Definition

     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.

Signs and symptoms

     Signs and symptoms of astigmatism may include:

• Distortion in portions of your visual field

• Blurring of vertical, horizontal or diagonal lines

• Eyestrain or fatigue

• Headaches

Causes

     There is no known reason why some people develop astigmatism and others do not. Some known causes of astigmatism include the weight of the upper lid on the eyeball, trauma to the eye and cuts or scarring on the cornea. The tendency to develop astigmatism is increased when one or more parents have the condition. Poor nutrition can make astigmatism worse.
 

Screening and diagnosis

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

• Keratometer. This instrument quantifies 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 which contact lenses might be best for you. Contact lenses are also used in a procedure called orthokeratology, or Ortho-K. 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, eyeglasses come in a wide variety.

     Refractive surgery
     This astigmatism treatment method corrects the problem 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. Alternatively, this same cut can be made with a special cutting laser. 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.

• Laser-assisted subepithelial keratomileusis (LASEK). In this procedure, a much thinner layer of cornea is folded back, which makes the eye less vulnerable to damage should an injury occur. LASEK may be a better option for people with thin corneas or those at high risk of an eye injury at work or from playing sports.

     Radial keratotomy is a procedure that was used in the past to correct astigmatism. However, because some people who had the surgery developed increasing farsightedness after an initially good result, it's generally not performed anymore.