You're sitting quietly at home when suddenly you notice a severe weakness in your arm or leg. Call medical help at once. You're experiencing a symptom that could be a stroke warning sign.

Quick action might well mean the difference between life and death. Most people now know the warning signs of a heart attack, and the need to seek treatment when they occur.

Few know, however, that there are also early warning signs for many of the 600,000 strokes that Americans will suffer this year.

A stroke is caused by a disruption of blood flow to the brain. The symptoms of stroke onset will vary, depending on which part of the brain is affected.

Early medical evaluation of these symptoms could be life-saving. If the stroke is caused by the rupture of a small blood vessel, emergency treatment can limit the damage. Studies are under way to determine whether quick use of clot dissolving drugs can reduce damage done by strokes in which a blood clot blocks a brain artery. The treatment now is regarded as experimental because of the worry over provoking bleeding.

One early warning sign of a possible future stroke is a TIA - transient ischemic attack - in which a small blood vessel is blocked for a few moments. Anyone who has a TIA is considered to be at higher risk of having a major stroke in the future.

Some common symptoms of a TIA include loss of vision in one eye, sometimes temporary, numbness or weakness of an arm or leg, slurred speech and difficulty or inability to speak. These symptoms may come and go, but should be regarded as a warning to seek immediate medical attention.

A person who experiences a TIA as warning

signs of a stroke may need to be treated preventively with anticoagulants or aspirin - drugs that reduce blood clotting. Factors that are known to increase the risk of stroke are high blood pressure, diabetes, heart disease, use of oral contraceptives and illicit drug use, especially cocaine.