As if migraines didn’t already cause enough suffering, a new study has found that people who experience the debilitating headaches are at much higher risk for Alzheimer’s disease.

The study from two Canadian universities — the Universities of Waterloo and Manitoba — found that migraine sufferers were three times more likely to have a general dementia diagnosis and four times more likely to have Alzheimer’s disease than peers without migraines. The headaches, however, were not associated with vascular dementia, a type of dementia caused by damaged blood vessels in the brain. Dementia is an umbrella term that includes both Alzheimer’s and vascular dementia as well as less common types of cognitive decline.

Put another way, the study found that 24% of participants with Alzheimer’s disease and only 10% of those without dementia had a history of migraines.

The study, being published Thursday in the International Journal of Geriatric Psychiatry, analyzed data from 679 older people who participated in the Manitoba Study of Health and Aging. The patients were followed for five years in the 1990s, and 51 developed dementia. Migraine was self-reported, but study subjects received extensive evaluations for dementia, said senior researcher Suzanne Tyas, an associate professor in the School of Public Health and Health Systems at the University of Waterloo.

Previous investigations into the relationship between migraines and dementia have yielded mixed results. This one was different because it followed a sample of community-based participants over time rather than focusing on patients who sought treatment for migraine. People who seek treatment may not be as representative of the general population, Tyas said.

Understanding risk factors for Alzheimer’s, which is untreatable, could help doctors identify patients who need more cognitive monitoring or should pay special attention to other health problems that can raise risk, she said. She did not know how migraine and Alzheimer’s could be related, but said they may share underlying causes that might help scientists find new targets for dementia prevention. She said it was a surprise that migraines did not raise risk for vascular dementia.

The results were most relevant for women, she said. In fact, there weren’t a lot of men who had migraines in their sample, and none of them got dementia. A larger study would be needed to explore sex differences. Both Alzheimer’s disease and migraines are more common in women.

Migraines have previously been linked to a higher likelihood of having heart attacks, stroke, seizures, and some mental health problems.

Migraines are severe headaches that can last for an hour to days. Overall, they affect 20% of women and 8% of men.

Stephen Silberstein, director of the Jefferson Headache Center, said he was skeptical of the study because only about a third of people who get migraines know they have them. They peak when women are ages 35 to 40, and as many as 40% of women have them. They become less common after menopause.

Doctors used to think that migraine was a disorder of blood vessels, Silberstein said. Now, "we think it is a disorder of hypersensitivity of the central nervous system."

The pain from migraines may cause temporary changes in thinking ability. "If you have headaches every day, all day, you don't think well," Silberstein said.

The data available for this study did not allow the researchers to analyze whether treating migraine made a difference, or whether the type and frequency of headaches mattered, Tyas said. Those are important questions for future research, she said.