Astigmatism is a type of distorted vision caused by a defect in the curvature of the cornea or lens of the eye. This prevents the rays of light entering the eye from being properly focused onto the retina at the back of the eyeball. Some rays are misplaced, causing the image to be partially out of focus.

Types

There are several types of astigmatism. Most people with astigmatism can see clearly those objects directly in front of them, but their peripheral vision is defective. Others have vertical astigmatism, in which the areas above and below their direct gaze are imperfectly visualized. Some may have horizontal astigmatism, in which the right and left sides of their field of vision are defective. The astigmatism may be diagonal as well.

Treatment

Fortunately, a defect in the curvature of the cornea or lens is usually uniform and can be easily corrected by eyeglasses or contact lenses. The fault is usually in the shape of the cornea (the clear "window" in front of the iris and pupil of the eye), which may be slightly flattened vertically or horizontally. The eyeglass lenses or the contact lenses are curved to adjust the angle of rays entering regions of the eye that are defective; the glasses or lenses do not bend the rays entering the areas of clear vision.

Although most people with astigmatism were born with a tendency toward the condition, a few cases are caused by eye disease or injury. In these situations, the condition causing the astigmatism may be more difficult to correct.

Regardless of the cause, astigmatism -- as with any vision problem -- should be corrected as early as possible. Children, in particular, are at risk for development of permanently defective vision if astigmatism goes uncorrected.