Nearsightedness, medically known as myopia, is the most common cause of poor eyesight, especially in younger adults. It refers to being able to see well at close range, but not at a distance. It is the opposite of farsightedness (hyperopia). Focusing ability can start to decline in childhood and continue to slide through the teenage years, usually stabilizing in the early 20s. After that, most people find they don't need to change their glasses or contact lens prescriptions very often, if at all.
Myopia affects about 30% of the population of Canada. We know that some ethnic groups are more prone than others. People of Chinese, Japanese, and Southeast Asian descent are especially likely to be myopic.
Myopia is caused when the shape of the eye doesn't permit it to focus on distant objects. The cornea is a special 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. 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. In myopia, either the cornea is too curved or the eye is too long front to back, so the light rays meet and cross in front of the retina. By the time the image reaches the retina, it's blurred.
Nearsightedness is at least partly genetic. It isn't a disease. Just as some people's noses are longer than others, so are some people's eyes. Some research suggests that people who do a lot of work requiring that they focus closely may be more likely to develop nearsightedness over time. Children who are born prematurely often develop eye conditions that affect the shape of the eye and may be more likely to develop nearsightedness.
Symptoms and Complications
People usually first notice myopia in their teenage years, when they may experience headaches and it becomes apparent that other people can see distant objects better than they do. This may come as a shock to a young person who has forgotten that they used to see more clearly as a child. Such a teenager may be amazed at the clarity of the world after they get their first lens prescription – often they were unaware that the human eye is capable of functioning that well. Other clues that a person may be developing nearsightedness include the need to squint to see clearly, frequent blinking and eye rubbing. A person who is nearsighted may also hold books close to read.
Myopia almost never worsens rapidly. Some diseases, like diabetes, can make nearsightedness develop faster, but underlying diseases more usually cause trouble at the back of the eye. This makes it hard for an adult to notice if their prescription needs changing. The only way to be sure is to get regular eye tests.
Myopia can lead to poor performance at school. If a student's grades start dropping off around age 10 or 12, there's a chance eyesight is involved, although certainly plenty of other factors can cause 12-year-olds to have problems at school.
Among adults, severe myopia can cause disability, primarily by preventing driving. This can usually be corrected.
Making the Diagnosis
The test for myopia is the well-known poster with rows of smaller and smaller letters, called the Snellen sight card. You wear a special set of lens frames, and different lenses are inserted until you see properly. This test can also detect astigmatism, in which a cornea that's irregularly shaped can't focus sharply at any range. People with myopia (and even people without myopia) often have some degree of astigmatism.
Treatment and Prevention
There are three possible treatments for myopia: glasses, contact lenses, and eye surgery.
Glasses can correct all vision errors, including hyperopia (farsightedness), myopia, and astigmatism. Bifocal lenses are particularly useful for older people with poor vision at long range and very close range. Looking through the upper lens helps long-range vision, while looking through the lower helps close-range vision.
Contact lenses can also correct these problems. Lenses with high correction factors (i.e., those made to correct very poor vision) are liable to be thicker and heavier than lenses that correct for less severe cases of myopia, and some people find them uncomfortable. This is particularly true of lenses that correct for astigmatism. Soft lenses are taking the place of hard and gas-permeable versions, as they're more comfortable and easier to adjust to, though they may be harder to clean. Disposable contact lenses, which can be worn and then thrown away, are also a popular option. Wearing lenses makes people somewhat more prone to eye infection. This risk can be minimized by cleaning them according to instructions, throwing away disposables on time, and never sleeping with the lenses in.
Many Canadians have opted for eye surgery, mostly to correct myopia. 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 and 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 modified version of LASEK that uses a mechanical device to remove the outer layer of the cornea.
These techniques are about equally effective in improving sight. Infection of the area that was operated on may occur a day or two after the operation, but antibiotics are usually given to prevent infection. Otherwise, if infection does occur, it's usually easily treated with antibiotics.
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. Another procedure involves the insertion of a tiny removable ring into the cornea, which leaves the central part untouched, reducing the risk of complications.
The operations themselves are quick (less than several minutes per eye) and painless, though the eye will sting for a few days afterwards. 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. In myopia, they often deliberately under-correct slightly, knowing that your eyesight will probably drift towards farsightedness as you age.
Finally, although laser surgery has been around for over 15 years, we still don't know the long-term consequences. While there's no reason to believe they'll have problems, we can't be absolutely sure. It is important to discuss potential complications thoroughly with an eye doctor before opting for laser surgery.
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