The
burden of stroke is increasing globally. Reports on seasonal variations
in stroke occurrence are conflicting and long-term data are absent.

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).

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.

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.