The cornea is the clear front layer on the surface of the eye that does most of the work of bending light as it enters the eye. Light must be bent properly so that it creates a sharp image. Theoretically, the eye's cornea should be perfectly round, like a slice off the surface of a ping-pong ball.
When parallel light rays pass through the cornea, they should be bent just enough to meet and focus on the retina (the light-sensitive membrane that lines the back of the eye). The lens is responsible for fine adjustments of focus.
Astigmatism is a condition in which the cornea is oval, like the bottom of a dessert spoon. This means that the light rays focus at different points in the eye. As a result, the focus is smeared so that while lines in one direction may be sharp, lines in another direction will be blurry.
We don't know what causes astigmatism, but we know it tends to run in families. Some people are born with astigmatism. Others get astigmatism later in life, sometimes after eye injury, surgery, or disease.
Everyone has a tiny degree of astigmatism – no cornea is perfectly round. It's only a problem when it interferes with vision. Astigmatism is not made worse by sitting too close to the television, squinting, or reading in low light.
Symptoms and Complications
If you have astigmatism, you may be able to focus on an object just fine, but the peripheral field of view won't be perfectly clear. Often, people with astigmatism have myopia (nearsightedness) as well. If their vision isn't corrected, everything will be blurred, but still some areas of the field of view will be more focused than others. People with astigmatism may experience headache or eyestrain.
Astigmatism shouldn't cause halos, dimming, brightness, or any changes to the outward appearance of the eye. These symptoms suggest other problems like glaucoma, cataracts, diabetic retinopathy, and conjunctivitis. The degree of astigmatism may change over the years, but there shouldn't be any sudden changes in ability to see and make out detail.
Making the Diagnosis
A thorough eye test will include various tests for astigmatism. This is usually done by an optometrist (a doctor who tests vision and prescribes glasses and contact lenses) or an ophthalmologist (a doctor who specializes in eye problems and eye surgery).
Treatment and Prevention
There are three ways to treat astigmatism: glasses, contact lenses, and surgery.
Glasses and contact lenses for people with astigmatism have an extra component called a toric lens. This bends light to counterbalance the effects of astigmatism. These lenses are usually a bit more expensive.
Toric contact lenses may be slightly heavier on one side. This side slips to the bottom of the eye, ensuring that the right part of the lens is over the right part of the eye. Some people have found these lenses less comfortable than normal lenses. But contact lens technology is improving all the time, so if you found toric lenses uncomfortable in the past, you might not if you try them now.
Surgery treats astigmatism by reshaping the surface of the eye. For people who are also nearsighted, the surgery can correct both the nearsightedness and astigmatism at the same time.
There are a few basic types of operations, which are usually done in an eye clinic:
- laser-assisted in situ keratomileusis (LASIK)
- laser-assisted subepithelial keratomileusis (LASEK)
- epithelial laser in situ keratomileusis (Epi-LASIK)
- photorefractive keratectomy (PRK or PK)
With LASIK, the doctor makes a small cut into the top layer of the cornea, then folds back this layer like a flap. A laser is then used to mould the inner tissue of the cornea underneath into a better shape. The flap is then replaced.
PRK is done with a computer-guided laser. A computer-controlled laser removes the epithelium (the very top protective layer over the cornea in the front of the eye), then reshapes the cornea by ablating (destroying) tissue in the periphery or outer part. The result is a cornea that is shaped to refract (bend) light correctly.
LASEK is a variation of LASIK and PRK. With LASEK, the flap created in the cornea is thinner, and the laser moulds the outer layer of the cornea (the epithelium).
Epi-LASIK is a modified version of LASEK that uses a mechanical device to remove the outer layer of the cornea.
Newer refinements of these procedures are constantly under development. One example is "wavefront" or "custom" LASIK, in which a computer maps the visual problems needing correction, increasing the accuracy of the result.
Surgery can only be performed on uninfected healthy eyes. It's not suitable for children and teenagers, whose vision is still changing.
Eye operations don't always produce the desired results. Study results vary, but most people who have laser surgery have improved vision. It is important to realize, however, that some people do not notice any improvement in their vision, and a few end up with poorer vision.
Laser surgery can reduce the quality of your best corrected vision (the absolute best you could see with the right lenses). It can also impair night vision. In addition, the eye can change again after the operation. Sometimes, it heals from surgery into much the same shape it had before, producing only temporary improvement. Because eye doctors like to err on the side of caution, they may under-correct vision, and you may need follow-up procedures. It is important to discuss any potential complications thoroughly with an eye doctor before opting for laser surgery.