Astigmatism occurs when the cornea is not uniformly curved, causing light rays to not converge at a single focal point and resulting in blurred vision. It is most often caused by an uneven curvature of the cornea (corneal astigmatism). Symptoms include distorted or blurred vision at all distances. Astigmatism is commonly measured using a keratometer and can be corrected using spectacle lenses, contact lenses, or refractive surgery. Prescribing cylinders for spectacle correction requires determining the axis of astigmatism and cylinder power through retinoscopy techniques.

1. ASTIGMATISM
Dr .Ibrahim ali
1st yr pg ophthalmology

2. Definition
 Astigmatism (from the Greek “a” meaning
absence and “stigma” meaning point)
 Astigmatism occurs when incident light rays
do not converge at a single focal
 point.The cornea of the normal eye has a
uniform curvature, with resulting equal
refracting
 power over its entire surface.
 Most astigmatic corneas are normal also.

3.  In some individuals, however, the cornea is not
uniform and the curvature is greater in one
meridian (plane) than another, much like a
football as a rugby ball.

4. Lightraysrefractedbythiscorneaarenot
broughttoasinglepointfocus,andretinalimagesfrom
objectsbothdistantandnearare
blurredandmayappearbroadenedorelongated.

6.  Most astigmatism is corneal in origin.
Lenticular astigmatism is a result of uneven
curvature and differing refractive indices
within the crystalline lens.
 internal or non-corneal
 astigmatism was found to be -0.46X98.2° for
right eyes and -0.50X99.4° for left eyes.

7. 1.2 Epidemiology - prevalence
 Astigmatism (more than 0.5 diopters) is a
commonly encountered refractive error,
 accounting for about 13 per cent of the refractive
errors of the human eye.
 Human infants exhibit both high prevalence and
high degrees of astigmatism, largely corneal in
origin
 In adult Americans, the prevalence of
astigmatism has been reported to be 20% higher
 among men than women but was not associated
with number of years of formal education

8.  astigmatism types, classification
 Ocular astigmatism can occur as a result of
unequal curvature along the two principal
 meridian of the anterior cornea (known as
corneal astigmatism)

9.  unequal curvatures of the front and back
surfaces of the crystalline lens,
 decentration or tilting of the lens or unequal
refractive indices across the crystalline lens
 (known as internal or non-corneal
astigmatism)

10.  Corneal astigmatism is often classified
according to the axis of astigmatism as being
either with-the-rule (WTR),
 oblique or against-the-rule (ATR)
 .

11. The principal meridians-the meridians of
maximum and
minimum corneal curvature-are usually at
right angles to each other in astigmatism
and are
usually (but not necessarily) in the
vertical and horizontal planes.

12.  Astigmatism can be described as regular or
irregular.
 In regular astigmatism, there are two principal
meridians separated by 90 degrees; the best
 spectacle-corrected visual acuity (BSCVA) is at
least 20/20 and, in the case of corneal
 astigmatism, corneal topography displays a
symmetrical bow—tie pattern.

13.  In regular astigmatism, the refractive power
varies successively from one meridian to the
next, and each meridian has a uniform
curvature at every point across the entrance
pupil.

14. Types of regular astigmatism
 Astigmatic eyes:
 Simple myopic astigmatism: one meridian focuses light
in front of the retina, the other on
 the retina;
 Simple hyperopic astigmatism: one meridian focuses
light on the retina, the other
 theoretically behind the retina;
 Compound myopic astigmatism: both meridians focus
light in front of the retina;
 Compound hyperopic astigmatism: both meridians focus
light theoretically behind the
 retina;
 Mixed astigmatism: one meridian focuses light in front of
the retina, the other behind the
 retina.

15.  In irregular astigmatism, which is less common, the
corneal “rugby ball” would appear out
 of its customary shape and/or bumpy.
 Irregular astigmatism can be regularly irregular or
irregularly irregular
 In regularly irregular astigmatism, two principal
meridians exist but are either asymmetrical or not
90 degrees apart and is typified by either unequal
slopes of the hemi meridians along a single meridian
(the “asymmetric bow-tie”) or hemimeridians of
equal slope but not aligned with
 each other (the “angled bow-tie” or nonorthogonal
astigmatism).

17.  Irregularly irregular astigmatism does not have
 identifiable prime meridians. In irregular astigmatism,
which can be clinically significant in
 conditions such as keratoconus and other corneal
ectasias; corneal basement membrane and
 stromal dystrophies; corneal scarring; and post-surgical
corneas (e.g., following penetrating keratoplasty, radial
keratotomy, and complicated refractive surgery), the
magnitude and the
 axis of astigmatism vary from point to point across the
entrance pupil

18. Symptoms
 Distortion or blurring of images at all distances is
one of the most common astigmatism
 symptoms.This may happen vertically,
horizontally, or diagonally.
 There can be
 indistinctness of objects, circles become elongated
into ovals and a point of light begins to
 tail off. Symptoms of eye strain such as headaches ,
 photophobia, and fatigue are also among the most
common astigmatism symptoms.
 Reading small print is difficult with astigmatism.

20. Keratometric
 Performed with a device called keratometer
or ophthalmometer, keratometry is the
 measurement of a patient’s corneal
curvature.As such, it provides an objective,
quantitative measurement of corneal
astigmatism, measuring the curvature in each
meridian as well as the axis.

21. Non-surgical treatment of astigmatism
 Retinoscopy of Regular Astigmatism
 Most eyes have some regular astigmatism. In such cases,
light is refracted differently by the 2 principal astigmatic
meridians.
 Sweeping the retinoscope back and forth measures the
power along only a single axis.
 Moving the retinoscope from side to side (with the streak
oriented at 90°) measures the optical power in the 180°
meridian. Power in this meridian is provided by a cylinder
at the 90° axis.

22. Finding the cylinder axis
 Before the powers in each of the principal
meridians can be determined, the axes of the
meridians must be determined. Four
characteristics of the streak reflex aid in this
 determination:
 1) Break. A break is observed when the streak is
not oriented parallel to 1 of the principal
meridians.The reflex streak in the pupil is not
aligned with the streak projected on the iris
and surface of the eye, and the line appears
broken.

23. The break disappears (ie, the line
appears continuous) when the projected
streak
is rotated to the correct axis.

24.  Width.The width of the reflex in the pupil
varies as it is rotated around the correct
 axis.The reflex appears narrowest when the
streak, or intercept, aligns with the axis.

26.  Intensity.The intensity of the line is brighter
when the streak is on the correct axis.
 Skew. Skew (oblique motion of the streak
reflex) may be used to refine the axis
 in small cylinders. If the retinoscope streak is
off-axis, it moves in a slightly different
 direction from that of the pupillary reflex.

27. The reflex and streak move in the same direction
when the streak is aligned with 1 of the
principal meridians.

28.  When the streak is aligned at the correct axis,
the sleeve may be lowered (Copeland
instrument)
 or raised (Welch Allyn instrument) to narrow
the streak, allowing the axis to be
 determined more easily

29.  The axis can be confirmed through a
technique known as straddling, which is
 performed with the estimated correcting
cylinder in place
 The retinoscope streak is turned 45° off-axis
in both directions, and if the axis is correct,
the width of the reflex should be equal in
both off-axis positions.

30.  If the axis is not correct, the
 widths are unequal in these 2 positions.
 The axis of the correcting plus-cylinder
should be moved toward the narrower reflex
and the straddling repeated until the widths
 are equal.
 This technique is often more accurate than
subjective cross-cylinder axis refinement.

32. Finding the cylinder power
 After the 2 principal meridians are identified,
the previously explained spherical techniques
 are applied to each axis:
 With 2 spheres. Neutralize 1 axis with a
spherical lens; then neutralize the axis 90°
 away..

33.  The difference between these readings is the
cylinder power.
 For example, if the 90° axis is neutralized
with a+ 1.50 sphere and the 180° axis is
neutralized with a +2.25 sphere, the gross
retinoscopy is+ 1.50 +0.75 x 180

34.  The examiner's working distance (ie, + 1.50) is
subtracted from the sphere to obtain the final
refractive correction: plano +0.75 x 180.
 With a sphere and cylinder. Neutralize 1 axis with
a spherical lens.To enable the
 use of with reflexes, neutralize the less plus axis
first.Then, with this spherical
 lens in place, neutralize the axis 90° away by
adding a plus cylindrical lens.The
 spherocylindrical gross retinoscopy is read
directly from the trial lens apparatus.

35. Common Guidelines for Prescribing
Cylinders for Spectacle Correction
 1. In children, give the full astigmatic correction.
 2. In adults, try the full astigmatic correction first.
Give warning and encouragement.
 If problems are anticipated, try a walking-around
trial with trial frames before
 prescribing.
 3.To minimize distortion, use minus cylinder
lenses and minimize vertex distances.
 4. Spatial distortion from astigmatic spectacle
corrections is a binocular phenomenon.
 Occlude 1 eye to verify that this is indeed the
cause of the patient's complaints.

36.  5. If necessary, reduce distortion still further by
rotating the cylinder axis toward
 180° or 90° (or toward the old axis) and/or by
reducing the cylinder power. Balance
 the resulting blur with the remaining distortion,
using careful adjustment of
 cylinder power and sphere. Residual astigmatism
at any position of the cylinder
 axis may be minimized with the Jackson cross-
cylinder test for cylinder power.
 Adjust the sphere using the spherical equivalent
concept as a guide, but rely on a
 final subjective check to obtain best visual acuity.

37.  6. If distortion cannot be reduced sufficiently
by altering the astigmatic spectacle
 correction, consider contact lenses (which
cause no appreciable distortion) or iseikonic
corrections.

38.  Refrence
 American academy clinical optics
 Duke-elder practise of refraction
 Elkington
 Google images