Dr June Spector and colleagues from Johns Hopkins University, Baltimore, conducted the largest meta-analysis to date of stroke risk in migraine sufferers, reviewing 21 studies involving 622,381 patients.

Previous studies have suggested that migraine sufferers may be at increased stroke risk, but Dr Spector and colleagues sought to quantify this effect.

They found that migraine sufferers were 2.4 times as likely as non-sufferers to experience a stroke.

'Migraine is a potentially modifiable risk factor that can be treated, and stroke risk can be reduced through reduction of other factors,' they said.

The researchers found that the association between migraine and stroke risk was strongest in studies involving women participants.

However, they were not able to analyse this difference in detail because there were no studies involving both men and women with results differentiated by sex.

Dr Spector and colleagues said there were several potential mechanisms that might explain the increased stroke risk in migraine sufferers.

Migraine may be associated with cerebrovascular changes which leave patients more likely to suffer a stroke, they said. Changes in platelet activation or aggregation may occur in migraine sufferers or migraine may be linked to changes in levels of compounds involved in clot formation.

However, Dr Spector and colleagues were not able to determine which of these mechanisms led to the increased stroke risk.

The researchers said the findings highlighted the importance of identifying high-risk migraine sufferers with other modifiable stroke risk factors.

Studies will now need to look at the effect of migraine treatment on stroke risk, they said.

• Am J Med Online 2010