Astigmatism is probably the most misunderstood vision problem. Like nearsightedness and farsightedness, astigmatism is a refractive error, meaning it is not an eye disease or eye health problem; 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 usually causes vision to be blurred or distorted to some degree at all distances.
Symptoms of uncorrected astigmatism are eye strain and headaches, especially after reading or other prolonged visual tasks.
Squinting also is a very common symptom.
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. 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 is called lenticular astigmatism, to differentiate it from the more common corneal astigmatism.
Types of Astigmatism
There are three primary 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 principal 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 front surface of the eye a football shape.
Irregular astigmatism can result from an eye injury that has caused scarring on the cornea, from certain types of eye surgery or from keratoconus, a disease that causes a gradual thinning of the cornea.