If you have heard the word astigmatism but never understood what it meant, you are not alone. Astigmatism is one of the most common vision problems and one of the least understood.
Refractive errors and astigmatism
Most people wear glasses because they are either nearsighted (able to see near) or farsighted (able to see at a distance). These conditions are called refractive errors because they are caused by conditions that affect how light is refracted or bent as it enters the eye. Astigmatism is another refractive error that can occur by itself or in combination with either nearsightedness or farsightedness.
Light entering the eye first passes through the clear front surface of the eye, which is called the cornea. The light then passes through the pupil and the lens of the eye before reaching the retina which is the lining on the inside of the back of the eye. Both the cornea and the lens work to focus the light entering the eye. The goal is to bring all the light to a clear focus at one point on the retina. When either the cornea or the lens is the wrong shape, the image is not focused clearly and vision is blurry.
What is astigmatism?
Astigmatism in the eye is most commonly caused by an irregular curve on the surface of the cornea. A normal cornea is shaped like a round ball where all the curves are the same. Astigmatism stretches the cornea so that it is shaped more like a football. This irregular shape means light passing through different parts of the cornea is focused differently. The result can be blurry vision. Astigmatism is often recognized when looking at parallel lines. The horizontal lines may appear clear while the vertical lines are blurry, or the other way around. Lines may also appear wavy, like looking in a fun-house mirror. The amount of astigmatism is often different between the two eyes, so each eye should be tested separately.
Some people never realize they have astigmatism because the shape of the cornea has only a minor effect on vision. Other people have significant blurring.