Clear outlook: Study finds no link between weather and stroke

July 11, 2002

DALLAS, July 12 - Challenging previous reports of a little-understood link between stroke and weather, a Canadian study found no discernible connection between the two, researchers report in the July issue of Stroke: Journal of the American Heart Association.

Calgary lies in the "Chinook belt" - an area where geological and atmospheric factors frequently cause rapid and extreme weather variations, causing what is known locally as the Chinook wind. A University of Calgary research team obtained hourly weather data for 1996 through 2000, and compared average daily strokes on Chinook and non-Chinook days with average daily temperatures, humidity, barometric pressure, and wind speed.

They tracked 182 Chinook days and more than 3,000 strokes, but found no association between stroke and weekly, monthly or seasonal weather changes, or between any type of stroke or weather parameter. The authors had doubts about earlier observational reports linking stroke and weather change because research findings across studies have been inconsistent.

Previous studies indicated that strokes are more prevalent in cooler months, but no type of stroke has been consistently linked to weather or seasonal changes in different areas, the researchers note.

The inconsistency suggests these reports may not have considered complex variables such as culture, while the lack of negative studies suggests potential publication bias, says Michael D. Hill, M.D., a neurologist with the Calgary Stroke Program in the university's department of clinical neurosciences.

If a relationship does exist between weather and stroke, it remains foggy, writes Myles D. Connor, FCP(SA) in an accompanying editorial. Connor, a stroke researcher and member of the neurology division at the University of the Witwatersrand, in Johannesburg, South Africa, says he has a number of criticisms of the Calgary study.

"Not only is it retrospective and therefore open to inaccuracies in stroke diagnosis and case ascertainment, but it is also not community-based," he writes. Despite the Canadian researchers' claim that most stroke patients in the region are admitted to their hospital, he says, there is no data to support the assertion. "As a result, patients with very severe strokes who died before admission or very mild strokes who stayed home may have been missed."

Connor recommends a systematic review of the literature and further community-based studies in other regions, with a range of weather conditions, to determine once and for all whether such an association exists or not.
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