Ambient temperature and bath related stroke subtypes in a Japanese Stroke Registry.

Bath-related stroke is an important but underrecognized clinical problem during colder seasons. In Japan and other East Asian regions, where hot-water immersion bathing is common, individuals often bathe under cold ambient conditions, which may induce substantial hemodynamic fluctuations. However, the association between ambient temperature and bath-related onset among stroke cases, particularly subtype-specific differences and temperature-related patterns, remains unclear. This registry-based, retrospective observational study included 14,772 hospitalized patients with stroke registered in the Tochigi Stroke Registry from nine municipalities. Daily minimum ambient temperature linked to the admission date was categorized into quartiles. Bath-related onset occurred in 431 patients (2.9%). A higher proportion of hemorrhagic stroke and a lower proportion of ischemic stroke were seen in bath-related onset cases than in non-bath-related onset cases. In age- and sex-adjusted subtype analyses, lower daily minimum temperature was associated with higher odds of bath-related onset among patients with intracerebral hemorrhage and ischemic stroke, particularly atherothrombotic infarction; however, these subtype-specific findings were exploratory, and formal interaction tests did not show statistically significant heterogeneity by stroke subtype. The proportion of bath-related onset increased as daily minimum temperature decreased, and the coldest quartile was associated with higher odds of bath-related onset compared with the warmest quartile in the multivariable model (adjusted odds ratio, 1.65; 95% confidence interval, 1.23-2.21). A similar direction of association was observed in analyses restricted to patients without conventional vascular risk factors, although event counts were limited. In sensitivity analyses additionally adjusted for calendar year and month, the estimates were attenuated and no longer statistically significant. Lower daily minimum ambient temperature was associated with higher odds of bath-related onset among hospitalized stroke cases, although this association was attenuated after adjustment for calendar year and month. Subtype-specific findings were exploratory and should be interpreted cautiously. These findings highlight the importance of further evaluating the role of thermal conditions in bathrooms and dressing rooms during colder periods.