Astigmatism

"What is a stigmatism?" is a question that optometrists hear every day, but actually it's spelled "astigmatism." Astigmatism is the most common vision problem, but most people don't know what it is.

It may accompany nearsightedness or farsightedness. Usually it is caused by an irregularly shaped cornea (called corneal astigmatism). But sometimes it is the result of an irregularly shaped lens, which is located behind the cornea; this is called lenticular astigmatism. Either kind of astigmatism can usually be corrected with eyeglasses, contact lenses or refractive surgery.

If you have only a small amount of astigmatism, you may not notice it or have just slightly blurred vision. But sometimes uncorrected astigmatism can give you headaches or eyestrain and distort or blur your vision at all distances.

It's not only adults who can be astigmatic. Dr. Karla Zadnik, an optometrist at Ohio State University School of Optometry, found in a recent study of 2,523 children that more than 28 percent of them had astigmatism. Children may be even more unaware of the condition than adults, and they are unlikely to complain of the blurred or distorted vision. Unfortunately, astigmatism can affect their ability to see well in school and during sports, so it's important to have their eyes examined at regular intervals in order to detect any astigmatism early on. Read more about the recommended frequency of eye exams for children.

Astigmatism occurs when the cornea is shaped more like an oblong football than a spherical baseball, which is the normal shape. In most astigmatic eyes, the oblong or oval shape causes light rays to focus on two points in the back of your eye, rather than on just one. This is because, like a football, an astigmatic cornea has a steeper curve and a flatter one.

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In regular astigmatism, the meridians in which the two different curves lie are located 180 degrees apart. In irregular astigmatism, the two meridians may be located at something other than 180 degrees apart; or there are more than two meridians. Regular astigmatism is usually easy to correct (see treatments below), but irregular astigmatism can be complicated and more difficult to correct, depending on the extent of the irregularity and its cause.

Usually astigmatism is hereditary: many people are born with an oblong cornea, and the resulting vision problem may get worse over time. But astigmatism may also 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.

Unless it is extreme, astigmatism can be compensated for satisfactorily with eyeglasses or contact lenses. If your eyeglass or contact lens prescription contains three parts rather than one, your eyecare practitioner has found some astigmatism in one or both of your eyes. A prescription with three parts looks like this:
-2.75 -1.25 x 90.

• The first part indicates your main spherical correction, meaning the amount of power (in diopters) required in a lens to sharpen your visual acuity to an acceptable level, usually 20/20. In this example, the person has myopia and requires a negative (concave) lens to correct it.
   
• Part two shows the extent of the astigmatism in diopters. Again, the minus sign means a concave lens is needed.
   
• Part three is the axis (in degrees) of the cylinder required to bend certain light rays to compensate for the cornea's oval shape.
   

Many people with astigmatism believe that they can't wear contact lenses or that only rigid contact lenses (RGPs, also called GP lenses) can correct astigmatism. (Read more about RGP contact lenses.)

This was true many years ago, but now there are soft contacts that correct astigmatism; they are called toric contact lenses. Toric lenses have a special correction built into them and may also contain a prescription for nearsightedness or farsightedness if you need it. (Read more about toric contact lenses.)

While soft torics work well for many people, if you have severe astigmatism, you'll likely do better with RGP contact lenses or eyeglasses. Your eyecare practitioner will advise you.

Depending on the type and severity of your astigmatism, you may also be able to have it corrected with refractive surgery. Discuss with your eye doctor which procedure is best to correct your astigmatism, and review your options in our LASIK & Vision Surgery section.