There may be a link between weather and the risk of suffering a stroke, say researchers who analyzed climate trends and hospital records on millions of Americans.
Cold weather, high humidity and big daily temperature swings seem to land more people in the hospital with strokes. As it got warmer, risk fell — 3 percent for every 5 degrees, the study found.
"Maybe some of these meteorological factors serve as a trigger," said Judith Lichtman, a Yale University stroke researcher who led the study. With global climate change and extreme weather like this week's freak storm in the South, "this could be increasingly important," she said.
Lichtman and colleagues from Harvard and Duke universities gave results of their study Wednesday at the American Heart Association's International Stroke Conference in San Diego. It is the largest and most detailed research on this issue.
Each year, about 800,000 Americans have a stroke. Most are due to clots that block a blood vessel to the brain, and high blood pressure is a major risk factor.
Some earlier studies found a seasonal trend to stroke rates, and there are biological reasons to think they are related, said one independent expert, Dr. Andrew Stemer, a neurologist at Georgetown University.
Blood vessels constrict in cold weather, which can raise blood pressure, he said. Extreme weather can trigger a stress reaction by the body, causing it to release substances "that not only increase the work of the heart" but make blood stickier and more likely to clot, Stemer said.
In cold weather "your body clamps down, there's cardiovascular stress," said Dr. Larry Goldstein, a Duke stroke specialist who worked on the study.
Conversely, "high humidity may cause dehydration," which also can raise the risk for clots and raise stress on the body, he said. "You know how you feel when you're out in hot, humid weather — you don't feel so hot."
Several of these same researchers published another study earlier this year that looked at stroke deaths from 1999 to 2006 among Medicare patients and found a pattern — higher rates in the winter, lower in summer and a small peak in July.