Ben Freeman, age 9, of Cheyenne, Wyoming, for his question:

WHAT CAUSES A STROKE?

A stroke is a serious medical condition that happens if the brain does not receive a proper amount of blood. There are three main causes of a stroke: there is a cerebral thrombosis, a cerebral hemorrhage or a cerebral embolism. Most strokes are caused by high blood pressure.

Every year more than 500,000 Americans suffer strokes and with more than 200,000 the attacks prove to be fatal. Most of the survivors of stroke suffer temporary or permanent disabilities and require extensive medical care.

Cerebral thrombosis happens when a blood clot forms in one of the major arteries that carries blood to the brain. In most cases, the clot builds up in a blood ve.ssel that has been narrowed by arteriosclerosis, which is a hardening of the arteries.

People with high blood pressure are especially likely to develope arteriosclerosis.

A cerebral hemorrhage happens when an artery in the brain ruptures. In many cases, the rupture results from high blood pressure. It can also result from an aneurysm, which is a weak spot on the wall of an artery that might swell like a bubble until it bursts.

A cerebral embolism, like cerebral thrombosis, involves a clot that blocks one of the major arteries of the brain. However, the clot forms in another part of the body. It then travels through the bloodstream until it clogs one of the arteries that supplies the brain.

A person who suffers a stroke may first experience weakness and paralysis on one side of the body. Other symptoms may include difficulty in speaking or understanding language and blind spots in vision.

In cerebral thrombosis, the symptoms may develop progressively during several hours or several days. In cerebral hemorrhage and cerebral embolism, the changes occur very rapidly. In either case, many victims eventually lose consciousness, and a large number of them die.

Recovery from a stroke depends on how much of the brain has been permenently damaged.

In a great many strokes, there isn't complete damage to the entire area of the brain that controls a certain activity. If enough of the area remains undamaged, the victim may be able to learn that activity again. Other parts of the brain also can learn to take over some of the jobs previously performed by the damaged area.

Rehabilitation should start as soon as the patient regains consciousness. At first, the treatment consists of stretching contracted muscles and moving the arm and leg on the affected side of the body.

High blood pressure, also called hypertension, is the basic cause of most strokes. Therefore, the control of this condition is the most effective way to prevent strokes.

Doctors often prescribe various medications and diets to control high blood pressure.