The study was approved by the Swedish Ethical Review Authority (Dnr 2012÷967−31÷3, with subsequent amendments including Dnr 2023−07233−02 and Dnr 2025−01810−02) with a waiver of informed consent. Reporting follows Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guidelines.⁷

This population-based cohort study included all residents of the Stockholm region younger than 40 years with saccular aSAH between January 1, 1967, and December 31, 2014. Cases were identified through the prospectively maintained Stockholm Intracranial Aneurysm Register, cross-linked with the Swedish Cause of Death Register⁸ and National Patient Register⁹ using codes from the relevant International Classification of Diseases. Sweden organizes neurosurgical care through 6 regional centers with defined catchment areas precluding referral bias, ensuring all regional residents were captured regardless of treating hospital or place of death, including prehospital deaths ascertained through mandatory autopsy. Capture-recapture analysis estimated case ascertainment completeness at 97.7%. All cases were manually verified against original medical records and national register data (eMethods 1 in Supplement 1).

Eligibility required ruptured saccular aneurysm confirmed by concurrent 4-vessel cerebral angiography and availability of original index imaging for de novo validation. Nonsaccular etiologies (arteriovenous malformation–related, dissecting, mycotic, or Moyamoya-associated aneurysms) were excluded. During 1967 to 1986, regional neurosurgical care was provided at Karolinska University Hospital (Karolinska) and Stockholm South General Hospital (SGH).¹⁰ The original-imaging requirement for de novo validation excluded a substantial proportion of SGH patients from this period, as films were generally not retained after the department’s closure in 1987; SGH patients with preserved imaging were eligible under identical criteria (eMethods 1 and eTable 6 in Supplement 1). Patients treated at other neurosurgical centers but followed up in the Stockholm region with original imaging available were also eligible.