ASTIGMATISM

					       Speaker
Abdullah Al Otaibi,MD
  Assistant Professor
Refraction:
The deviation of a ray of light passing from
one medium into another is called
refraction.
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"Refraction" refers to the
way the eye focuses light,
the source of everything
we see. There are three
basic elements that
determine the eye's
ability to focus light
How the eye works The
eye is a very intricate
optical system similar to a
camera. In a normal eye,
light rays pass through
the cornea, pupil and lens,
focusing sharply and
precisely on the retina.
Visual Acuity
The visual acuity test
measures the smallest letters
that you can read on a
standardized chart at a
distance of 20 feet.
 VISUAL ACUITY
 Snellen chart :
 20/ 20
 20/ 40
 20/100
 V.A which improve with pinhole refractive error
Convex lens :
Parallel rays → into a focus.
Concave lens:
Diverge rays of light.
Anterior corneal surface produces
most of the eye`s refracting power ≈
40 Diopter.
Aqueous, lens + vitreous provide the
remaining ≈ 20 Diopter.
In emmetropia the principal focus is on the
plane of the retina.
In myopia the principal focus is anterior to
the retina.
High axial length.

Strong refractive power.
Clinical features
 patients usually have problem in seeing at
  distance.
 The vision at near is normal.
 4 myopic will see clear at ¼.
 25 C.M
 6 myope?
V.A. improves with pinhole.
Myopes squeeze their eyes to see clearly.
In hypermetropia the principal focus is
behind the retina, and is therefore virtual.
Causes of hypermetropia:


Decreased Axial length
Weak refractive power .
After cataract surgery , lens is removed and
  we have to substitute it with :
  Intra ocular lens.
  Contact lens.
  Glasses.
              ASTIGMATISM
 Regular: The directions of greatest and least curvature
  are always 90 degree apart.
 Corrected by a cylindrical spectacle lens.


 Irregular:The principal meridians are not perpendicular
  to one another.
 Example: Corneal scar due to trauma or keratoconus.
 Corrected by hard lens.
THE DEVELOPMENTAL EVOLUTION OF
THE REF. STATE

 Low grade hyperopia is the usual ref. State in
  infancy and childhood.
 As the child grows;changes in axial
  length,cornea,and lens compensate for each
  other toward emmetropia.
ANISOMETROPIA
 Difference in the refractive errors of the two eyes.
 Uncorrected anisometropia in children may lead to
  amblyopia.
 Unilateral aphakia.
Anisometropia : achild 3-y-old,
OD: +3
OS:+1
Amblyopia OD.
ACCOMMODATION
 Flxible mechanism by which the eye can change
  its ref power.
 Contracture of ciliary muscle relaxation of the
  zonules more biconvex state of the lens.
 Presbyopia
y to focus the eye when viewing objects at close
 When an attempt is made to look at a near object, the lens
 ye changes shape (accommodates). This shape change
a result of impulses sent to muscles inside of the eye. As
 closer during viewing, impulses are also sent to the external
 the eye to converge or turn the eyes inward in order to avoid
ouble image. This convergence system allows us to keep
 n of objects when viewing from distance to near. These two
 Accomodation and Convergence work together and provide
 ne another to maintain one clear image when viewing objects
                                                                 The lens changes shape du
                                                                 accommodation.
AGE   AVERAGE ACCOMODATION
           AMPLITUDE
 8             14
12             13
16             12
20             11
24             10
32              8
40              6
44              4
52             2.5
56              2
64              1
Example
 A 20 year old c / o decrease vision for distance.
 V.A: 20 / 80 ; PH 20 / 30
 Ref. Error.
 Auto.ref.     -3
 Auto.ref. –3 –2 x 180.
 Astigmatism

                 -3
                  -2


                 -3
 Pt who had difficulty in reading.
 V/A 20 / 100 PH 20/40.
 Auto.ref: +4.
 Convex glasses.

                         +4
• 15 y old boy ;VA 20/100 PH 20/40 ;at 33cm 20/20
• MYOPIA
• REF.ERR -3
A 50 –y- old man started to have difficulty in
  reading.
VA 20 /20
PRESBYOPIA.
CONTACT LENSES
SOFT: spherical ref error .
HARD:keratoconus.
TORIC:spherocylidrical ref error.
RISK OF CONTACT LENSES.
Refractive surgery alters the curvature of the cornea to
allow light rays to come to focus closer to the retina, thus
improving uncorrected vision. In myopia, the central corneal
curvature is flattened; in hyperopia, the central corneal
curvature is made steeper; and in astigmatism, the cornea
is made more spherical.
RK.
PRK.
Lasik.
up to eight incisions are placed
in a spokelike pattern in the
peripheral cornea using a
guarded diamond blade
Photorefractive keratectomy (PRK) is used for persons
with low to moderate myopia. The excimer laser is used
to flatten the central corneal tissue through
photoablation. The excimer laser uses an argon
fluoride gas mixture to create ultraviolet energy that is
able to break intermolecular bonds with submicron
precision. Each pulse of the laser removes 0.25 µ of
corneal tissue. The corneal epithelium is removed
before photoablation and generally takes 3 days to
regenerate. During the procedure, laser delivery to the
cornea usually lasts < 1 min. PRK can safely treat
higher degrees of myopia than can radial keratotomy,
with > 90% of people seeing 20/40 or better without
glasses.
In laser in situ keratomileusis (LASIK), a flap of
corneal tissue is created with a microkeratome, is
turned back, and then the stromal bed is sculpted
with the excimer laser. In the great majority of
cases, the flap adheres tightly to the stromal bed
without the need for sutures. Because the surface
epithelium is not disrupted centrally, visual
recovery is rapid. Most people notice a significant
improvement in their vision 1 day postoperatively.
LASIK can be used to treat myopia, astigmatism,
and hyperopia.

				
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