Risk of growth of tiny (≤3 mm) unruptured intracranial aneurysms: a retrospective cohort study of 228 tiny aneurysms

BACKGROUND: Tiny unruptured intracranial aneurysms (UIAs), defined as ≤3 mm, are increasingly detected due to advancements in imaging. However, the optimal management approach remains uncertain. Studies have questioned the necessity of long-term radiological monitoring, because of presumed low risks of growth, yet robust data are limited. AIMS: This retrospective cohort study aimed to estimate the 5-year growth risk of conservatively managed tiny UIAs and assess potential differences in growth between men and women. METHODS: We conducted a single-centre retrospective cohort study including adult patients diagnosed with one or more tiny saccular UIAs at a tertiary academic hospital between 2008 and 2020. Patients were included if at least two imaging studies (baseline and ≥1 follow-up) were available. Growth was defined as a ≥1 mm increase in size or a morphological change. Growth risk was calculated, and cumulative incidence was estimated using Kaplan-Meier analysis. Sex differences were analysed using log-rank tests. RESULTS: A total of 182 patients (142 women; median age, 54 years) with 228 tiny UIAs were followed for a median of 5.5 years. Growth was observed in 29 UIAs (12.7%). Kaplan-Meier analysis showed cumulative growth of 9.7% (95% CI: 5.1%-14.2%) at 5 years. Growth rates were comparable between men and women (p = 0.63). CONCLUSIONS: Approximately one in eight tiny UIAs demonstrated growth over long-term follow-up, indicating a clinically relevant growth risk despite small initial size. No sex-related differences in growth were observed. These findings support the importance of long-term radiological monitoring. Future studies should investigate whether tiny UIAs may be safely followed with longer surveillance intervals than larger UIAs.Data access statement:The data underlying this study are not publicly available due to institutional and ethical restrictions but may be accessed upon reasonable request and subject to approval by the appropriate review committees.