In Astigmatism, light rays entering the eyes are not focussing on a single common point rather one meridian fall in front and another meridian falls behind.
What causes Astigmatism
Most of the times this occurs because of the irregular cornea but can sometimes due to the curvature of the lens. A normal cornea is round, curved like basketball but in astigmatism, cornea curves more in one direction than other like a football. This makes dioptric power difference between the two meridians. Therefore it bends the light rays more in one meridian than other. So it formed multiple points rather than single common focal points. This causes the image to be blurred and stretched out both in distance and near.
Types of Astigmatism
Clinically there are three primary types of astigmatism1)Primary Care Optometry (5th ed). Theodore P. Grosvenor. pg.18.
- Simple Astigmatism
- Compound Astigmatism
- Mixed Astigmatism
If the eye is focussing one meridian on the retina and the other meridian either in front or behind the retina, then the person is said to have a simple astigmatism.
- Simple Myopic Astigmatism
If the eye is focussing on meridian on the retina and the other meridian in front of the retina then the person has simple myopic astigmatism.
- Simple Hyperopic Astigmatism
If the eye is focussing on meridian on the retina and the other meridian behind the retina then the person has simple hyperopic astigmatism.
- If the eye is focussing both the meridian (f1 & f2) in front of the retina then the person has compound myopic astigmatism.
- If the eye is focussing both the meridian (f1 & f2) behind the retina then the person has compound hyperopic astigmatism.
If the eye is focussing one meridian (f1) in front of the retina and another meridian (f2) behind the retina then the person has mixed astigmatism.
How astigmatism is Diagnosed.
The optometrist performs a various diagnostic procedure to detect your astigmatism. Examinations like visual acuity, keratometry, and refraction.
Your optometrist shows a letters or numbers in distance chart or from the monitor and asks you to read. By doing so, he is checking your vision. Visual acuity is noted down in a fraction (for example 6/24). The top number denotes the standardized testing distance and the bottom number denotes how many lines you have read from the chart. A person with 6/24 vision, by definition, they can read at 6 meters what the normal individual can read at 24 meters.
Keratometry/ Corneal topography
A keratometry is an instrument which measures your anterior corneal curvature particularly for assessing extent and axis of your astigmatism. Corneal topography is a technique for mapping the surface curvature of the cornea, which gives the more detailed shape of the cornea.
It is generally done using a handheld light instrument called retinoscope or sometimes using an automated computerized instrument to measure the power of your eye. Your optometrist places series of lenses in front of your eye and asks you to read the letters in the distance chart until you get is a crisp and clear vision.
How it is treated?
With the results of the above diagnostic procedures, your optometrist can determine whether you have astigmatism. If you have astigmatism you optometrist may recommend any one of the below options,
People with astigmatism generally wore eyeglasses which have special cylindrical power lenses. This lens corrects your cylindrical error and helps you to regain your clear vision.
Some people may prefer contact lens rather than the eyeglasses. Both the eyeglasses and the contact lens corrects your cylindrical lens. But some prefer contact lenses because of their cosmetic appearance. Another advantage of using contact lens is that it provides more field of view. Despite their advantages, there are some disadvantages also. Since the contact lens is worn directly the eyes it requires regular cleaning habits to safeguard your eyes from infection. This is the major disadvantage of the contact lens use.
Astigmatism can also be corrected by reshaping the cornea with LASIK (laser in-situ keratomileusis) or PEK (photorefractive keratectomy) surgeries.
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|1.||↑||Primary Care Optometry (5th ed). Theodore P. Grosvenor. pg.18.|