Vertebrobasilar and Middle Cerebral Aneurysms Carry Highest Case Fatality Rates

Anatomical Determinants of Survival in Subarachnoid Hemorrhage

Aneurysmal subarachnoid hemorrhage remains a catastrophic clinical event, accounting for a significant portion of the global stroke burden and carrying high rates of mortality and long-term disability [1]. While evidence-based guidelines emphasize early, aggressive care to mitigate complications like delayed cerebral ischemia (a secondary reduction in blood flow that can lead to permanent brain damage), the initial rupture often proves fatal before medical intervention is possible [2]. Current pharmacological strategies, such as the administration of nimodipine to improve functional outcomes, primarily target hospitalized patients who have survived the initial ictus [3]. Although population-based data suggest that overall case fatality rates (the proportion of patients with a condition who die from it) have gradually declined over the last four decades, there remains substantial variation between different regions and patient cohorts [4]. A comprehensive analysis of whole-population data from Finland and New Zealand has now clarified how specific anatomical rupture sites influence survival, highlighting the critical role of prehospital mortality.

Population-Wide Analysis of Ruptured Intracranial Aneurysms

To capture the true mortality burden of subarachnoid hemorrhage, researchers analyzed externally validated administrative databases to identify all hospitalized and nonhospitalized cases in Finland and New Zealand between 2001 and 2017. By including prehospital deaths, this approach avoids the survivorship bias inherent in hospital-only registries. The analysis captured a total cohort of 13,470 subarachnoid hemorrhage cases, including 5,056 cases originating from New Zealand. The study population had a median age of 58 years, and women comprised 61.3% of the total cases, reflecting the established demographic distribution of the disease. Ruptured intracranial aneurysms were categorized using International Classification of Diseases, 10th Revision (ICD-10) codes into four primary anatomical locations: the anterior communicating artery, the internal carotid artery, the middle cerebral artery, and the vertebrobasilar artery. To ensure diagnostic precision, these location-specific diagnoses were cross-referenced with external hospital datasets, population registries, and autopsy records. This rigorous validation is critical for practicing physicians because it ensures that the mortality risks associated with specific vascular territories are based on confirmed anatomical sites rather than administrative estimates. The primary outcome measure was the overall 30-day case fatality rate. To account for potential confounding variables, the researchers computed risk ratios adjusted for age, sex, and country using a Poisson regression model (a statistical method that calculates the relative risk of an event across different groups while controlling for multiple variables). These calculations were reported with 95% confidence intervals to isolate the independent effect of aneurysm location on survival outcomes.

Mortality Risks by Anatomical Site

The anatomical site of the rupture proved to be a critical determinant of whether a patient survived long enough to receive inpatient neurosurgical or endovascular intervention. Among the 13,470 cases, the distribution of ruptured intracranial aneurysm locations was 29.5% in the middle cerebral artery, 26.6% in the anterior communicating artery, 18.4% in the internal carotid artery, 11.5% in the vertebrobasilar artery, and 14.0% in other or unspecified locations. The researchers found that subarachnoid hemorrhage case fatality varies significantly by aneurysm location, with the highest mortality observed in the posterior circulation. The overall 30-day case fatality rate was highest for vertebrobasilar artery ruptures at 54.1%, followed by middle cerebral artery ruptures at 40.5%. In contrast, the 30-day case fatality rates for anterior communicating artery and internal carotid artery ruptures were notably lower, recorded at 29.1% and 28.5%, respectively. To understand the timing of these fatalities, the researchers calculated sudden death rates, defined as deaths occurring before admission to a hospital ward. The data demonstrate that the high case fatality rates in vertebrobasilar and middle cerebral artery locations are primarily driven by high rates of sudden death. Specifically, the sudden death rate for vertebrobasilar artery ruptures was 33.0%, and the sudden death rate for middle cerebral artery ruptures was 21.6%. These figures represent a substantial increase in prehospital mortality compared to other sites. For instance, the sudden death rate for anterior communicating artery ruptures was 11.9%, while the rate for internal carotid artery ruptures was the lowest among the primary categories at 9.9%.

Temporal Trends and Clinical Implications

Longitudinal analysis revealed significant improvements in survival for specific aneurysm locations over the nearly two-decade study period. Between the 2001 to 2003 interval and the 2015 to 2017 interval, the overall 30-day case fatality rates for vertebrobasilar artery ruptures declined by 24% (95% CI, 13% to 34%). During the same timeframe, the 30-day case fatality rates for middle cerebral artery ruptures decreased by 15% (95% CI, 5% to 24%). While the location-specific differences in case fatality rates remained consistent across both the Finland and New Zealand cohorts, the researchers noted a geographic discrepancy in survival improvements. Specifically, temporal decreases in case fatality were observed only in the Finnish cohort, suggesting that regional differences in healthcare delivery, prehospital management, or secondary prevention may influence long-term outcomes. The study also identified a shifting epidemiological profile regarding the distribution of rupture sites. Between the 2001 to 2003 period and the 2014 to 2017 period, the proportion of vertebrobasilar artery ruptures increased by 35.9%, rising from 10.3% to 14.0% of all cases. This trend is particularly concerning for clinicians given that these posterior circulation events carry the highest risk of sudden death. Because the high mortality associated with vertebrobasilar and middle cerebral artery aneurysms is largely driven by prehospital deaths, it remains unclear whether patients with these high-risk aneurysm locations would benefit more from improved primary prevention or enhanced prehospital emergency response strategies. Crucially, the authors caution that these epidemiological findings cannot be directly applied to guide the preventive treatment of unruptured intracranial aneurysms in daily practice, as the dynamics of rupture risk differ from post-rupture survival.

References

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2. Connolly ES, Rabinstein AA, Carhuapoma JR, et al. Guidelines for the Management of Aneurysmal Subarachnoid Hemorrhage. Stroke. 2012. doi:10.1161/str.0b013e3182587839

3. Dayyani M, Sadeghirad B, Grotta JC, et al. Prophylactic Therapies for Morbidity and Mortality After Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Network Meta-Analysis of Randomized Trials. Stroke. 2022. doi:10.1161/strokeaha.121.035699

4. Mahlamäki K, Rautalin I, Korja M. Case Fatality Rates of Subarachnoid Hemorrhage Are Decreasing with Substantial between-Country Variation: A Systematic Review of Population-Based Studies between 1980 and 2020.. Neuroepidemiology. 2022. doi:10.1159/000526983