In their study of two U.S. states, researchers whose report appeared in the journal Neurology found that the rate of strokes among adults younger than 55 nearly doubled between 1993 and 2005.

Among whites aged 20 to 54, the rate rose from 26 strokes for every 100,000 people, to 48 per 100,000. Among African Americans, it climbed from 83 to 128 per 100,000.

The researchers said they could only speculate on possible explanations. One might be that doctors are detecting strokes in young people more often, both as a result of better brain-imaging technology and being more vigilant.

"But I really don't think that's the major reason," said lead researcher Brett Kissela, of the University of Cincinnati College of Medicine. "We're definitely seeing a higher incidence of risk factors for stroke now."

Those risk factors include obesity, diabetes and high blood pressure.

"And if you're developing them at the age of 20, then you may have a stroke at a younger age too," Kissela said.

But a researcher not involved in the study agreed that better diagnosis and a real increase in young people's risk of stroke are both probably at work.

"Now MRI allows us to detect smaller strokes," said Mitchell S.V. Elkind, of Columbia University in New York, who co-wrote an editorial published with the study.

These include subtle symptoms like mild degrees of blurry vision, weakness or numbness. In the past, doctors might not have thought "stroke" when a relatively young person had them - and MRI scans were not used often back in the 1990s.

The study was based on nearly 5,900 Ohio and Kentucky adults who suffered a first-time stroke between 1993 and 2005. Over that time, 20 to 54-year-olds accounted for a growing proportion of strokes - from 13 percent in 1993 to almost 19 percent by 2005.

The study group came from only two U.S. states, but both Kissela and Elkind said the findings likely reflect what's happening nationally. A government study last year found a similar pattern nationwide.

Kissela's team found that in 1993-1994, only 18 percent of all stroke patients in their study had an MRI. By 2005, that figure had risen to 58 percent.

"But that probably doesn't explain it all," Elkind said, noting that drug abuse can also cause strokes. "We know there's been an increase in obesity and diabetes."

Kissela said the findings underscore the importance of a healthy lifestyle and that younger adults shouldn't see themselves as "invincible" and get to the doctor if they do in fact have health problems like high blood pressure or cholesterol.

"It's a small percentage of young people who have strokes, but it can happen," Kissela said. SOURCE: bit.ly/IUcacJ

(Reporting from New York by Amy Norton at Reuters Health; editing by Elaine Lies)

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