Seasonal Variation in the Occurrence of Stroke in a Finnish Adult Population

Abstract

Background and Purpose Seasonal influence on the incidence of and mortality from cerebrovascular disease has been reported during the last three decades, but generally with some discrepancy in results, particularly regarding stroke subtypes. The aim of this study was to examine seasonal variation in the incidence, mortality, and case-fatality rate of stroke in data from the FINMONICA population-based stroke register. Methods During 1982 to 1992, 15 449 stroke events were registered in the monitored populations aged 25 to 99 years in three geographic areas of Finland: the provinces of Kuopio and North Karelia in eastern Finland and the Turku/Loimaa area in southwestern Finland. We analyzed the seasonal and monthly incidence, mortality, and case-fatality rate of stroke and subtypes of stroke by pooling the data for the three areas and stratifying by sex and age. Results The rate of occurrence of ischemic stroke events was 12% (95% confidence interval [CI], 5% to 20%) greater in men and 11% (95% CI, 4% to 19%) greater in women in winter than in summer. For intracerebral hemorrhage, we observed a 28% (95% CI, 3% to 58%) greater rate of occurrence in men and a 33% (95% CI, 6% to 66%) greater rate of occurrence in women in winter than in summer. The occurrence of subarachnoid hemorrhage did not vary significantly by season. The greater incidence of ischemic strokes in winter was particularly prominent among men aged 25 to 64 years and less prominent in elderly men and in women. The 28-day case-fatality rate of ischemic stroke showed significant seasonal variation only in women ( P =.001), with the lowest rate in summer. Conclusions There is a significantly greater incidence of ischemic strokes and intracerebral hemorrhages during winter in Finland. Further research that also takes meteorologic and sociodemographic factors into account is warranted, since it may help to determine new ways to prevent strokes.