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Q: Please explain the cause and prevention of retinal tears.

-- E.N., Hadley, N.Y.

A: The retina is a thin, semi-transparent layer of nerve endings and supporting tissue that lines the back of the eyeball. The nerve endings (the rods and cones) respond to different wavelengths of light and send signals to the brain through the optic nerve.

The back of the eyeball if filled with a gooey substance called the vitreous humor. It serves to keep the eye round and also to hold the retina in place. When there is a shift in the vitreous, changes in vision occur.

Fortunately, such shifts are usually very small and only cause what are called floaters. These look like pieces of lint on a movie projector that causes spots to move across the vision. They may even resemble circles or spiders.

If the shift is great enough, a detachment may take place. A tearing of the retina is also called a detachment, because the retina has to pull away from or detach from the back of the eyeball for a noticeable tear to take place.

Symptoms of a detachment include: a gradual or sudden increase in floaters in one eye; a gradual or sudden increase in flashes in one eye; and, more ominously, the sudden appearance of a cloud or curtain being drawn over one eye from any direction.

Since it's highly unlikely to have a retinal detachment in two eyes at the same time, its symptoms will only occur in one eye at a time. One way to determine whether you suffer from the condition, therefore, is to look first through one eye and then the other.

There are a couple of ways in which a retina can become detached. The most common way is for a slight tear to occur in the retina, allowing fluid to seep behind it and push it away from the back of the eyeball. Another cause is the contraction of scar tissue, such as occurs in people with bleeding caused by diabetes.

Minor detachments often can repair themselves and cause minimal problems. Major detachments are often permanent and can cause partial to complete blindness.

The retina is very delicate tissue, so fixing it requires very high-level skills and precise instruments. In almost all cases of retinal detachment -- and many times for the prevention of detachment -- a laser is used to "spot weld" the retina in place.

Whether laser welding will help prevent further detachment depends on the cause of the detachment, as well as the overall health of the retina.

Certainly you need to talk with the ophthalmologist who has been working on your eyes to help you evaluate the benefits and risks of any prophylactic measures.

Update on Vioxx: If you are using this medication, I hope you have already heard that Vioxx is being removed from the market. Merck, the drug's manufacturer, decided to do so days after learning the results of a recent study. The study found that patients who took a high daily dose of Vioxx for three years had twice as many heart attacks and strokes as patients who took a placebo.

It should be noted, however, that the death rate for those taking Vioxx was no different than for those taking a placebo. Moreover, the incidence of strokes and heart attacks among the study's participants was so low that the overall increase in risk for those who took Vioxx amounted to less than 1 percent.

There are two other drugs in the same class of drugs as Vioxx. These drugs are called Cox-2 inhibitors. Many may wonder if they should be worried about taking these other two drugs (Bextra and Celebrex). Although this is of concern, to date no good medical study has proved that these drugs increase the risk of heart attacks or strokes.

Regardless of what future studies may find, there are, fortunately, many other drugs (e.g., aspirin, naproxen and ibuprofen) that adequately treat the pain and inflammation that the Cox-2 drugs are used for -- and often they are much less expensive.

Write to Allen Douma in care of Tribune Media Services, 2225 Kenmore Ave., Suite 114, Buffalo, N.Y. 14207.

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