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Q: I had my first retinal tear about 12 years ago and the second one about seven years later. I've now had two in the last year. I still have good vision and my eye pressure is normal. Please explain the cause and prevention of retinal tears.

A: Tears of the retina are also called detachments, since the retina has to pull away from the back of the eyeball for a noticeable tear to take place.

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 is filled with a gooey substance called the 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, it causes visual changes.

Fortunately, shifts are usually very small and only cause what are called floaters. These are like pieces of lint on a movie projector that causes spots to move across the vision. If the shift is great enough, a detachment may 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, these changes only occur in one eye at a time. By looking through one eye and then the other, you can make this determination.

There are three ways in which a retina can become detached. The most common way is for a slight tear to occur in the retina, then fluid seeps behind it and pushes it away from the back of the eyeball.

Another way is the through contraction of scar tissue, such as occurs in people with bleeding caused by diabetes. And the third way is by excess fluid formation in the tissue behind the retina, which builds up and pushes the retina away.

Minor detachments that can repair themselves may 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 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 in your case will depend on the cause of your detachment as well as the condition of your 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 blood pressure: Blood pressure readings may depend on where you get them done. For example, some people are more nervous at a doctor's office so their blood pressure is higher than when taken by a drugstore-based machine.

Blood pressure readings can also vary from one arm to another. In order to best compare how your blood pressure is doing from visit to visit, it's important to have it taken on the same arm each time.

But sometimes there is a large difference in blood pressure readings between arms. In a recent study, it was found that about 20 percent of people had either a systolic blood pressure difference of greater than 20 or a diastolic difference of greater than 10.

The study also found that those with these greater differences also had an increased incidence of problems related to the heart or blood vessels.

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