Astigmatism

ASTIGMATISM

Dr Rajvin Samuel

1 1 st yr Pg Ophthalmology

• Definition :

A defect of an optical system causing light rays from a point source to fail to meet in a focal point resulting in a blurred and imperfect image.

Rays of light converge forming Focal lines

The word is derived from Greek α –without and stigma – Spot

• Astigmatism begins in the 1 st yrs of life > 1 D• This arises from recti muscles pulling upon

delicate infant sclera in different gazes• This astigmatism helps to attain position of

best focus• This incidence of astigmatism decreases with

age

• A toric lens is a lens with two different powers in two orientations perpendicular

to each other

• A toric surface resembles a section of the surface of an doughnut where there are two regular radii, one smaller than the other one.

• Conus of Sturm:- Geometric configuration of light rays

emanating from single point source & refracted by spherocylindrical lens

Focal interval of Sturm :- Distance between 2 focal lines

• Circle of least diffusion At the dioptric mean of focal lines the cross

section of sturms conoid appears as circular patch of light rays – best overall focus

• Based on axis of the principal meridians

• Regular astigmatism – principal meridians are perpendicular– With-the-rule astigmatism– Against-the-rule astigmatism 2]

– Oblique astigmatism

• Irregular astigmatism - principal meridians are not perpendicular

• Based on focus of the principal meridians

• Simple astigmatism– Simple hyperopic astigmatism– Simple myopic astigmatism

• Compound astigmatism– Compound hyperopic astigmatism– Compound myopic astigmatism

• Mixed astigmatism

Regular Astigmatism : • Correctable by Spherocylindrical lenses Etiology : 1. Corneal - abnormalities of curvature [common]

2. Lenticular is rare. It may be: i. Curvatural - abnormalities of curvature of lens as seen in lenticonus. ii. Positional - tilting or oblique placement of lens , subluxation.

3. Retinal - oblique placement of macula [rare]

• Symptoms :

Blurring of vision Asthenopic symptoms Tilting of head Squinting [Half closure of eyelid]

Investigations:

• Retinoscopy• Keratometry • Computerized corneal Tomography• Astigmatic fan test• Jackson cross cylinder

Irregular Astigmatism• Etiology : Corneal [ Scars , Keratoconus , flap complications, marginal degenration , Lenticular [Cataract maturation]

Symptoms : Defective vision Distorsion of objects Polyopia

Investigations: - Placido's disc test reveals distorted circles

- computerized corneal topography

Treatment :• Optical treatment : RGP contact lenses Hybrid contact lenses Scleral lenses Piggyback lens

• Phototherapeutic keratectomy

• Corneal Cross-Linking With Riboflavin and Ultraviolet Irradiation

• Penetrating Keratoplasty [PKP]

• Intacs corneal implants

• Deep Anterior Lamellar Keratoplasty [DALK]

• Descemet's Stripping Endothelial Keratoplasty [DSEK]

Residual astigmatism :

The amount of astigmatism that still remains after correction of a refractive error.

In the case of correction of corneal astigmatism using rigid contact lens ,lenticular residual astigmatism is exposed.

Treatment

• Eyeglasses• contact lenses• refractive surgery

• Astigmatism correction requires prescription of convex cylindrical lenses at 180 +/- 20 deg or concave cylindrical lenses at 90 +/- 20 deg with the rule and vice versa.

Contact lenses• Toric contact lenses Soft lenses [SL] Rigid gas permeable lenses [RGP]RGP do not conform to the asymmetry of corneal surface but

replaces it totally and also provides clarity of vision ,more durable.

Soft lenses are more comfortable to wear ,easy to fit, adhere more tightly to cornea .

Refractive surgeries• Astigmatic Keratotomy• Photoastigmatic refractive Keratectomy [PRK]• Relaxing incisions with compression sutures• LASIK surgery

THANK YOU