Astigmatism is an eye condition in which the vision tends to be blurred, in most cases because the eyball itself is slightly out of shape. This condition which often begins early in life, can usually corrected by suitable glasses. These must be fitted by a specialist after proper testing of the vision.
What Causes Astigmatism?
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.
(To understand what meridians are, think of the front of the eye like the face of a clock. A line connecting the 12 and 6 is one meridian; a line connecting the 3 and 9 is another.)
The steepest and flattest meridians of an eye with astigmatism are called the principal meridians.
In some cases, astigmatism is caused by the shape of the lens inside the eye. This type of astigmatism is called lenticular astigmatism, to differentiate it from the more common corneal astigmatism.
There are several types of astigmatism:
- Myopic astigmatism. One or both principal meridians of the eye are nearsighted. (If both meridians are nearsighted, they are myopic in differing degree.)
- Hyperopic astigmatism. One or both principal meridians are farsighted. (If both are farsighted, they are hyperopic in differing degree.)
- Mixed astigmatism. One prinicipal meridian is nearsighted, and the other is farsighted.
Astigmatism also is classified as regular or irregular. In regular astigmatism, the principal meridians are 90 degrees apart (perpendicular to each other). In irregular astigmatism, the principal meridians are not perpendicular. Most astigmatism is regular corneal astigmatism, which gives the eye a football shape.