Astigmatism, a defect in the outer curvature on the surface of the eye that causes distorted vision. In the normal eye, light rays coming from a single point are bent, or refracted, toward each other by the cornea (the transparent outer covering of the eye). As the rays pass through the inner parts of the eye, the lens (transparent tissue that changes shape to help focus light rays) bends the rays still further, focusing them to a point on the retina (the membrane at the back of the eye that transmits images of external objects to the optic nerve). In a patient with an astigmatism, the cornea or sometimes the lens of the eye is curved abnormally. This causes light rays to refract unevenly inside the eye. While some light rays focus on the retina, other light rays focus in front of or behind the retina, resulting in blurred vision.

An astigmatism is usually present at birth, but sometimes is caused by disease or injury to the eye later in life. Most patients with an astigmatism can see clearly objects directly in front of them. However, their peripheral vision (the outer edge of the field of vision) is distorted. The effects of an astigmatism are particularly noticeable when patients observe a pattern of straight lines. For example, if they have a horizontal astigmatism, vertical lines appear sharp and clear, while horizontal lines outside their direct focus will seem blurred. An astigmatism also may be vertical or diagonal. An astigmatism may also occur in combination with nearsightedness (myopia) or farsightedness (hyperopia).

In mild cases of astigmatism, the eye may adjust to the slight distortion without correction. In more pronounced cases, an astigmatism may be corrected easily with eyeglasses or contact lenses.