Like nearsightedness and farsightedness, astigmatism is a refractive error, meaning it is 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 or behind the retina (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 is also a common symptom of uncorrected astigmatism.
What Causes Astigmatism?
Astigmatism is commonly caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round ball shape, it is curved like a rugby ball, with one meridian being significantly more curved than the meridian perpendicular to it.
To understand what meridians are, think of the eye like a Globus. A line between north and south is one meridian; the line from east to west is another.
Astigmatism may also be caused by the shape of the natural lens inside the eye. This type of astigmatism is called lenticular astigmatism. Lenticular astigmatism may eliminate corneal astigmatism in exceptional cases.
Types of Astigmatism
The steepest and flattest meridians of an eye with astigmatism are called the principal meridians.
• 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 is also 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. Irregular astigmatism can result from an eye injury that has caused scarring on the cornea, from particular complications of eye surgery or from corneal conditions such as keratoconus.
Astigmatism is detected during an eye exam with the same instruments and techniques used for the detection of nearsightedness and farsightedness. Most astigmatism is corneal astigmatism, and corneal topography mapping is useful in the diagnosis.
Treatment of Astigmatism
Astigmatism, like nearsightedness and farsightedness, can be usually corrected with eyeglasses, contact lenses, refractive laser surgery, or during cataract surgery by making small incisions in the cornea or by implanting a special (toric) intraocular lens.