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Astigmatism
By Mayo Clinic staffMayo Clinic Health Manager
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Definition
Astigmatism is a common, mild and easily treatable imperfection in the curvature of your eye. The condition can cause 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.
Astigmatism blurs your vision at all distances. Astigmatism is often present at birth and may occur in combination with nearsightedness or farsightedness. Often it's not pronounced enough to require corrective action. When it is, your treatment options include corrective lenses and surgery.
Symptoms
Signs and symptoms of astigmatism may include:
- Distortion in portions of your visual field
- Blurred vision
- Eyestrain
- Headaches
When to see a doctor
If your quality of vision detracts from your enjoyment of activities or interferes with your ability to perform everyday tasks, see an eye doctor. An eye doctor can determine the degree of your astigmatism and advise you of your options to correct your vision.
If you're a healthy adult older than 40, have your eyes examined about every two to four years until age 65. After age 65, have them checked every one to two years for signs of eye disease or problems. If you have eye problems, such as astigmatism, you may need to have your eyes checked more frequently. If you're at risk of certain eye diseases, such as glaucoma, or you have diabetes, check with your doctor to see how often you need to have your eyes examined.
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 smooth 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, the light rays aren't refracted properly. This causes 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. 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. Blurred vision may occur more in one direction — 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. When your eye is in a relaxed state, light is focused behind the back of your eye, making nearby objects blurry.
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.
Tests 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.
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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 your cornea. Observation through the keratoscope of the reflection of light from your 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 measure the change in corneal surface curvature, in a process called corneal topography.
Treatments and drugs
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 are:
- 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 help compensate for the uneven shape of your eye.
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 your eye less vulnerable to damage should an injury occur. LASEK may be a better option if you have a thin cornea or if you're 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, it's not commonly performed anymore.
- Astigmatism. American Optometric Association. http://www.aoa.org/x4698.xml. Accessed Oct. 27, 2008.
- Preferred practice pattern: Comprehensive adult medical eye evaluation. San Francisco, Calif.: American Academy of Ophthalmology. http://one.aao.org/asset.axd?id=6749e4a0-6a09-4795-8f8d-aec60dc937c9. Accessed Oct. 27, 2008.
- Abad JC, Azar DT. Current concepts, classification and history of refractive surgery. In: Yanoff M, et al. Ophthalmology. 2nd ed. St. Louis, Mo.: Mosby; 2004. http://www.mdconsult.com/das/book/body/108512830-3/764482214/1197/19.html#4-u1.0-B0-323-01634-0..50019-5--cesec5_324. Accessed Oct. 27, 2008.
- Preferred practice pattern: Refractive errors and refractive surgery. San Francisco, Calif.: American Academy of Ophthalmology. http://one.aao.org/asset.axd?id=b5e06307-8a97-4675-8174-367ed4eb355b. Accessed Oct. 27, 2008.
