Background: The decision of whether to treat incidental intracranial saccular aneurysms is complicated by limitations in current knowledge of their natural history. We combined individual patient data from prospective cohort studies to determine predictors of aneurysm rupture and to construct a risk prediction chart to estimate 5-year aneurysm rupture risk by risk factor status.
Methods: We did a systematic review and pooled analysis of individual patient data from 8382 participants in six prospective cohort studies with subarachnoid haemorrhage as outcome. We analysed cumulative rupture rates with Kaplan-Meier curves and assessed predictors with Cox proportional-hazard regression analysis.
Findings: Rupture occurred in 230 patients during 29,166 person-years of follow-up. The mean observed 1-year risk of aneurysm rupture was 1·4% (95% CI 1·1-1·6) and the 5-year risk was 3·4% (2·9-4·0). Predictors were age, hypertension, history of subarachnoid haemorrhage, aneurysm size, aneurysm location, and geographical region. In study populations from North America and European countries other than Finland, the estimated 5-year absolute risk of aneurysm rupture ranged from 0·25% in individuals younger than 70 years without vascular risk factors with a small-sized (<7 mm) internal carotid artery aneurysm, to more than 15% in patients aged 70 years or older with hypertension, a history of subarachnoid haemorrhage, and a giant-sized (>20 mm) posterior circulation aneurysm. By comparison with populations from North America and European countries other than Finland, Finnish people had a 3·6-times increased risk of aneurysm rupture and Japanese people a 2·8-times increased risk.
Interpretation: The PHASES score is an easily applicable aid for prediction of the risk of rupture of incidental intracranial aneurysms.
Funding: Netherlands Organisation for Health Research and Development.
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