The findings, published in the December issue of the Journal of Neurology, Neurosurgery & Psychiatry, echo earlier reports of a relationship between migraine with aura and ischemic stroke.
While emphasizing that the risk of stroke is generally small among women who get migraines, the researchers did find increased risk depends on the type and frequency of migraine.
The researchers, from Radcliffe Infirmary in Oxford and the Imperial College of Science, Technology & Medicine in London, compared 86 women admitted to the hospital with a first-time ischemic stroke with 214 women admitted for other illnesses during the years 1990-93. All were between the ages of 20 and 44.
The risk of ischemic stroke (in which the arterial blood supply is blocked) increased by a factor of 8 in those women whose first migraine was accompanied by aura and by 10 in those whose first migraines with aura happened more than once a month. If migraines had persisted more than 12 years, the stroke risk was four times higher. The most recent study is a more detailed analysis of 300 of the women who took part in the much larger World Health Organization Collaborative Study of Cardiovascular Disease and Steroid Hormone Contraception.
While the use of oral contraceptives is known to increase slightly the risk of stroke in women with migraines, and starting on oral contraceptives is likely to change the type of migraine from one without aura to one with it, the study authors found no increase in stroke risk associated with a change in the type of migraine.
And the researchers emphasize that the absolute risk of stroke in women with migraines is still very small, about 15 per 100,000 women-years. Absolute risk is defined as the actual number of exposed and unexposed people in the groups being compared who actually got the disease.
The take-home point, the authors note, is that the initial migraine type may be important to know to evaluate a woman's later stroke risk.
About 28 million Americans suffer from migraine, according to the National Headache Foundation, and women are more often affected than are men. About one fifth of migraine sufferers have auras, the warning signs that can include visual disturbances such as wavy lines or flashing lights as well as disturbances in smell, taste or touch beginning 20 minutes to an hour before the migraine.
"It's an interesting study," says Dr. Seymour Diamond, executive chairman of the National Headache Foundation in Chicago and director of the Diamond Headache Clinic. However, he says the research finding is not new, as the researchers themselves note.
Overall, says Diamond, who has been a headache expert for more than 40 years, research suggests those with migraines have a mild to moderate increased risk of stroke. Exactly why is not known, he says.
"I believe the nervous system of migraineurs is probably more susceptible to vascular things happening," he adds.
Those who have migraine with aura have "more neurological phenomenon associated with it that would give an increased tendency to have a vascular event," Diamond says.
He echoes the researchers' reassurance that the absolute risk is very low. "I've seen all types of migraine, and I can count on one hand the number of migrainous strokes I have seen," he says.
As a rule, Diamond generally advises younger women with a history of migraine who want to take oral contraceptives to take the lowest estrogen dose possible.
However, Michael John Coleman, executive director of the Migraine Awareness Group, says the study results should be a wake-up call to women on the pill who smoke cigarettes and have migraines.
They should stop smoking immediately and switch to the lowest pill dose possible, he says.
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