Variation in ischemic stroke frequency in Japan by season and by other variables

Abstract

Background and purpose: It is unclear whether acute ischemic stroke exhibits a seasonal pattern in Japan. The aim of the present study was to elucidate seasonal differences in acute ischemic stroke.

Methods: Our study enrolled 12,660 patients with ischemic stroke (7943 men, 4717 women; mean age, 70.1 years, S.D. 11.5; median 70; range 18-107). We divided the year into four parts: spring (March-May); summer (June-August); fall (September-November); and winter (December-February). Time of stroke onset was divided into three subgroups: daytime (08:00-16:00), evening (16:00-24:00), and night (24:00-08:00). We examined the association between clinical characteristics, season, and time of stroke onset.

Results: Stroke occurred least frequently in spring (22.9%), followed by winter (25.3%), fall (25.8%), and summer (26.0%) (P<0.001). No differences in age, National Institutes of Health Stroke Scale (NIHSS) score, modified Rankin Scale (m-RS) score, history of stroke/transient ischemic attack (TIA), or risk factors for stroke were observed among the four seasons. Stroke in men (63.8% vs. 62.4%; P<0.01), lacunar stroke (LS) (41.2% vs. 39.4%, P<0.01), atherothrombotic stroke (ATS) (34.0% vs. 32.3%; P<0.01), and nighttime stroke (26.5% vs. 24.8%; P<0.05) were observed more frequently in summer compared to other seasons. This contrasts with the findings for stroke in women (39.0% vs. 36.7%; P<0.05), cardioembolic stroke (CES) (23.4% vs. 20.6%; P<0.05), and daytime stroke (47.4% vs. 45.0%; P<0.05), which were more frequent in winter.

Conclusions: Acute ischemic stroke displays seasonal characteristics according to gender, stroke subtype, and time of stroke onset. These results may have important clinical implications in ischemic stroke prevention.