Symptoms include photophobia, redness, and blurred vision; in some cases, there are also
"floaters" and possibly pain. The history of trauma differentiates this condition from other
types of granulomatous uveitis; other differentiating factors include its bilateral, diffuse, and
acute nature. (see also Wounds)

A group of symptoms that occur together; may affect the whole body or any of its parts.
The following selected syndromes affect the eye.
Syndrome                General Body Characteristics                Ocular Involvement
Batten-Mayou            Cerebral degeneration; amaurotic
                        familial idiocy; appears in childhood;
                        early death.
Bourneville's Disease   Inherited as autosomal dominant;            Retinal tumors.
(also called            adenoma sebaceum (large
Tuberous Sclerosis)     "blackheads"); central nervous system
                        tumors; renal tumors; multiple lung
                        cysts; seizures; mental retardation;
                        appears at birth or within the first few
                        years; death in teens.
Coat's Disease          --                                          Chronic, progressive, inflammatory
                                                                    condition of the retina; exudations
                                                                    cause retinal detachment; iritis;
                                                                    glaucoma; cataracts; occurs mostly in
Down's Syndrome         Small stature; flattened/round              Hyperplasia of iris; narrow palpebral
                        mongoloid faces; saddle nose; thick         fissures; high myopia; strabismus;
                        lower lip; large tongue; soft, seborrheic   cataracts; gray spots on the iris.
                        skin; smooth hair; obesity; small
                        genitalia; short fingers; simian fold;
                        congenital heart anomalies; mental
                        retardation; psychic disturbances.
Edward's (Trisomy       Mental and physical retardation;            Corneal and lenticular opacities;
18)                     congenital heart defects; renal             unilateral ptosis; optic atrophy.

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Syndrome               General Body Characteristics                 Ocular Involvement
Galactosemia           Allergy to milk; autosomal recessive         If untreated, cataracts develop.
                       inheritance pattern. If not identified and
                       treated (withdrawal of milk and milk
                       products), can cause enlarged liver and
                       mental retardation.
Grave's                An endocrine problem; pretibial              Lid retraction; proptosis; extraocular
(Hyperthyroidism)      myxedema (clubbing of the fingers)           muscle involvement; corneal
                                                                    involvement; optic nerve involvement;
                                                                    most common in women.
Hurler's (Type I)      Autosomal recessive; gargoylism; thick       corneal clouding; buphthalmos;
                       tongue; puffy cheeks; unbilical hernia;      megalocornea; esotropia; slight ptosis;
                       flat nose-bridge; dwarfism; mental           pigmentary retinopathy; optic atrophy.
Laurence-Moon-Biedl    Autosomal recessive; obesity; mental         Retinitis pigmentosa.
                       retardation; polydactyly; hypogenitalism.
Lowe's                 X-linked; cerebral defects, mental           congenital cataracts; infantile glaucoma;
                       retardation; dwarfism; renal dysfunction;    nystagmus; occur only in males.
                       high early mortality rate.
Marchesani's           Autosomal recessive; multiple skeletal       Spherophakia; ectopia lentis, which
                       abnormalities; short and stocky with         leads to lenticular myopia; iridonesis;
                       well developed muscles; hands and feet       glaucoma (which resists treatment);
                       spade-shaped; childhood X-rays show          poor visual prognosis.
                       delayed carpal and tarsal ossification.
Marfan's               Autosomal dominant; arachnodactyly           Dislocated lens (usually superiorly and
                       (increased length of long bones,             nasally); severe refractive errors;
                       especially in fingers and toes); scanty      megalocornea; cataract; uveal
                       subcutaneous fat; relaxed ligaments;         colobomas; secondary glaucoma.
                       congenital heart disease; spine/joint
                       deformities; high infant mortality rate.
Patau's (Trisomy 13)   Cerebral defects; cleft palate; heart        Anophthalmos; microphthalmos; retinal
                       lesions; polydactyly; hemangiomas;           displasia; optic atrophy; coloboma of
                       death by 6 months.                           uvea; cataracts.
Refsum's               Spinocerebellar ataxia; deafness;            Retinal degeneration.
Rubella                Maternally induced infection of fetus;       Bilateral cataracts; uveal colobomas;
                       heart abnormalities; hearing defects;        searching nystagmus; microphthalmus;
                       sometimes mental retardation.                strabismus; retinopathy; glaucoma.
Spielmeyer-Vogt        Autosomal recessive; central nervous         Cerebromacular degeneration, involving
                       system disorders; mental deterioration;      the outer layers of the retina; pallor of
                       spasticity; epilepsy and seizures;           the optic disk.
                       appears in childhood; death in teens.
Bielschowsky's         (similar to Spielmeyer-Vogt)                 Cerebromacular degeneration, involving
                                                                    all layers of the retina.
Stevens-Johnson        Hypersensitivity to drugs or food (rash      Purulent conjunctivitis; occlusion of
                       develops).                                   lacrimal gland ducts (which produces a
                                                                    "dry eye;" corneal ulcers and
Still's Disease        Juvenile rheumatoid arthritis, especially    Chronic uveitis; cataracts; secondary
                       in the knee joints.                          glaucoma; band-shaped keratopathy;
                                                                    ocular complications occur three times
                                                                    as often in girls.
Sturge-Weber           Autosomal dominant; port-wine-stain          Unilateral infantile glaucoma on the
                       angioma on one side of the face; central     affected side; appears at birth; choroidal
                       nervous system disorders (including          hemangioma.
                       jacksonian convulsive seizures; no
                       treatment; death by age 30.

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Syndrome              General Body Characteristics                    Ocular Involvement
Tay-Sachs             Autosomal recessive; mental and                 Cerebromacular degeneration;
                      physical deterioration in Jewish infants,       degeneration of inner layer of the retina;
                      first 2-3 years of life; death shortly after.   retinal pigmentary changes; optic
                                                                      atrophy; characterized by "cherry red"
                                                                      spot in macula.
Turner's (monosomy)   Affects females; retarded growth;               pterygium colli; epicanthus; ptosis;
                      rudimentary ovaries and female                  colour blindness
                      genitalia; amenorrhea.
Usher's               Congenital hearing loss to some                 Retinitis pigmentosa.
                      degree, but usually deteriorates in teen
Wilson's              Recessive inheritance pattern;                  Pigment ring in periphery of cornea
                      neurological symptoms (chorea,                  (progressive, but rarely occludes the
                      spasticity, dysarthia, dysphagia);              pupil); sometimes cataracts.
                      cirrhosis of the liver; faulty renal
                      function; linked to defects in copper

Tectopontine Tract
Relays information from the colliculus to the cerebellum for further coordination of
eye and head movement. See Superior Colliculus

Tectospinal Tract
Axons from the colliculus to the upper spinal cord where the neck motor neurons
are located. Involved in reflex control of head and neck movements

Instrument to measure the pressure inside the eye. See also glaucoma.

Test. Measurement of intraocular pressure.

An acronym for several organisms (Toxoplasmosis, Other viral infections, Rubella,
Cytomegalovirus, Herpes) which can infect the unborn child and cause among
other defects, vision impairment.

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                                              A type of granulomatous uveitis
                                              caused by a protozoan parasite
                                              (Toxoplasma gondii); rats, birds, or
                                              humans can become infected by
                                              ingesting the parasite which is found in
                                              cat faeces; ingestion of raw meat is
                                              another source of the disease There
                                              are two types: congenital and
                                              acquired. The congenital type is a
                                              result of intrauterine infection; the
                                              inflammatory process is more severe
                                              and is often bilateral; chorioretinitis is
almost always present Cerebral involvement, with radiopaque calcification and
mental retardation occurs in about 10% of the congenital types. The acquired type
may appear at any age and is usually
milder; it is often unilateral and frequently
does not involve the central nervous
system. The eye may not even be involved
in the acute stage, but chorioretinitis may
occur in the chronic form. Typically, the
chorioretinitis progresses to a healed stage,
leaving scars on the retina and choroid. If
the macula has been involved, loss of
central vision is permanent. The condition is
not progressive, but usually chronic; new
lesions may develop later. (see also
                                                            Toxoplasmose retinal scarring

Trabecular meshwork
Mesh-like structure inside the eye at the iris-scleral junction of the anterior chamber angle.
Filters aqueous fluid and controls its flow into the canal of Schlemm, prior to its leaving the
anterior chamber.

A form of bilateral kerato-conjunctivitis which causes corneal scarring; at its onset, it
resembles conjunctivitis with symptoms of tearing, photophobia, pain, swelling of the
eyelids, and superior keratitis; as it passes through four stages, the conjunctival tissues
become follicular, heal, and finally scar. Lacrimal glands and ducts are often affected as
well; the upper lid may turn inward and the lashes then abrade the cornea; corneal
ulceration results, becomes infected, and ultimately scars. When scarring is extensive,
blindness results. The disease is spread by contact; flies and gnats may also transmit it.

Following a moving object using eyes and/or head movements.

Tracking Problems
There are two types of eye movements. One is a smooth tracking motion and is
called a pursuit and the other is a jump from one target to another. Accurate
movements are required so as not to skip words or loose ones place.

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Trichromatic Colour Vision
Colour vision based on three cone photoreceptors pigments in the retina, as in humans.

Eyeglass lens that incorporates three lenses of different powers. The main portion is
usually focused for distance (20 ft.), the centre segment for about 2 ft., and the lower
segment for near (14 in.).

May occur in any part of the eye or its related structures; may be benign or malignant. Lid
tumors of the benign type include nevi, warts, molluscom contagiosum, xanthelasma, and
hemangioma; malignancies of the lids include carcinoma, sarcoma, and melanoma. The
conjunctiva is also a site for a number of both benign and malignant growths. The cornea
is subject to extensions of conjunctival carcinomas and melanomas Intraocular tumors are
more difficult to differentiate (between the benign and malignant types) since biopsy is
difficult, if not impossible. Orbital tumors are difficult to identify until they are large enough
to displace the eyeball. The CAT scan and untrasonography are often used as diagnostic
tools, and help to locate the tumor for surgical exploration. (see also Retinoblastoma)

Tunnel Vision

The loss of peripheral vision so that only a small area of central vision remains.
The affected individual has the impression of looking through a tunnel. Has
consequences for night vision and for safety and mobility. See also retinitis

Turned Eye
See strabismus.

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Ultraviolet radiation:
radiant energy with a wavelength just below that of the visible light. UV-c is the
shortest wavelength at 200-280nm and is absorbed by the atmosphere before
reaching the surface. Extremely damaging to living tissue. UV-b, a t 280-315nm is
"burning rays" of the sun and is damaging to most living tissue. UV-a, at 315-
400nm is "tanning rays" of the sun and is somewhat damaging to certain tissues.
UV radiation has been described as a contributing factor to the processes which
result in ARMD and cataracts and causes exposure keratitis. See Action Spectra.

The collective name for the iris, ciliary body and choroid.

Uveal tract
Pigmented layers of the eye (iris, ciliary body, choroid) that contain most of the
intraocular blood vessels.

                                              Broadly defined: an inflammation of the uveal
                                              tract (one or all three parts); has many causes,
                                              but often the cause is unknown; common
                                              mainly among young and middle-age groups.
                                              Two types are distinguished: non-
                                              granulomatous (which occurs mainly in the iris
                                              and ciliary body) and granulomatous (which
                                              commonly occurs in the posterior, or
                                              choroid/retina area). Non-granulomatous uveitis
                                              is the more common type and is typified by
                                              acute onset, pain, photophobia, blurred vision, a
                                              small and irregular pupil, and a marked
                                              circumcorneal flush; there is usually no vitreous
haze; it is usually unilateral. Recurrence is common, but the prognosis is good.
Granulomatous uveitis is characterized by an insidious onset, minimal or no pain, only
slight photophobia, blurred vision, a small and irregular pupil, and often a vitreous haze;
prognosis is fair to poor. Uveitis is associated with other diseases (e.g., rheumatoid
arthritis, tuberculosis, toxoplasmosis, histoplasmosis, toxocariasis, pars planitis,
sympathetic ophthalmia, sarcoidosis). Anterior uveitis may cause glaucoma or cataract,
vitreous degeneration, and retinal detachment. Posterior uveitis nearly always involves
both choroid and retina (chorioretinitis), leaving scars and scotomas.

Visual acuity. Assessment of the eye's ability to distinguish object details and
shape, using the smallest identifiable object that can be seen at a specified
distance (usually 20 ft. or 6 metres)

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Visual Attention
Clearly, everything that is presented to our retina is not necessarily seen, and thus
things can be ignored. The simple fact that we possess the faculty of attention
seems to indicate that not everything in the world is of importance to us. We only
pay attention to the things we care about. This would explain the fact that people
have a general tendency to pay more attention to people of the same age and
social group. The truth is, most of what goes on in the world goes unnoticed to us.

Vision Development (in infants)
Infants in the age of 0-2 months have dichromatic vision. This is to say that they only have
two types of functional cones, ones that are especially sensitive to reddish light, and ones
that are especially sensitive to greenish light. Starting from around the 2nd or 3rd month,
infants' functionality of the cones that are sensitive to bluish light develops and they
become trichromatic like normal adults.

Visual acuity / contrast sensitivity
The visual acuity of infants from the ages of 0-6 months is around 20/500. This numerical
representation of their acuity means that these infants can see the same level of detail
from 20 feet that an adult with normal vision can detect from 500 feet. This acuity is
considered to be legally blind. Although these infants may not be able to detect much
detail, they can however, detect motion fairly well. At around 7 months, the acuity of
infants improves to around 20/40. There are several reasons why the acuity of infants is so
low, but the three main reasons are as follows.
1) Cones - cell migration
The falloff of the density of cones from the fovea to the periphery is much more gradual
than in mature eyes. As infants grow and develop, the cones migrate closer into the fovea,
and the falloff density of cones from the fovea to the periphery becomes much steeper.
2) Cell elongation
In a newborn infant, the cone cells are much wider and shorter than those in mature eyes.
Therefore, these cones detect the ambient light, and directional sensitivity is terrible. Light
coming from a wide range of positions will be detected by the same cones because of their
wideness and lack of directional sensitivity. As the infants grow older, the photoreceptor
cells grow longer.
3) Cortical branching
This reason is the "catch-all" of explaining why infants have such low visual acuity,
although it is important. As the infant develops, more complex pathways begin to form in
their visual system, to allow for the detection of more detail.

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Accommodation / convergence
Newborn infants have no accommodative ability, or ability to change the thickness of their
lens. Infants have one set accommodative setting, where things around 2.5 feet away are
seen most clearly. In addition, a baby's eyes do not have the ability to rotate, and thus
their convergence ability is non-existent as well. If they want to focus on something, they
must move their entire head, and cannot just rotate their eyes to fixate on an object. By 1
month, infants have little accommodative/convergence ability, and by 3 months, their
accommodative and convergence abilities are good.
Stereopsis is being presented with two slightly different images from each eye, and
transforming that into a singular knowledge of depth. Both images coming from each eye
are slightly different because the eyes are not located in the exact same positions.
Newborn infants do not have the ability to derive depth information from stereopsis.
Starting around months 4-5, infants have pretty good understanding of depth from retinal
disparity, and before this time there is a critical developmental period in which they must
experience retinal disparity. If the infants do not experience retinal disparity (ex. wearing
an eye patch for an extended period of time), they never fully develop the ability to derive
depth information from stereopsis.

                         Visual Field
                         The extent of space seen by an eye which is looking straight
                         ahead. Fields can be mapped by computer. The diagram
                         shows severely restricted visual fields.
                         The different causes and types of visual field loss depend on the
                         part of the visual pathway that is damaged.
                          Causes of damage to the Retina or Optic Nerve
                          There are many different causes of damage to the retina and optic
nerve. Some of the more common causes include:
•       Glaucoma
•       Toxoplasma Infection
•       Coloboma
•       Tumours
These conditions may cause patches of visual field loss. The macula is the central bit of
the retina. This bit is for seeing in fine detail and in colour. It is used for things like reading,
watching TV and recognising faces. If this bit of the retina gets damaged then the child is
very likely to complain of blurred vision. If a bit of retina away from the centre is affected
then the child may not notice the visual field loss at all.
If damage occurs at the optic chiasm a special type of visual field loss may occur. The
child will lose the outside bits of the visual world on both sides. It is called a bi-temporal
hemianopia. ‘Bi’ means both right and left sides. ‘Temporal’ means the outside bit of
vision. ‘Hemi’ means half and ‘anopia’ means no vision. Damage to the chiasm may occur
from small growing tumours. This is very rare in children.
Causes of damage to the brain
There are many different causes of damage to the visual pathway in the brain. The
damage can also occur at different times in the child’s life. Often it is not known what the
cause of the damage to the brain is.
Most often damage occurs while the unborn child is still in the womb. The reasons for
damage to the brain while in the womb are often not known or well understood. Some

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cases might be due to the lining of the womb becoming infected. This type of damage can
also be associated with early (premature) birth.
Damage can also occur around the time of birth. If the baby becomes ‘tired’ during birth,
damage can sometimes occur to the brain. This might be due to poor blood supply to the
baby or difficulty with breathing.
Most children even if they do have problems in the womb or during birth do not have any
problems with vision or the brain.
There are many other causes of damage to the brain that can affect vision after a child is
born such as:
•      Infection (meningitis, encephalitis and infected intracranial aneurysms)
•      Damage to the drainage of water from the brain (hydrocephalus)
•      Serious head trauma
•      Brain tumours
If a part of the right visual pathway in the brain is damaged the child may develop left
visual field loss. If a part of the left visual pathway is damaged the child may develop right
visual field loss. If the top part of the visual pathway is damaged the child may develop
visual field loss in the bottom part of the visual field. If the top part of the visual pathway is
damaged the child may develop visual field loss that affects the bottom of the visual field.
The relationship between visual pathway damage and visual field loss is all opposite to
what you might think.

Vision Impairment
Colenbrander (1977) modified the W.H.O's Classification of
Visual Performance to provide a useful frame of reference
for educational purposes. The following three categories are
universally accepted:
1.   Blindness (moderate, severe, total)
     The visual disability of this group is such that they
     require low vision aids and the use of other senses even
     for gross visual tasks. Most features of the visual
     environment are not available because of profoundly low
     visual acuity or severely restricted visual field.
2.   Low Vision (moderate, severe)
     The visual disability of this group is such that they require low vision aids for
     detailed visual tasks. Some features of the visual environment may be
     available, but without training, little attention is given to them.
3.   Normal (normal, near normal)
Members of this group, often with low vision aids, can perform all visual tasks.
They are aware of and consciously attend to many features of the visual
environment but training could improve visual functioning.

Visual Evoked Response (V.E.R.)
Also known as Visual Evoked Potential (V.E.P.)
A computerized recording of electrical activity from the brain resulting from light
flashes to the retina. Used to assess the quality of the visual message from the
retina to the brain.

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Visual Acuity
The eye's ability
to distinguish
detail. Distance
and near acuity
must be
separately as
they may differ.

   Distance Acuity Tests

Visual Cortex
The part of the brain where visual information is registered.

Visual System
A combination of the optics (eyes, retinas) transmission (optic nerves) and
reception (visual cortex).

Visual Phosphene
The visual perception resulting from non-photopic stimulation such as electrical
stimulation of the cortex or pressing on the eyeball. Brinkdley & Lewin 1968) and
Bak et al (1990) have shown that stimulation of the cortex produces visual

Visual Perception
The term given to what the brain does with the image received, or how it responds
or reacts to it. Visual Perception refers largely to learned behaviour, and the way
in which the child constructs reality based on visual information. It is said that no
two people perceive the world in exactly the same way, as no two people have
had exactly the same visual (or sensory) experiences.
Although it is unclear exactly how perception occurs, it is clear that without
perception, all the information in the world would be meaningless to us. The
process of perception allows us to take sensory information and translate that into
a meaningful understanding by combining the sensory information with a memory
base that is continually expanding.
The mind can only see what it is prepared to see. – Edward de Bono
It is very hard to see something that has no shape or name - James Elkins
Vision is the primary medium of thought - Rudolf Arnheim
Visual literacy is the ability to understand and to use images, including the ability to think,
learn and express oneself in terms of images - R A Braden & J A Hortin

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Not all perceptions are logical. The brain will struggle to make sense of some
visual stimuli:
                          Three-prong fork and two sticks?.
    Incongruous design elements on opposite ends of the parallel lines create confusion.

Vision Therapy
Exercises prescribed by an
orthoptist or optometrist to
correct poorly developed
vision in children.

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Visual Pathways

Our attempt to understand the visual pathways is very much like approaching a
machine about which we know nothing but its basic functions. By way of analogy,
we all know how to operate a car (more or less) and recognize its basic function,
to get us from one place to another. Now let us assume that we knew absolutely
nothing but this and, for some reason, decided we should learn every detail of the
cars inner workings. We might start by trying to figure out the parts of the car
devoted to this central function and those that are not. We remove the bumper,
horn, AC, and windshields and find the car still runs splendidly. We remove small
parts of the engine, piece by piece until the car no longer starts, analogous to the
lesion studies in the brain. Slowly we begin to understand what parts of the car
(brain) are involved in locomotion (vision). In vision research we are very much at
this stage of the game, still wanting to know what each part actually does, when it
does it, and how all the individual parts act in concert. It is the hope of many
researchers that a careful investigation into the structure and function of the visual
pathways using chemical, electrophysiological, genetic, and behavioural

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approaches will culminate in a true understanding of how the brain provides us
with this most crucial of sensory capabilities, vision.

Vitreous (or vitreous humour)
The transparent jelly-like substance which fills the portion of the eye between the
crystalline lens and the retina.

Vitreous detachment.
Separation of vitreous gel from retinal surface. Usually innocuous, but can cause retinal
tears, which may lead to retinal detachment. Frequently occurs with aging as the vitreous
liquefies, or in some disease states, e.g. diabetes and high myopia.

See exotropia.

May be non-penetrating or penetrating; if non-penetrating, can be due to abrasion (usually
corneal or conjunctival), contusion, or burns. Although abrasion injuries are painful, they
usually heal quickly after treatment Contusions may result in hemorrhages (a "black eye,"
subconjunctival bleeding, vitreous and or retinal hemorrhaging), ruptures (of the cornea,
root of the iris or iris sphincter, dislocation of the lens, choroidal rupture and/or retinal
detachment), paralysis or spasm of the muscles of accommodation, or optic nerve injury.
Secondary complications of contusion may be traumatic cataract or secondary glaucoma.
Thermal burns damage eye tissue much as they do other skin tissues, but corneal burns
may result in opaque scar tissue. Ultraviolet irradiation (e.g., unfiltered ocular exposure to
an electric welding arc) can produce a superficial keratitis (which is painful but usually
heals quickly). Solar macular burns cause permanent visual damage; excessive X-ray
exposure produces later cataractous changes.
Penetrating injuries include corneal and conjunctival foreign bodies (which usually heal
well and produce no visual loss unless the central cornea is involved), lacerations (both of
the lids and of the eyeball), and intraocular foreign bodies. The prognosis for lacerations
depends on the extent (depth) of the wound, and whether there has been any loss of
ocular contents.

Intraocular foreign bodies are the most serious type of penetrating wounds. Identification
and localization of the object is essential, since some materials (e.g., glass or porcelain)
may be tolerated and left alone, while others (e.g., copper or iron) must be removed
because of later chemical reactions within the eye. Moreover, damage to ocular structure
and/or tissue must be detected prior to treatment decision.

Anatomy. Radially arranged fibres that suspend the lens from the ciliary body and hold it in

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