Seasonality of stroke in Finland

Introduction: The burden of stroke is increasing globally. Reports on seasonal variations in stroke occurrence are conflicting and long-term data are absent. Methods: A retrospective cohort study using discharge registry data of all acute stroke admissions in Finland during 2004-2014 for patients >= 18 years age. A total of 97,018 admissions for ischemic stroke (IS) were included, 18,252 admissions for intracerebral hemorrhage (ICH) and 11,271 admissions for subarachnoid hemorrhage (SAH). Results: The rate of IS admissions increased (p=0.025) while SAH admission rate decreased (p<0.0001), and ICH admission rate remained stable during the study period. The lowest seasonal admission rates were detected in summer and the highest in autumn for all stroke subtypes. Seasonal variation of IS was more pronounced in men (p=0.020), while no sex difference was detected in ICH or SAH. The seasonal patterns of in-hospital mortality and length of stay (LOS) differed markedly by stroke subtype. Diagnoses of hypertension, atrial fibrillation, or diabetes showed no seasonality. Conclusions: All major stroke subtypes occurred most commonly in autumn and most infrequently in summer. Seasonality of in-hospital mortality and length of hospital stay appears to vary by stroke subtype. The seasonal pattern of ischemic stroke occurrence appears to have changed during the past decades. KEY MESSAGES All major stroke subtypes (ischemic stroke, intracerebral hemorrhage, subarachnoid hemorrhage) occurred most frequently in autumn and least frequently in summer. Seasonal patterns of in-hospital mortality and length of stay differed markedly by stroke subtype. The seasonal pattern of ischemic stroke occurrence in Finland seems to have changed compared to 1982-1992.