People who suffer migraines are about twice as likely as people without the painful headaches to suffer a stroke caused by a blood clot, according to a new research review.
The analysis, which combined the results of 21 previous studies, confirms a connection between migraines and ischemic stroke - the most common form of stroke, occurring when a clot disrupts blood flow to the brain.
Across the studies, migraine sufferers were about twice as likely to suffer an ischemic stroke as people without migraines, according to findings published in the American Journal of Medicine.
Experts are not sure why the relationship exists, and it is not yet known whether the migraines themselves directly lead to strokes in some people.
It's likely, however, that a common underlying process contributes to both migraines and stroke risk, said Dr Saman Nazarian, the senior researcher on the new study and an assistant professor at Johns Hopkins University School of Medicine in Baltimore.
He said health that for now the bottom line for migraine sufferers is that they should be particularly vigilant about controlling any modifiable risk factors for stroke that they may have.
Some of those risk factors include high blood pressure, smoking and diabetes.
"The main thing I would want (people) to take away from this is that if they get migraines, they should address stroke risk factors," Nazarian said.
"They should not smoke and they should watch their blood pressure and have it treated if it is high."
Experts also generally say that people with migraines should remember that while the headaches are linked to a relatively increased risk of stroke, the absolute risk to any one person remains fairly low.
In one recent study of 6,100 adults with migraines, for example, 2% reported a history of stroke, versus 1.2% of 5,243 adults who did not suffer from migraines.
The current findings are based on 21 international studies conducted between 1975 and 2007 and involving more than 622,000 adults with and without migraines.
Most of the studies took into account a number of factors that might help explain any connection between migraine and stroke risk -- such as age, high blood pressure, diabetes, smoking habits and weight.
Even with those factors considered, migraines themselves were linked to a two-fold increase in stroke risk when Nazarian's team combined the results of all 21 studies.
The precise cause of migraines is not fully understood, but the pain involves constriction, and then swelling, of brain blood vessels. One theory is that people with migraine may have dysfunction in the blood vessels throughout the body, which may explain the increased risk of stroke and, as some previous studies have found, heart attack.
No one yet knows whether treating and preventing migraine attacks can do anything to affect people's risk of cardiovascular problems.
On one hand, researchers have noted, drugs that prevent migraine attacks could theoretically lower the risk of cardiovascular problems.
On the other hand, certain medications might have negative effects; some anti-inflammatory painkillers have been linked to ardiovascular risks, while migraine drugs known as "ergots" tend to constrict blood vessels throughout the body.