Astigmatism

By Kausar

Ali

Student at Doctor of optometry

PEF

University

Astigmatism Contents of lecture :A. What is astigmatism. B. Incidence. C. Optics. D. Etiology.E. Classification.F. Signs & Symptoms. G. Diagnosis . H. Treatment .

What is Astigmatism : Astigmatism is a refractive error of the eye

in which there is a difference in degree of refraction in different meridians .

(i-e The eye has different focal points in different planes ) .

For example: The image be clearly focused on the retina in the horizontal (sagittal)plane,but not on the retina in the vertical (tangential ) plane .

Astigmatism causes difficulties in seeing fine details and in some cases vertical lines (e.g walls) may appear to be leaning over .

Most astigmatic corneas have two curves, a steeper curve and a flatter curve . This causes light to focus on more than one point in the eye resulting in blurred vision .

The cornea is normally spherical although In people with astigmatism it may be shaped like a rugby ball or oval instead of tennis ball .

In astigmatism the front surface of the eye has an irregular shape like

An egg verses a non-astigmatic eye, which is shaped like an orange .

NON Astigmatic

Astigmatic

Optics of Astigmatism : An astigmatism, the rays of light

from one sector fall on one point & rays of another sector falls on anther point . In other words a point focus of light cannot be formed upon the retina .

The configuration of the rays refracted through the astigmatic surface (toric surface ) is called strums conoid.

Thus there are two focal points separated from each other by a focal interval of strum .

The length of this focal interval is the measure of the degree of astigmatism and the correction of the error can only be accomplished by reducing these two foci in to one.

Incidence: No eye is perfectly stigmatic as

almost all the individual have a minor degree of physiological astigmatism .

About 60% cases of refractive error have astigmatism which needs to be corrected .

Occurs in equal frequency in males & females .

Approximate distribution according to a degree of astigmatism is .

The most common type is compound myopic followed by compound hyperopic, mixed, simple myopic & simple hyperopic .

0.25 to 0.5 50%0.75 to 1.0 60%1.0 to 4.0 24%>4.0 1.0%

On study as With the rule 38% Against the rule 30% Oblique 32% Etiology : Corneal Astigmatism .

It occurs due to abnormalities of curvature of cornea .Most common cause of Astigmatism

Foe example : ( keratoconus pterygium, mild corneal opacities , chalizon .

Lenticular Astigmatism: It is comparatively rare it may be . Curvatural ……. Lenticonus . Positional ……. Congenital tilting Traumatic subluxation of lens . Index ……. Developing

cataract nuclear sclerosis .

Severity of Astigmatism : The severity of astigmatism can be classified as follows :

Mild Astigmatism < 1.00 D Moderate 1.00 to 3.00 D Severe Astigmatism 2.00 to 3.00

D Extreme Astigmatism >3.00 D .

Classification of Astigmatism:

Astigmatism

( Regular ) (Irregular)

Mixed ( Myopic & Hyperopic) Compound (Myopic & Hyperopic)

Simple

Astigmatism –Based on asymmetry of structure .

Corneal Astigmatism – astigmatism due to an irregularly shaped cornea.

Lenticular Astigmatism – astigmatism due to an irregularly shaped lens .

Astigmatism – Based on axis of the principle meridian .

Regular Astigmatism . Against the rule. with the rule .

Oblique astigmatism. Bi oblique astigmatism. Irregular astigmatism. Regular astigmatism : The astigmatism is said to be

regular if there is different refraction by the eyes in to meridia at right angle to each other .

Can be corrected with spectacles Normally, horizontal curvature of

cornea is flatter than vertical &

This is attributed press of lid on the corneal surface . This is physiological so , vertical cornea should be more curve than horizontal in normal eyes .On the basis it has two types .

With the rule Against the rule With the rule (Direct Astigmatism ) • Principle meridia are at right angle to

each other .

• Vertical curve is more than horizontal .

• Concave cylinder is prescribed in horizontal axis ( 180* ) and convex are prescribed at vertical axis (90*)

• Normally the vertical meridian is rendered 0.25 D more convex than horizontal by the pressure of fleshy upper eyelid .

Against the rule Astigmatism : ( indirect Astigmatism )

• Principle meridia are at right angle to each other .

• Horizontal curve is more than vertical .

• Convex cylinder is prescribed in horizontal axis (180*)and concave are prescribed in vertical axis 90*

• Usually associated with old age .

Oblique Astigmatism :• A type of astigmatism in which

principle meridia are not horizontal or vertical but are at right angle to each other (45 & 135 )

• Usually symmetrical in both the eyes (cylinder required at 30* in one meridia and 150* at other merida)

Oblique AstigmatismSymmetrical

Complementary

Bi Oblique Astigmatism : • In this type of astigmatism the two

principle meridia are not at right to each other .

• One meridia a 30% & other at 100%

o Irregular Astigmatism: o It is characterized by an irregular

change of refractive of the refractive power in different meridia .

o There are multiple meridia which admit no geometrical analysis .

o Cannot be corrected with spectacles o It occurs due to corneal scars during of

cataract etc . Astigmatism – based on focus of the principle

meridians: Simple Astigmatism . simple hyperopic astigmatism . Simple myopic astigmatism . Compound Astigmatism .

Compound hyperopic astigmatism . Compound myopic astigmatism .

Mixed Astigmatism .

Simple Astigmatism : In simple astigmatism one of the foci

falls on retina & other focus in front or behind the retina .

This leads to one meridian being emmetropic & other being myopic (one of the foci falls on retina & other focus in frontof retina & hyperopic one focus on retina and other behind the retina ) so called as simple myopic astigmatism & simple hyperopic astigmatism respectively

It can be with the rule or against the rule .

-2.0 D cylinder at 90* is example of simple myopic astigmatism .

+2 D cylinder at 90* is example of simple hyperopic astigmatism .

Compound Astigmatism : Neither of the two foci falls on the

retina . The condition is know as compound

hyperopic if both foci are at back of retina.

It can be with the rule or against the rule

-3 Ds with -2Dc at 90 * is example of compound myopic astigmatism

+ 3 Ds with +2 Dc at 90* is example of compound myopic astigmatism .

In mixed astigmatism one of the two foci lies at back while other at front of the retina .

It can be with the rule or against the rule .

-3 Ds with +8 Dc at 90* is an example of mixed astigmatism .

If cylindrical power is less than spherical power than it is not mixed but compound astigmatism .

For example -3 Ds with 1 Dc at 180* sounds as it is mixed astigmatism but actually it is compound astigmatism as cyl is less than sphere .

Residual Astigmatism : The largest element of the total

astigmatism is due to anterior corneal surface .

While the other component like : Posterior corneal surface . Lens Refractive indies . Constitute the residual astigmatism Residual Astigmatism –total corneal

astigmatism , Sign & Symptoms : Type of the symptoms produced

are:

1. blurring of vision : Transient blurring of vision in low

astigmatism . Relieved by closing / rubbing the eyes . Circles elongate into ovals A point of light appears tailed off . A line appears as a succession of

strikes fused into a blurred image . 2.Asthenopia symptoms . More marked in patient with low

astigmatism .

Severe in hyperopic astigmatism . Tiredness of eyes . Headaches ( from mild frontal ache

to explosions of pain ) Nervous Distribution . Dizziness Fatigue Irritability .3.Tilting of the head : Some patient with high oblique

astigmatism may hold the head tilted

to one side to reduced image distortion . Some children may even develop

scoliosis . ( The condition of slide to slide spinal curve is called scoliosis . On an x rays the spine of an individual with scoliosis looks more like an S or a C than a straight line.

4.Half closure of the lids. Seen in patient with high

astigmatism .

This is to make a sort of stenopaeic slit & cutting out the rays from one meridian.

This is also cause Asthenopia symptoms .5.Reading material is held too close : Reading material is held too close to the

eyes by the patient to achieve but large image just like myopic .

6. Blurring and itching : May be seen in patients with low

astigmatism B/c of rubbing eyes . Falling of eyelashes . Hyperemia , styes and chalizon .

Diagnosis : VA with and without correction

monocularly . Pin hole VA . Retinoscopy . Keratometry . Keratoscopy with placido disc, Computerized corneal

topography /videography.

Jackson cross cylinder . Astigmatic fan & block . Trail and error technique ( axis then

power ) Maddox v Stenopaeic slit . Treatment: Spectacles ‘ Contact lenses Refractive surgery Keratoplasty .