Long-Term Subarachnoid Hemorrhage Risk and Survival after Microsurgical Clipping of Unruptured Intracranial Aneurysms

Background and PurposeProphylactic microsurgical clipping for unruptured intracranial aneurysms (UIAs) effectively prevents rupture, yet its long-term impact on cerebrovascular and systemic health remains uncertain. We assessed subarachnoid hemorrhage (SAH) risk, cerebrovascular outcomes, and systemic disease incidence after clipping under stringent surgical selection criteria. MethodsWe retrospectively analyzed 930 patients (990 procedures) with asymptomatic anterior-circulation UIAs treated between 2003-2025, totaling 7,638 patient-years (median follow-up 8.3 years). The primary endpoint was postoperative SAH; secondary endpoints included all-stroke, malignancy, cardiovascular disease, dementia, and all-cause mortality. Incidence rates (per 1,000 patient-years) with 95% CIs were derived using Byars approximation; rate ratios (RRs) versus natural-history cohorts (UCAS, SUAVe) and standardized incidence ratios (SIRs) versus Japanese population registries were computed using exact Poisson methods. ResultsTen SAH events occurred (1.31/1,000 patient-years; 95% CI, 0.63-2.41). Compared with UCAS and SUAVe, clipping reduced SAH risk by 86-93% (RR 0.14 vs UCAS; 0.24 vs SUAVe; both p<0.001), with the lowest risk in patients without untreated aneurysms (0.64/1,000 patient-years). Nevertheless, age-adjusted analyses revealed persistent cerebrovascular vulnerability: SAH incidence exceeded the general population (SIR 3.22; 95% CI, 0.88-8.25; p=0.075), and all-stroke incidence was doubled (SIR 2.11; 95% CI, 1.50-2.90; p<0.001). Dementia incidence in the [&ge;]70-year subset was significantly lower (SIR 0.34; 95% CI, 0.20-0.53; p<0.001). Cardiovascular disease risk was unchanged (SIR 1.41; p=0.221). A modest malignancy excess (SIR 1.36; p=0.009) likely reflected surveillance bias rather than treatment-related risk. ConclusionsMicrosurgical clipping provides durable long-term protection against SAH, reducing rupture risk by >90% versus natural history. However, residual cerebrovascular risk--driven partly by de novo aneurysm formation--underscores the need for lifelong vascular risk management and imaging surveillance even after successful aneurysm treatment.