Seasonal and Temperature-Related Variation in Subarachnoid Hemorrhage Hospital Admissions: A Nationwide Ecological Time-Series Analysis in Brazil, 2020-2024.

BACKGROUND: Subarachnoid hemorrhage (SAH) carries high mortality worldwide. Seasonal patterns have been documented in temperate regions, but evidence from tropical and subtropical populations remains limited. We investigated associations between meteorological variables and SAH hospital admissions across diverse climate zones in Brazil. METHODS: We conducted an ecological time-series analysis linking nationwide SAH hospitalizations [International Classification of Diseases Version 10 (ICD-10; I60.0-I60.9)] from Brazil's Unified Health System with meteorological data from the National Institute of Meteorology (January 2020-November 2024). Quasi-Poisson generalized linear models were used to estimate relative risks (RR) per 1 °C decrease in mean minimum temperature, overall and stratified by season. Sensitivity analyses excluded the first coronavirus disease 2019 (COVID-19) pandemic year. RESULTS: Among 9903 SAH hospital admissions over 59 months, seasonal variation was observed, with the highest proportion occurring in autumn (27.2%, n = 2697) and the lowest in spring (23.4%, n = 2316; Chi-squared p < 0.001). The mean summer-winter difference in minimum temperature was 3.6 °C. Overall, the temperature-SAH association did not reach statistical significance [RR per 1 °C decrease: 1.019; 95% confidence interval (CI): 0.994-1.045; p = 0.14]. In season-stratified analyses, winter was the only season demonstrating a significant inverse association (RR = 1.133; 95% CI 1.008-1.275; p = 0.037). This finding was robust to the exclusion of the pandemic year 2020 (RR = 1.140; 95% CI 1.013-1.282; p = 0.030). In-hospital case fatality did not differ significantly across seasons (range 5.2-5.7%; p = 0.38). CONCLUSIONS: SAH hospital admissions in Brazil exhibit modest seasonal variation, with a significant association with temperature observed only during the winter months. These findings extend evidence on environmental determinants of SAH to a tropical and subtropical setting, though the small effect size and ecological design warrant cautious interpretation.