In order to see clearly, the eye must be able to focus light into a single plane onto the back of the eye at the retina. The word astigmatism comes from the Greek "a" meaning "without" and "stigma" meaning "spot." In astigmatism, a point (or spot) of light is focused at two different planes, causing blurred vision. An optical system (or eye) without astigmatism has only one focus for all rays of light. An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. For example, if an optical system with astigmatism is used to form an image of a plus sign, the vertical and horizontal lines will never be in focus at the same time, since they are in sharp focus at two distinctly different distances. In an eye without astigmatism, the surface of the cornea is shaped like a Ping-Pong ball, where all the curves are the same. This is called a spherical surface. In an eye with astigmatism, the surface of the cornea is shaped more like a football, where there are two different surface curves located 90 degrees apart. This is called a toric surface.

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Most astigmatism does not have a recognized cause but merely is an anatomical imperfection in the shape of the cornea, where the front curvature of the cornea is toric, rather than spherical. A small amount of astigmatism is considered normal and does not represent a disease of the eye. This type of astigmatism is extremely common and frequently is present at birth or has its onset during childhood or young adulthood. There is some hereditary basis to most cases of astigmatism, and most people with astigmatism have it in both eyes. Astigmatism is often associated with myopia (nearsightedness) or hyperopia (farsightedness). Astigmatism can increase in amount during the growing years. In regular astigmatism, the meridians in which the two different curves lie are located 90 degrees apart. Most astigmatism is regular. In irregular astigmatism, the two meridians may be located at something other than 90 degrees apart or there are more than two meridians. A scar in the cornea, resulting from an injury or infection, or a disease called keratoconus may also cause astigmatism. This type of astigmatism is usually irregular.

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