Seasonal variation in spontaneous intracerebral hemorrhage frequency in Chengdu, China, is independent of conventional risk factors

Seasonal variations in the incidence of intracerebral hemorrhage (ICH) have been extensively evaluated in studies conducted in various parts of the world. However, seasonal variation of ICH in China has not been comprehensively reported in the English literature. The aim of the present study was to examine the seasonal variation of spontaneous ICH incidence and the 28-day case-fatality rate in a Chinese population. We also examined whether this variation was modified by conventional stroke risk factors (hypertension, diabetes mellitus, smoking, drinking alcohol). From 2006 to 2009, 1615 hemorrhagic stroke events (male, 965; female, 650) were registered in residents of the Jinjiang and Wuhou districts, Chengdu, China. Seasonal variation was calculated for the whole period. Incidence rates (per 100,000 person-seasons) and 95% confidence intervals (CI) were calculated by gender and age for winter, spring, summer, and autumn. After stratifying patients by their risk factor history, odds ratios and 95% CI of having a stroke in autumn, winter, and spring were calculated, with summer serving as a reference. Among the seasons, the incidence per 100,000 person-years of spontaneous cerebral hemorrhage was highest in the winter (45.5, 95% CI, 38.4–52.7) and lowest in summer (24.1, 95% CI, 21.9–26.2). Winter incidence was highest in both men and women, and in subjects younger than 60 years as well as subjects 60 years or older. The winter excess in stroke incidence was observed regardless of the presence or absence of risk factors. Factors that explain this excess need further investigation.