Astigmatism

AstigmatismIt is a type of refractive error which is one of the most common eye related problems nowadays. Normally the eye of a person is like a sphere. The front part of the eye (lens) converge the light rays coming from an object and point them to the back part (retina) of the eye.

Due to astigmatism the cornea of the eye is abnormally curved which causes symptoms. When the light rays enter the cornea, they are refracted more in one direction, allowing only one part of the object to be focused at a time. So the light rays do not converge at a one point but forms focal lines.

Causes

The exact cause of astigmatism is unknown. It is basically caused by an irregular curvature of cornea or lens. It can be present since birth or it can be along with nearsightedness (myopia) or far sightedness (hypermetropia).

Types

Astigmatism is of two types i.e Regular astigmatism and Irregular astigmatism.

Regular Astigmatism

It is caused by to Corneal Astigmatism (results due to the abnormalities of curvature of cornea) and Lenticular astigmatism (results due to the abnormalities of curvature of the lens, tilting or oblique placement of the lens and variable refractive index of the lens).

Irregular Astigmatism

It is caused by Curvatural irregular astigmatism (It is found in patients with keratoconus or with extensive corneal scars) and Index Irregular astigmatism (It is due to variable refractive index in different parts of the lens).

Sign and Symptoms

Patients mainly have a problem of blurred vision due to which he or she develops eye strain, headache, fatigue, squinting etc.

Diagnosis

Following tests are performed to examine the eyes:

Visual Acuity Test1. Visual acuity test: It is done by reading the numbers on each line on a chart called Snellen’s chart. The numbers gets progressively shorter in size on each line.

2. Ophthalmometer: Also known as keratometer. It measures the radius of curvature of cornea and can assess the extent of curvature of cornea.

3. Retinoscopy: It reveals different power in two different axis.

Treatment

Optical treatment: It comprises of cylindrical lens which replaces the anterior surface of the cornea for refraction. Spectacles or contact lenses can be used. Rigid contact lenses correct upto 2-3 of regular astigmatism while soft contact lenses can correct only little astigmatism.

Surgical treatment: They are quite effective. Some of these are Photorefractive keratectomy, LASEK ( laser epithelial keratomileusis) and Laser in situ keratectomy (LASIK).

Reference

1. Parsons, Disease of the Eye (Elsevier, 21 Edition) p54-55.

2. A.K Khurana, MD “Ophthalmology” (New Age International Publisher, Inc 2012) p36-38.

3. Image used on this page is taken from UMMC, copyright to University of Maryland Medical Center (www.umm.edu) and astigmatism picture copyright to Accuvision (www.accuvision.co.uk).

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