This is the VOA Special English Health Report.

Strokes are a major cause of death and disability. A stroke is a loss of blood flow in the brain. There are two kinds: An ischemic stroke happens when a blood vessel in the brain gets blocked. A hemorrhagic, or bleeding, stroke happens when a blood vessel breaks.

People are more likely to die from a bleeding stroke. But ischemic strokes are more common, and doctors may be able to treat them.

A drug called tPA can break up blood clots. But traditional guidelines say not to use tPA if more than three hours have passed after the first signs of a stroke.

There is a risk that giving a patient a strong blood thinner during a stroke can cause bleeding inside the brain. The longer the wait, experts say, the more likely that the risks of treatment will outweigh the benefits.

But recent findings have suggested that tPA may be effective in saving brain tissue even if three to four and a half hours have passed.

Some studies have failed to produce clear evidence to support treatment after three hours. But scientists reported that the evidence was stronger when they combined the results of the four major studies done so far. The new findings appeared in the journal Stroke.

The researchers said tPA improved the chances of a successful result by thirty-one percent and produced no change in the death rate.

Maarten Lansberg at the Stanford University medical school in California worked with scientists from Belgium and Germany. One of them worked for a company that makes tPA for use in Europe. The United States National Institutes of Health paid for the study.

If you think someone is having a stroke, you should seek help immediately. The warning signs usually appear suddenly. These include trouble walking, weakness especially on one side of the body, difficulty seeing and difficulty speaking.

Yet people who seem healthy can suffer a stroke without even knowing it. A study published in Stroke last year involved about two thousand people with an average age of sixty-two. Brain imaging showed that nearly eleven percent of them had suffered what is known as a silent stroke.

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