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					ASTIGMATISM
Astigmatism is a type of refractive error wherein the
refraction varies in the different meridia. Consequently,
the rays of light entering in the eye cannot converge
to a point focus but form focal lines. Broadly, there
are two types of astigmatism: regular and irregular.
REGULAR ASTIGMATISM
The astigmatism is regular when the refractive power
changes uniformly from one meridian to another (i.e.,
there are two principal meridia).
Etiology
1. Corneal astigmatism is the result of abnormalities
of curvature of cornea. It constitutes the most
common cause of astigmatism.
2. Lenticular astigmatism is rare. It may be:
i. Curvatural due to abnormalities of curvature of
lens as seen in lenticonus.
ii. Positional due to tilting or oblique placement of
lens as seen in subluxation.
iii. Index astigmatism may occur rarely due to
variable refractve index of lens in different meridia.
3. Retinal astigmatism due to oblique placement
of macula may also be seen occasionally.
Types of regular astigmatism
Depending upon the axis and the angle between the
two principal meridia, regular astigmatism can be
classified into the following types :
1. With-the-rule astigmatism. In this type the two
principal meridia are placed at right angles to one
another but the vertical meridian is more curved than
the horizontal. Thus, correction of this astigmatism
will require the concave cylinders at 180° ± 20° or
convex cylindrical lens at 90° ± 20°. This is called
'with-the-rule' astigmatism, because similar astigmatic
condition exists normally (the vertical meridian is
normally rendered 0.25 D more convex than the
horizontal meridian by the pressure of eyelids).
2. Against-the-rule astigmatism refers to an
astigmatic condition in which the horizontal meridian
is more curved than the vertical meridian. Therefore,
correction of this astigmatism will require the
presciption of convex cylindrical lens at 180° ± 20° or
concave cylindrical lens at 90° ± 20° axis.
3. Oblique astigmatism is a type of regular
astigmatism where the two principal meridia are not
the horizontal and vertical though these are at right
angles to one another (e.g., 45° and 135°). Oblique
astigmatism is often found to be symmetrical (e.g.,
cylindrical lens required at 30° in both eyes) or
complementary (e.g., cylindrical lens required at 30°
in one eye and at 150° in the other eye).
4. Bioblique astigmatism. In this type of regular
astigmatism the two principal meridia are not at right
angle to each other e.g., one may be at 30o and other
at 100°.
Optics of regular astigmatism
As already mentioned, in regular astigmatism the
parallel rays of light are not focused on a point but
form two focal lines. The configuration of rays
refracted through the astigmatic surface (toric
surface) is called Sturm’s conoid and the distance
between the two focal lines is known as focal interval
of Sturm. The shape of bundle of rays at different
levels (after refraction through astigmatic surface) is
described on page 25.
Refractive types of regular astigmatism
Depending upon the position of the two focal lines in
relation to retina, the regular astigmatism is further
classified into three types:
1. Simple astigmatism, wherein the rays are focused
on the retina in one meridian and either in front
(simple myopic astigmatism – Fig. 3.30a) or behind
(simple hypermetropic astigmatism – Fig. 3.30b) the
retina in the other meridian.
OPTICS AND REFRACTION 37
2. Compound astigmatism. In this type the rays of
light in both the meridia are focused either in front or
behind the retina and the condition is labelled as
compound myopic or compound hypermetropic
astigmatism, respectively (Figs. 3.30c and d).
3. Mixed astigmatism refers to a condition wherein
the light rays in one meridian are focused in front and
in other meridian behind the retina (Fig. 3.30e). Thus
in one meridian eye is myopic and in another
hypermetropic. Such patients have comparatively less
symptoms as 'circle of least diffusion' is formed on
the retina (see Fig. 3.15).
Symptoms
Symptoms of regular astigmatism include: (i) defective
vision; (ii) blurring of objects; (iii) depending upon
the type and degree of astigmatism, objects may
appear proportionately elongated; and (iv) asthenopic
symptoms, which are marked especially in small
amount of astigmatism, consist of a dull ache in the
eyes, headache, early tiredness of eyes and
sometimes nausea and even drowsiness.
Signs
1. Different power in two meridia is revealed on
retinoscopy or autorefractometry.
2. Oval or tilted optic disc may be seen on
ophthalmoscopy in patients with high degree of
astigmatism.
3. Head tilt. The astigmatic patients may (very
exceptionally) develop a torticollis in an attempt
to bring their axes nearer to the horizontal or
vertical meridians.
4. Half closure of the lid. Like myopes, the astigmatic
patients may half shut the eyes to achieve the
greater clarity of stenopaeic vision.
Investigations
1. Retinoscopy reveals different power in two
different axis (see page 548)
2. Keratometry. Keratometry and computerized
corneal topotograpy reveal different corneal
curvature in two different meridia in corneal
astigmatism (see page 554)
3. Astigmatic fan test and (4) Jackson's cross cylinder
test. These tests are useful in confirming the power
and axis of cylindrical lenses (see pages 555, 556).
Fig. 3.30. Types of astigmatism : simple myopic (A); simple
hypermetropic (B); compound myopic (C); compound
hypermetropic (D); and mixed (E).
38 Comprehensive OPHTHALMOLOGY
Treatment
1. Optical treatment of regular astigmatism comprises
the prescribing appropriate cylindrical lens,
discovered after accurate refraction.
i. Spectacles with full correction of cylindrical power
and appropriate axis should be used for distance
and near vision.
ii. Contact lenses. Rigid contact lenses may correct
upto 2-3 of regular astigmatism, while soft contact
lenses can correct only little astigmatism. For
higher degrees of astigmatism toric contact lenses
are needed. In order to maintain the correct axis
of toric lenses, ballasting or truncation is required.
2. Surgical correction of astigmatism is quite
effective. For details see page 48.
IRREGULAR ASTIGMATISM
It is characterized by an irregular change of refractive
power in different meridia. There are multiple meridia
which admit no geometrical analysis.
Etiological types
1. Curvatural irregular astigmatism is found in
patients with extensive corneal scars or
keratoconus.
2. Index irregular astigmatism due to variable
refractive index in different parts of the crystalline
lens may occur rarely during maturation of
cataract.
Symptoms of irregular astigmatism include:
  Defective vision,
  Distortion of objects and
  Polyopia.
Investigations
1. Placido's disc test reveales distorted circles (see
page. 471)
2. Photokerotoscopy and computerized corneal
topography give photographic record of irregular
corneal curvature.
Treatment
1. Optical treatment of irregular astigmatism consists
of contact lens which replaces the anterior surface
of the cornea for refraction.
2. Phototherapeutic keratectomy (PTK) performed
with excimer laser may be helpful in patients with
superficial corneal scar responsible for irregular
astigmatism.
3. Surgical treatment is indicated in extensive
corneal scarring (when vision does not improve
with contact lenses) and consists of penetrating
keratoplasty.

				
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posted:9/16/2012
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Description: Common Eye Disease