Astigmatism usually occurs when the front surface of the eye, the cornea, has an irregular curvature. Astigmatism is one of a group of eye conditions known as refractive errors. Refractive errors cause a disturbance in the way that light rays are focused within the eye. Astigmatism often occurs with nearsightedness and farsightedness, conditions also resulting from refractive errors. Astigmatism is not a disease nor does it mean that you have "bad eyes." It simply means that you have a variation or disturbance in the shape of your cornea. Astigmatism is a mild and easily treatable imperfection in the curvature of your eye. The condition can result in blurred vision.

Astigmatism occurs when the front surface of your eye (cornea) or the lens, inside your eye, has a slightly different surface curvature in one direction from the other. Instead of being even and smooth in all directions, the surface may have some areas that are flatter or steeper. When the cornea has a distorted shape, you have corneal astigmatism. When the lens is distorted, you have lenticular astigmatism. Either type of astigmatism can cause blurred vision.

Astigmatism blurs your vision at all distances. Astigmatism is usually present from birth and may occur in combination with nearsightedness or farsightedness. The condition tends to remain constant, neither improving nor deteriorating much over time.

Most people have some degree of astigmatism. Often it's not pronounced enough to require corrective action.

Signs and symptoms of astigmatism may include:

• Distortion in portions of your visual field

• Blurring of vertical, horizontal or diagonal lines

Causes

Your eye has two parts that focus images — the cornea and the lens. In a perfectly shaped eye, each of these focusing elements has a perfectly smooth curvature like the surface of a rubber ball. A cornea or lens with such a surface curvature bends (refracts) all incoming light the same way and makes a sharply focused image on the back of your eye (retina).

However, if your cornea or lens isn't evenly and smoothly curved, light rays aren't refracted properly, and you have a refractive error. Astigmatism is one type of refractive error. In astigmatism, your cornea or lens is curved more steeply in one direction than in another. Uncorrected astigmatism blurs your vision. Typically, the blurred vision occurs more in one direction than in another — either horizontally, vertically or diagonally.

Astigmatism may occur in combination with other refractive errors, which include:

• Nearsightedness (myopia). This occurs when your cornea is curved too much or your eye is longer than normal. Instead of being focused precisely on your retina, light is focused in front of your retina, resulting in a blurry appearance for distant objects.

• Farsightedness (hyperopia). This occurs when your cornea is curved too little or your eye is shorter than normal. The effect is the opposite of nearsightedness. Light is focused beyond the back of your eye, making nearby objects blurry. You're able to see faraway objects clearly.

In most instances, astigmatism is present at birth. Sometimes, astigmatism develops after an eye injury, disease or surgery. Astigmatism isn't caused or made worse by reading in poor light, sitting too close to the television or squinting.

When to seek medical advice

If your degree of astigmatism is pronounced enough so that you can't perform a task as well as you want, or if your quality of vision detracts from your enjoyment of activities, see an eye doctor. An eye doctor can determine the degree of your astigmatism and advise you of your options to correct your vision.

Astigmatism changes slowly, if at all. Having regular eye examinations is a good way to detect vision changes, so that you can get corrective lenses or update your lens prescription.

If you're a healthy adult, have your eyes examined every three to five years until age 50. After age 50, have them checked more frequently for signs of eye disease or problems. If you have refractive or other eye problems, such as astigmatism, have your eyes checked every two years, or as often as your eye doctor recommends.

Screening and diagnosis

Your eye doctor may use these tools in examining your eyes:

•  Keratometer. With keratometry, your doctor uses an instrument called a keratometer to quantify the amount and orientation of corneal astigmatism by measuring reflected light from the surface of your cornea.

•  Keratoscope and videokeratoscope. These devices are used to detect and quantify corneal surface curvature and the presence of astigmatism. A keratoscope uses light to project rings on the cornea. Observation through the keratoscope of the reflection of light from the cornea and inspection of the shape and spacing of the rings provide information about the degree of astigmatism. A keratoscope fitted with a video camera is called a videokeratoscope. A videokeratoscope is the most common instrument used to quantify the change in corneal surface curvature, in a process called corneal topography.

Treatment

The goal of treating astigmatism is to address the uneven curvature that's causing your blurred vision. Treatments include wearing corrective lenses and undergoing refractive surgery.

Corrective lenses
Wearing corrective lenses treats astigmatism by counteracting the uneven curvature of your cornea. Types of corrective lenses include:

• Contact lenses. Contact lenses can correct both corneal and lenticular astigmatism. A wide variety of contact lenses are available — hard, soft, extended wear, disposable, rigid gas permeable and bifocal. Ask your eye doctor about their pros and cons and what might be best for you. Another type of procedure — orthokeratology, or Ortho-K — also uses contact lenses. In orthokeratology, you wear rigid contact lenses for several hours a day until the curvature of your eye improves. Then, you wear the lenses less frequently to maintain the new shape. If you discontinue this treatment, your eyes return to their former shape.

•  Eyeglasses. An alternative to contact lenses is eyeglasses. Like contact lenses, the variety of eyeglasses is wide.

Refractive surgery
This treatment method corrects astigmatism by reshaping the surface of your eye. Refractive surgery methods include:

• LASIK surgery. Laser-assisted in situ keratomileusis (LASIK) is a procedure in which a doctor uses an instrument called a keratome to make a thin, circular hinged cut into your cornea. The surgeon lifts the flap and then uses an excimer laser to sculpt the shape of the cornea under the flap. An excimer laser differs from other lasers in that it doesn't produce heat.

• Photorefractive keratectomy (PRK). In PRK, your surgeon removes the outer protective layer of the cornea before using an excimer laser to change the curvature of the cornea.

LASIK and PRK are good for correcting low to moderate astigmatism in people with nearsightedness. The results aren't as good in farsighted people with astigmatism.

LASIK and PRK have replaced radial keratotomy. Radial keratotomy fell into disfavor in the 1980s after some people developed increasing farsightedness after an initially good result. Radial keratotomy involves making several incisions in your cornea to flatten it. The incisions radiate out like spokes of a wheel.

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Treatments