Scientists say they have found a strong correlation between the presence of migraine headaches and the prevalence of strokes. American and Canadian researchers say migraine sufferers are twice as likely to have strokes than those not suffering the painful headaches.

“The results of our study strongly suggest that migraine may be an independent risk factor for stroke,” said Ali Samii, a neurologist at the University of Washington, writing in the British Medical Journal. He and colleagues believe the heightened risks come from reduced blood flow to the brain caused during migraine episodes.

Patients who have light effects, also called auras, face even heightened risk of stroke. The researchers reviewed 14 other studies to reach their conclusions.

The cause of migraine headaches is unknown, according to experts, who said more research needs to be done regarding possible links between migraine and stroke risks.