A long airplane flight can increase the risk of having a rare type of stroke, a new study says.

The stroke type occurs in those who have a deformity called a patent foramen ovale, an opening between two of the heart's chambers. While up to 30 percent of the general population has this condition, most are unaware they do. And long-term air travel boosts the risk of getting this type of stroke when the abnormality is present, the authors say.

While the link between air travel and blood clots in the lungs has been widely studied and publicized, a connection between flying and strokes has not been fully explored till now.

The new study, appearing in the June 24 issue of Neurology, may be the first to report on it, the researchers say.

But the study's co-author says the findings should not alarm most passengers.

"It's a real problem but a small one," says Dr. Stephen W. Borron, an associate clinical professor of emergency medicine and medicine at George Washington University School of Medicine in Washington, D.C.. "There are millions of passenger miles [logged] without any problems."

Borron and his colleagues from Avicenne Hospital in Bobigny, France, examined passengers arriving at Paris' Charles de Gaulle Airport over an eight-year period, from 1993 to 2000. Of the 155 million passengers, 65 were tended by a medical team for lung blood clots. And of those, four passengers, or 6 percent of those with lung blood clots, had strokes.

All four of those had the patent foramen ovale, the researchers say.

What happens, the researchers say, is that air travel can increase the risk of developing blood clots in the legs, which can then travel to the lungs and block an artery. Sometimes, the heart openings can allow the blood clot to enter the brain blood vessels and cause a stroke.

Putting the risk in perspective, Borron says, "Passengers traveling long distances, over 3,500 miles, appear to be at some increased risk, albeit small, of having a blood clot travel to the lungs. If they have patent foramen ovale, there is also a risk that the clot could travel to the brain. It must be emphasized that the number of people who have blood clots during air travel is low."

At greater risk, he says, is anyone who has had surgery within the previous month, those with cancer or previous blood clots, pregnant women, and anyone who has been immobile longer than three days.

"The results of this study are not surprising," says another expert, Dr. Cathy Helgason, director of the cerebrovascular service at the University of Illinois College of Medicine in Chicago.

Most people with the heart abnormality would not know they have it, Borron says, unless they have had an echocardiogram of the heart or cardiac catheterization.

Both Borron and Helgason advise passengers flying more than 3,500 miles to take precautions.

"Anyone traveling long distances should be aware of the findings of this study," Helgason says. "Frequent walks up and down aisle, exercise of the lower extremities and arms, as well as maintenance of good hydration under such circumstances, should play a role in prevention."

"If you are taking a long flight, make sure you get a little exercise on the plane," Borron agrees.

If you can't walk up and down the aisle, he adds, do in-seat exercises, moving your ankles to keep the blood flowing.

In an earlier study of lung blood clots, Borron found that 85 percent of those affected had not moved around during the lengthy flight.

More information

For more information on stroke risk factors, click on the National Institute of Neurological Disorders and Stroke or the American Heart Association.