Increased incidence of subarachnoid hemorrhage during cold temperatures and influenza epidemics.

OBJECTIVE This study investigated whether the increased incidence of aneurysmal subarachnoid hemorrhage (SAH) in winter is related to temperature or increased incidence of influenza. Such relationships may elucidate the pathogenesis of intracranial aneurysm rupture. METHODS A nationwide sample of 18,714 patients with SAH was linked with weekly temperature and influenza-like illness consultation data. Poisson regression analyses were used to calculate incidence density ratios (IDRs) with corresponding 95% CIs for the association of SAH incidence with temperature and influenza epidemics; IDRs were adjusted for study year (aIDR). In addition, SAH incidence data from 30 European population-based studies were linked with daily temperature data from the European Climate Assessment. RESULTS The aIDR for SAH during influenza epidemics was 1.061 (95% CI 1.022-1.101) in the univariable and 1.030 (95% CI 0.989-1.074) in the multivariable analysis. This association declined gradually during the weeks after epidemics. Per 1°C temperature drop, the aIDR was 1.005 (95% CI 1.003-1.008) in the univariable and 1.004 (95% CI 1.002-1.007) in the multivariable analysis. In the European population-based studies, the IDR was 1.143 (95% CI 1.129-1.157) per 1°C temperature drop. CONCLUSIONS The incidence of SAH is increased during cold temperatures and epidemic influenza. Future studies with individual patient data are needed to investigate causality between temperature or influenza and SAH.