This prospective cohort study examined the lifetime cumulative incidence, risk factors, and rupture rate of de novo intracranial aneurysm formation in 544 survivors of saccular aSAH who were younger than 40 years at the index event (1967–2014, Stockholm, Sweden), with radiological follow-up extending up to 57 years.
Over 13,738 person-years, 19.5% (106/544) developed 158 de novo aneurysms; the 40-year Fine-Gray cumulative incidence was 30.2% (95% CI, 24.7%–35.7%) with no plateau. Independent risk factors were female sex (HR 1.76), smoking (HR 1.75), and family history (HR 2.17). The annual rupture rate was 153 per 100,000 person-years, but **no ruptures occurred under systematic surveillance**; one-third of ruptures happened >20 years post-index.
- Single-region (Stockholm) registry may limit generalizability to other populations. - Eligibility required original imaging/records, potentially introducing selection bias. - Smoking cessation data were observational; confounding cannot be excluded.
Young aSAH survivors face a lifelong, non-plateauing risk of new aneurysm formation — counsel patients that risk extends well beyond 20 years. Prioritize smoking cessation as a modifiable target, and support lifelong radiological surveillance, especially in women and those with a family history.
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