Astigmatism is a refractive error, meaning it is not an eye disease or eye health problem, but rather it's simply a problem with how the eye focuses light. In an eye with astigmatism, light fails to come to a single focus on the retina to produce clear vision. Instead, multiple focus points occur, either in front of the retina or behind it, or both. Astigmatism is very common, with more than 25% of the population having in one, or both eyes.
Astigmatism usually is caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football, with one meridian being significantly more curved than the meridian perpendicular to it.
There are three primary types of astigmatism:
- Myopic astigmatism. One or both principal meridians of the eye are nearsighted.
- Hyperopic astigmatism. One or both principal meridians are farsighted.
- Mixed astigmatism. One principal meridian is nearsighted, and the other is farsighted.
Astigmatism, like nearsightedness and farsightedness, usually can be corrected with eyeglasses, contact lenses or refractive surgery. In addition to the spherical lens power used to correct nearsightedness or farsightedness, astigmatism requires an additional cylinder lens power to correct the difference between the powers of the two principal meridians of the eye.