Background: Previous studies suggest that case mortality of aneurysmal subarachnoid hemorrhage (aSAH) has decreased during the last decades, but most studies have been unable to assess case severities among individual patients. We aimed to assess changes in severity-adjusted aSAH mortality in patients admitted to intensive care units (ICUs).

Methods: We conducted a retrospective, register-based study by using the prospectively collected Finnish Intensive Care Consortium database. Four out of five ICUs providing neurosurgical and neurointensive care in Finland participated in the Finnish Intensive Care Consortium. We extracted data on adult patients admitted to Finnish ICUs with aSAH between 2003 and 2019. The primary outcome was 12-month mortality during three periods: 2003-2008, 2009-2014, and 2015-2019. Using a multivariable logistic regression model-with variables including age, sex, World Federation of Neurological Surgeons grade, preadmission dependency, significant comorbidities, and modified Simplified Acute Physiology Score II-we analyzed whether admission period was independently associated with mortality.

Results: A total of 1,847 patients were included in the study. For the periods 2003-2008 and 2015-2019, the mean number of patients with aSAH admitted per year increased from 81 to 123. At the same time, the patients' median age increased from 55 to 58 years (p = 0.001), and the proportion of patients with World Federation of Neurological Surgeons grades I-III increased from 42 to 58% (p < 0.001). The unadjusted 12-month mortality declined from 30% in 2003-2008 to 23% in 2015-2019 (p = 0.001), but there was no statistically significant change in severity-adjusted mortality.

Conclusions: Between 2003 and 2019, patients with aSAH admitted to ICUs became older and the proportion of less severe cases increased. Unadjusted mortality decreased but age and case severity adjusted-mortality remained unchanged.