When small aneurysms bleed: Functional outcomes and the limitations of size-based risk stratification

Small intracranial aneurysms (≤5 mm) are often considered low risk and less clinically significant. While rupture risk has been extensively studied, the severity of clinical outcomes following rupture remains uncertain. This study assessed whether aneurysm size independently predicts functional outcome after aneurysmal subarachnoid haemorrhage (aSAH), with a focus on post-rupture morbidity.

This retrospective cohort study included 421 patients with ruptured saccular intracranial aneurysms treated at a tertiary neurovascular centre between July 2015 and June 2024. Aneurysms were stratified by maximum diameter (≤5 mm vs >5 mm) based on pre-treatment imaging. The primary outcome was functional dependence at 90 days, defined as a modified Rankin Scale (mRS) score greater than 2. Multivariable logistic regression was used to identify independent predictors of poor outcome.

Of 421 patients, 178 (42.3 %) had aneurysms measuring ≤5 mm. Baseline clinical grade, radiographic severity, rates of hydrocephalus, vasospasm, rebleeding, and in-hospital mortality were similar across size groups. Larger aneurysms (>5 mm) were associated with worse functional status at discharge (59.3 % vs 46.6 %; P = 0.010), but 90-day outcomes were comparable (40.3 % vs 35.4 %; P = 0.303). Aneurysm size was not independently associated with poor functional outcome (P = 0.304). Independent predictors included age, history of prior SAH, WFNS grade IV-V, intracerebral haemorrhage, rebleeding, vasospasm, modified Fisher grade 3-4, and the need for external ventricular drainage. Rupture size varied by anatomical location (P = 0.010), with smaller aneurysms more frequently observed in the anterior choroidal and pericallosal arteries.

Small aneurysms accounted for over 40 % of ruptured cases and, once ruptured, resulted in clinical outcomes similar to larger aneurysms. These findings challenge the assumption that rupture from small aneurysms is less clinically significant and underscore the limitations of using aneurysm size alone for risk stratification and outcome prediction.