Clear vision requires the cornea and lens of the eye to focus light perfectly on the retina. For this to happen, the cornea and lens must be symmetrical in shape.
Astigmatism occurs when the cornea or lens (or both) are not perfectly symmetrical. Instead of having an equal shape across their entire surface, they are shaped more like a football or the back of a spoon. Astigmatism is the loss of focus (or refractive error) caused by this irregular shape.
In astigmatism, the difference in curvature in different meridians of the eye (due to the irregular shape of the cornea and/or lens) causes a difference in the power of the eye along each meridian.
The steepest and flattest meridians of an eye are called its principal meridians. The principal meridians are perpendicular to each other (i.e. separated by 90 degrees). There are five types of astigmatism based on the amount of nearsightedness or farsightedness in each of the principal meridians:
- In simple myopic astigmatism, one principal meridian is free of nearsightedness or farsightedness, but the other meridian is nearsighted.
- In compound myopic astigmatism, both meridians are nearsighted, but one is more nearsighted than the other.
- In simple hyperopic astigmatism, one meridian is free of nearsightedness or farsightedness, but the other is farsighted.
- In compound hyperopic astigmatism, both meridians are farsighted, but one is more farsighted than the other.
- In mixed astigmatism, one meridian is nearsighted and the other is farsighted.
How common is astigmatism?
Most people have some degree of astigmatism, but it is frequently so mild that no corrective lenses are needed. It has been reported that an estimated 40 percent of people who wear corrective lenses (eyeglasses or contact lenses) have astigmatism.1
What are the symptoms of astigmatism?
Mild astigmatism may produce no symptoms. With moderate or severe astigmatism the following symptoms may occur:
- Eyestrain or fatigue
- Blurred vision at all distances
- Holding reading material closer than normal
Who is at risk?
Anyone can have astigmatism. A family history of astigmatism is an added risk factor.
How is astigmatism detected?
Astigmatism is detected by a comprehensive eye exam performed by an optometrist or ophthalmologist. Children with mild or moderate astigmatism can sometimes pass a school vision screening. Therefore, all preschoolers should have a thorough eye exam to evaluate their vision and rule out the presence of astigmatism.
How is astigmatism treated?
Astigmatism can be corrected with eyeglasses, contact lenses or refractive surgery.
1Bausch & Lomb Toric Awareness & Usage Study (2000).