Abstract TP121: Temporal Trends in Seasonal Variation of Frequency for Ischemic Stroke and Intracerebral Hemorrhage Admissions in a Nationwide Japanese Registry, 2005–2023

Introduction: Seasonal variation in stroke incidence has often been reported, but over the past two decades, climate and behavior have changed markedly, including global warming and COVID-19–related stay-at-home orders. We evaluated temporal trends in the seasonal distribution of hospital admissions for ischemic stroke and intracerebral hemorrhage (ICH) from 2005 to 2023. Methods: We analyzed patients in the Japan Stroke Data Bank, a multicenter prospective registry, from 2005 to 2023. We calculated seasonal proportions of admissions for ischemic stroke and ICH overall and by ischemic stroke subtype (large-artery atherosclerosis [LAA], cardioembolic [CE], and lacunar infarction). Seasons were defined as spring (March–May), summer (June–August), autumn (September–November), and winter (December–February). Seasonal distributions were summarized within four epochs (2005–2009, 2010–2014, 2015–2019, 2020–2023). Results: Overall, 228,127 cases were included. The ICH proportion was highest in winter (22.3%) and lowest in summer (17.5%) (p<0.001). This winter-peak/summer-low pattern persisted across epochs, with odds ratios for summer vs winter ranging from 0.70 to 0.76. The winter peak in ICH strengthened after 2015, and the same tendency was observed during the COVID era (2020–2023). In contrast, seasonal variation in ischemic stroke was small. By ischemic stroke subtype, lacunar infarction was modestly lower in winter (spring–autumn vs winter odds ratios, 1.05–1.08), whereas CE was modestly higher in winter (summer–autumn vs winter odds ratios, 0.91–0.92). Over time, lacunar seasonality attenuated, with no significant differences during 2020–2023, while the CE winter peak gradually strengthened and was most evident in 2020–2023. LAA showed no significant seasonal variation across epochs. Conclusions: Across 2005–2023, the ICH winter-peak/summer-low pattern persisted. Although overall seasonal variation in ischemic stroke was small, the winter peak of CE and winter low of lacunar persisted; CE seasonality slightly strengthened, whereas lacunar seasonality weakened over time. These directions were consistent during the COVID era, indicating that the pandemic did not introduce a fundamentally different seasonal pattern.