Researchers studied more than 7,000 patients who showed up with headaches at a Boston emergency department over a seven-year period to see if any environmental factors -- such as temperature change, air pollution or changes in barometric pressure -- might be at play.
Of all the factors explored, they found that higher air temperature in the 24 hours prior to the hospital visit was most closely associated with headache symptoms. They found a 7.5 per cent higher risk of severe headache reported for each temperature increase of five degrees Celsius.
The findings of the study, led by researchers at Beth Israel Deaconess Medical Center, have been published in the journal Neurology.
"I think there's strong lore both amongst patients and frankly amongst doctors about the effects of weather on headaches," said Dr. Kenneth Mukamal, first author of the study, and an internist at the medical centre.
"Many previous studies have been limited in so far as we conduct them by asking people, for example, to keep diaries. But there's a limited number of people you can ask to keep diaries, and if they think that certain things are affecting their headaches, then you may introduce bias just because they know they're being studied."
So for this study, Mukamal took a slightly different approach that allowed for a look at a large population, and took advantage of "excellent" monitoring techniques for meteorological and air pollution levels in the greater Boston area.
Of the patients who were discharged with a primary diagnosis of headache from May 2000 to December 2007, 2,250 were diagnosed with a migraine while 4,803 were diagnosed with a tension or unspecified headache.
"For the migraine headaches ... there was a stronger effect of temperature for those headaches," Mukamal said from Boston.
"For the non-migraine headaches, which are going to be a grab-bag of sinus headaches, tension headaches and other things, in that case there did seem to be an effect of temperature but also an effect of barometric pressure, or lower pressure, at least a couple of days before."
Dr. Werner Becker, a professor in the department of clinical neurosciences at the University of Calgary, said most migraine patients complain that weather triggers their attacks, or at least some of them.
"Some migraine patients call themselves human barometers -- when the pressure drops they tend to get headaches -- yet they found a slight association between barometric pressure falls and non-migraine headaches, but not with migraine itself."
The data from this new study are limited by the fact that most people with migraine attacks never visit the emergency department.
"Had they been able to take these 7,000 patients and know about all of their migraine headaches, perhaps they might have found a stronger correlation with some kind of weather change," he suggested.
Mukamal noted that a positive association was previously seen in a Montreal study between the number of emergency department visits for headache and changes in the atmospheric pressure.
But Mukamal said that study, and one done in Ottawa, did not find an association between changes in temperature and incidence of headaches.
Mukamal said the findings give doctors something more to think about when they're trying to decide what's triggering somebody's headaches.
"We know that exercise can trigger heart attacks, but we also know that there are medications that specifically seem to prevent the triggering effect of exercise," he said.
"Well, now I think we have the chance to investigate whether there are medications for migraine that might specifically prevent the triggering related to temperature."
Becker said some patients take prophylactic medications for months at a time to prevent attacks. But other kinds of migraine medication should not be taken too frequently, he said.
"If patients take their medication more than 10 days a month they become at risk for getting more and more migraines," he said.
"So you can imagine if they start taking medication whenever it gets hot in some parts of the U.S., they're going to end up taking medication many, many days a month, which is not a good thing."
-- The Canadian Press