Seasonal variation in the occurrence of ischemic stroke and subarachnoid hemorrhage in Siberia, Russia. A population-based study.

Epidemiological studies on the relationship between stroke occurrence and the seasons in different countries produced inconsistent results and little is known about these associations in a general population. We report a population-based study of 214 patients with first-ever ischemic stroke (IS, data for 1992) and 64 patients with first-ever subarachnoid hemorrhage (SAH) registered in the 25-74 years old population of Oktiabrsky District of Novosibirsk, Russia in 1982-92. IS and SAH incidence in four seasons (winter, spring, summer, autumn) was evaluated by means of a chi-square test. Poisson regression analysis was used to compute the rate ratios (RRs) and corresponding confidence intervals (CIs) for the occurrence of IS and SAH in winter, spring, and autumn compared with summer. Seasonal variations in the occurrence of IS were significant for the group of young men (25-64 years) and the group of older women (65-74 years) only. The age and sex adjusted RR of the occurrence of IS in winter was 49% greater than in summer (95%CI 1-119%). When men and women were analyzed separately and the rates were adjusted for age, a significantly higher risk of IS was found only in men (RR = 2.48; 95% CI 1.27-4.83) in spring compared with summer. No seasonality was observed for the occurrence of SAH in both men and women. Our findings indicate that there is a significantly greater incidence of ischemic stroke during winter in Siberia, Russia, whereas the incidence of SAH does not show a seasonal variation.