This is the VOA Special English Health Report.

　　A stroke is a loss of blood flow in the brain. There are two kinds of stroke. One is an ischemic stroke. It happens when a blood vessel in the brain gets blocked. The other kind is called a hemorrhagic, or bleeding, stroke. This happens when a blood vessel breaks.

　　Strokes can cause death or disability. Bleeding strokes are more likely to kill than ischemic strokes. But ischemic strokes are more common -- and doctors can treat them with a drug that breaks up blood clots.

　　If you think someone is having a stroke, you should seek help immediately. Experts at the United States National Institutes of Health say the treatment has to begin within three hours. But they say people need to get to the hospital within one hour so doctors have time to examine them.

　　Usually the warning signs appear suddenly. These include trouble walking, weakness especially on one side of the body, difficulty seeing and difficulty speaking.

　　Yet a recent study showed that seemingly healthy middle-aged people can suffer a stroke without immediately knowing it. The study involved about two thousand people. They were the children of men and women who took part in a major research project in Massachusetts called the Framingham Heart Study.

　　The average age in the new study was sixty-two. The people were examined every four to eight years. They were given M.R.I., magnetic resonance imaging, tests to inspect for damaged brain tissue and signs of stroke.

　　The imaging showed that nearly eleven percent of those with no reported stroke had suffered a silent cerebral infarction, or silent stroke. Silent strokes are brain injuries likely caused by a blockage that limits blood supply to the brain.

　　The researchers reported a link between silent stroke and a condition called atrial fibrillation. Atrial fibrillation is the most common cause of abnormal heartbeat in older adults.

　　Sudha Seshadri at the Boston University School of Medicine says atrial fibrillation more than doubled the risk of a stroke. Doctor Seshadri says the findings show the need for early testing and treatment of conditions that could lead to heart disease.

　　The study appeared in Stroke, a journal of the American Heart Association.

　　Other risk factors for a silent stroke are high blood pressure, heart disease, diabetes and tobacco use.

　　And that's the VOA Special English Health Report. For more health news, go to voaspecialenglish.com. I'm Steve Ember.