When the Weather Turns Cold: Seasonal Patterns of Subarachnoid Hemorrhage in a Tropical Country

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. Objectives We investigated associations between meteorological variables and SAH hospital admissions in Brazil across diverse climate zones. Methods We conducted an ecological time-series analysis linking nationwide SAH hospitalizations (ICD-10: I60.0–I60.9) from Brazil's Unified Health System with meteorological data from the National Institute of Meteorology, 2020–2024. Generalized additive models with distributed lag structures were used to assess the relationships between temperature, humidity, atmospheric pressure, and monthly SAH incidence. Air pollution analysis was attempted but revealed critical surveillance gaps. Stratified analyses explored effect modification by season, age, and region. Results Among 3,323 SAH admissions, incidence peaked during autumn (26.4%) and winter (25.4%) when mean temperatures were 3.4°C lower than summer. Temperature showed inverse associations with admissions in autumn (β = −8.3 per °C; 95% CI: −11.2, − 5.4) and winter (β = −7.1 per °C; 95% CI: −9.8, − 4.4). Effects were stronger in southern regions and among individuals aged 65 years or older. Air quality data were available for only three months, which precluded analysis and highlighted infrastructure deficiencies. Discussion SAH admissions in Brazil increase during colder months with a dose-response relationship. Findings align with cold-induced sympathetic activation and blood pressure elevation. The absence of integrated air pollution monitoring represents a major surveillance gap. Results support the incorporation of meteorological indicators into neurocritical care planning and underscore the urgent need for an environmental health data infrastructure in Latin America.