Overview of Astigmatism
Astigmatism causes blurry vision when looking at objects that are close (e.g., reading a letter) and objects that are far away (e.g., reading a road sign). Vision with astigmatism is not necessarily uniformly blurry; some distances are more out of focus than others.
The cornea is normally smooth and uniformly curved on all sides. In astigmatism, the cornea is irregularly curved. Instead of being round, it may be shaped more like a football. This irregular shape causes light to scatter and to bend, or refract, improperly as it passes through the cornea.
Instead of focusing directly on the retina, some light rays focus in front of and some focus behind the retina. These multiple focal points distort vision. In some cases, an irregularly curved lens produces minor degrees of astigmatism.
Astigmatism is usually present at birth and often goes unnoticed until the child begins school or starts to read. The condition may worsen over time, but generally remains the same.
Incidence and Prevalence of Astigmatism
Some experts believe that most people have some degree of astigmatism. Some people experience no symptoms, and others have severely impaired vision. Astigmatism often occurs with nearsightedness or farsightedness, or both.
Risk Factors for Astigmatism
Because astigmatism is often present at birth, many believe that there is a hereditary risk factor. It also may be development as the child's eye grows.
Astigmatism can be acquired through an injury that causes the lens or the cornea to become distorted. This includes trauma and scarring, as well as surgical procedures involving those structures.
A condition called keratoconus may be a risk factor for astigmatism. In keratoconus, the cornea becomes thin and cone-shaped. This condition may be inherited or result from chronic eye rubbing. It is more common in women and usually occurs in early adolescence.
Diabetes also may lead to astigmatism because high blood sugar levels can cause changes in the shape of the lens.
Signs and Symptoms of Astigmatism
People with mild astigmatism may experience headaches, eyestrain, fatigue, or blurred vision at certain distances. Severe astigmatism produces distorted images and blurring at all distances.
A diagnosis of astigmatism can be made only after a basic eye examination performed by an ophthalmologist or an optometrist.
A variety of tests may be used to diagnose astigmatism. One that is commonly used is the astigmatic dial, which shows a series of lines radiating from a center. People with astigmatism usually see some lines more clearly than others.
Treatment for Astigmatism
Almost all degrees of astigmatism can be corrected with prescription eyeglasses or contact lenses. Wearing the glasses or contact lenses allows the patient to experience normal, or dramatically improved, vision.
A number of surgical procedures are used to correct refractive errors. Some forms of astigmatism are surgically correctable. Surgery is not recommended for people under the age of 18 because their eyes are still growing.
There is no way to prevent astigmatism.