I recently got my eyes examined and the optometrist says I have astigmatism. What is this?
There are several structures of the eye involved in vision. For us to see clearly, light from objects in our field of vision hits the eye and is refracted (bent) by the cornea and the lens so that it eventually focuses on the retina. The cornea serves to refract 2/3 – 3/4 of the light so that it focuses on the retina. The cornea is usually spherical – round like a tennis ball. However in most cases it’s not perfectly round, it’s a little elliptical (egg shaped), so that it has a steep curve and a flatter curve. This means that the light is refracted into two or more points on the retina instead of one and you will see a blurry image, both for distance and near. This condition is called astigmatism.
If you look at a star at night, instead of it looking round, it may look oval shaped, or as if it has a tail, depending on the amount of astigmatism you have.
Most people have small amounts of astigmatism, but the optometrist would not usually mention it to the patient. It becomes more important when there are significant amounts of astigmatism, as the patient needs to be informed about problems adapting to the glasses. With high amounts of astigmatism, patients have to be warned that things will look high and low or may appear tilted when they first get their glasses.
If the cornea is round, but steeper than normal, the light will be bent too much and the light will focus in front of the retina. This condition is called myopia, or nearsightedness. If the cornea is round but flatter than it should be, the light will focus behind the retina, called hyperopia or farsightedness. So you can be nearsighted alone if the cornea is spherical, but be nearsighted with astigmatism if the cornea is elliptical. Most astigmatism is corneal. In a few cases, the curvature of the lens is irregular and this leads to lenticular astigmatism.
How common is astigmatism? Can it be corrected?
Most people have some degree of astigmatism, but it is frequently so mild that no corrective lenses are needed. It has been reported that an estimated 40 per cent of people who wear corrective lenses (eyeglasses or contact lenses) have astigmatism. Mild astigmatism may produce no symptoms. With moderate or severe astigmatism the following symptoms may occur: eyestrain or fatigue, squinting, blurred vision at all distances, holding reading material closer than normal.
Astigmatism is detected by a comprehensive eye exam performed by an optometrist or ophthalmologist. Children with mild or moderate astigmatism can sometimes pass a school vision screening. Therefore, all preschoolers should have a thorough eye exam to evaluate their vision and rule out the presence of astigmatism.
People assume that by wearing glasses their eyes get better, or they get ‘corrected.’ This is not the case. Glasses are used to change the direction of the light hitting the eye so that it can then be focused on the retina where it’s supposed to be.
So if you have astigmatism, you can get glasses that ‘correct’ for the corneal irregularities by changing the direction of the light, but they cannot change the fact that your eyes are shaped irregularly to begin with, ie they cannot ‘correct’ your eyes.
