Subarachnoid hemorrhage accounts for roughly five percent of all strokes worldwide, yet it punches far above its weight in mortality, with case fatality rates approaching thirty-five percent and many survivors left with lasting disability. The most common cause is the rupture of an intracranial aneurysm, a balloon-like bulge in the wall of a cerebral artery that has been silently weakening for years. While hypertension, smoking, and aneurysm characteristics are established risk factors, researchers have long suspected that environmental triggers, particularly sudden shifts in weather, may tip fragile vessels over the edge. Evidence from Europe and East Asia has repeatedly shown that cold weather is associated with spikes in hemorrhagic stroke, but nearly all of that work comes from high-latitude countries where winter temperatures plunge more than fifteen degrees below summer levels.

Brazil offered the research team, led by Daniela Laranja Gomes Rodrigues of Hospital Alemão Oswaldo Cruz and Universidade Federal de São Paulo, a fundamentally different test case. The country sprawls across equatorial, tropical, and subtropical climate zones, and during the study window from January 2020 to November 2024, the national difference in mean minimum temperature between summer and winter averaged just 3.6 degrees Celsius. If a temperature signal could be detected against such a mild seasonal gradient, it would suggest that even modest cooling might be enough to destabilize vulnerable cerebral blood vessels, a finding with potentially far-reaching implications as climate patterns shift and as health systems in warm regions plan for seasonal surges.

The researchers assembled their dataset from two national repositories. Hospitalization records came from DATASUS, the informatics backbone of Brazil’s Unified Health System, which captures care for the roughly seventy-five percent of Brazilians who rely on public healthcare. They identified 9,903 admissions for subarachnoid hemorrhage between January 2020 and November 2024 using the ICD-10 codes I60.0 through I60.9, an average of about 168 admissions per month. Meteorological data came from the National Institute of Meteorology, whose network of roughly five hundred automatic weather stations provided complete monthly temperature coverage for all 27 Brazilian federative units, eliminating the need for interpolation or imputation.

The statistical machinery behind the analysis was a quasi-Poisson generalized linear model, a workhorse of environmental epidemiology designed for count data that show more variability than a simple Poisson distribution can accommodate. Indeed, the dispersion parameter in this dataset was 4.55, indicating substantial overdispersion that the quasi-Poisson framework handled appropriately. The team’s prespecified primary effect measure was the relative risk per one degree Celsius decrease in mean minimum temperature, calculated overall and then separately for each Southern Hemisphere season: autumn from March to May, winter from June to August, spring from September to November, and summer from December to February.

The headline result is striking in its selectivity. Across the entire study period, the overall association between falling temperature and hemorrhage admissions pointed in the expected direction but did not reach statistical significance, with a relative risk of 1.019 per degree of cooling and a confidence interval spanning 0.994 to 1.045. But when the models were stratified by season, winter stood alone. During the coldest months, each one degree Celsius drop in mean minimum temperature was associated with a 13.3 percent increase in subarachnoid hemorrhage admissions, a relative risk of 1.133 with a confidence interval of 1.008 to 1.275 and a p-value of 0.037. No comparable association emerged in spring, summer, or autumn, and the overall Pearson correlation between minimum temperature and monthly admissions was weak at negative 0.192.

The seasonal pattern in raw admission counts told a consistent story. Autumn accounted for the largest share of admissions at 27.2 percent, followed by winter at 25.3 percent, while spring saw the fewest at 23.4 percent, a distribution that was statistically significant by chi-squared testing. The time-series plots showed recurrent fluctuations with visual concordance between temperature nadirs and admission peaks, particularly between May and August. Critically, the winter finding survived a rigorous sensitivity check: when the pandemic year 2020 was excluded to rule out disruptions in healthcare access, the winter association not only held but strengthened slightly, to a relative risk of 1.140. A parallel analysis using mean maximum temperature instead of minimum temperature produced a directionally similar estimate that approached but did not reach significance.

What could explain why cold snaps might rupture aneurysms? The biological plausibility rests on well-documented cardiovascular physiology. Acute cold exposure triggers sympathetic nervous system activation, peripheral vasoconstriction, and a rise in systemic vascular resistance, all of which transiently elevate blood pressure. Blood pressure variability is a recognized precipitating factor for aneurysmal rupture, and seasonal increases in prothrombotic and inflammatory markers have been described in the literature, though these mechanisms remain incompletely characterized. In essence, a cold morning may act as a physiological stress test that a weakened arterial wall fails. The researchers are careful to note, however, that their ecological design, which links population-level counts to population-level temperatures rather than tracking individual exposures, cannot directly demonstrate these mechanisms at work.

One reassuring finding concerned outcomes rather than incidence. In-hospital case fatality rates, calculated from 548 deaths over the study period, did not differ significantly across seasons, ranging narrowly from 5.2 percent in summer to 5.7 percent in winter. The authors caution that these are aggregate monthly ratios rather than patient-level outcomes, so they cannot speak to whether disease severity at presentation varied by season. Still, the stability of fatality rates suggests that the winter surge reflects more admissions rather than more lethal ones, which has practical value for hospital planners: neurocritical care units in resource-constrained systems can anticipate modest but predictable winter increases in bed occupancy, intensive care demand, and neurosurgical volume.

The authors are refreshingly candid about the limits of their work. The dataset captures only public-system admissions, excluding private care and prehospital deaths, so overall case ascertainment may approach only half of true national incidence, though validation studies support DATASUS reliability for temporal trends. The ICD-10 code I60.x bundles aneurysmal and nonaneurysmal bleeds together, individual clinical data such as hypertension status and aneurysm characteristics were unavailable, and with only fifteen winter observations across five years, statistical power was limited and the winter confidence interval was wide, leaving open the possibility of chance variation. Monthly aggregation may also obscure the acute cold exposures that matter most physiologically. The team calls for case-crossover studies with daily exposure resolution, extension to ischemic stroke and intracerebral hemorrhage, and integration of air pollution data, which were too sparse nationally to include here, a gap that itself underscores the need for stronger environmental surveillance in middle-income countries. If replicated, the finding that a mere 3.6 degree seasonal swing leaves a detectable fingerprint on brain-bleed admissions would be a sobering reminder that even mild climate variability shapes human health in unexpected ways.

Subject of Research: Seasonal and temperature-related variation in subarachnoid hemorrhage hospital admissions in Brazil

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

Article References: Rodrigues, D. L. G., de Andrade, J. B. C., & Silva, G. S. (2026). Seasonal and Temperature-Related Variation in Subarachnoid Hemorrhage Hospital Admissions: A Nationwide Ecological Time-Series Analysis in Brazil, 2020–2024. Neurocritical Care. https://doi.org/10.1007/s12028-026-02610-3

Image Credits: AI Generated

DOI: 10.1007/s12028-026-02610-3

Keywords: subarachnoid hemorrhage, stroke, temperature, seasonality, Brazil, winter, DATASUS, quasi-Poisson model, time-series analysis, neurocritical care, aneurysm rupture, climate and health