Long-term Cerebrovascular Risk after Microsurgical Clipping of Unruptured Anterior Circulation Aneurysms: Low Subarachnoid Hemorrhage Incidence and Persistent Stroke Susceptibility over 7,600 Patient-years

Microsurgical clipping is an established treatment for unruptured intracranial aneurysms; however, long-term cerebrovascular outcomes beyond aneurysm obliteration remain incompletely characterized in surgically treated patients, particularly regarding non-hemorrhagic cerebrovascular events. We retrospectively analyzed 930 patients (990 procedures; 655 females, 275 males; mean age, 62.8 years) with asymptomatic anterior circulation unruptured intracranial aneurysms treated between 2003 and 2025, with a total follow-up of 7,638 patient-years (median, 8.3 years). The primary endpoint was postoperative subarachnoid hemorrhage, and secondary endpoints included all-stroke events and all-cause mortality. Incidence rates with 95% confidence intervals were calculated, and rate ratios versus natural-history cohorts (Unruptured Cerebral Aneurysm Study and Small Unruptured Intracranial Aneurysm Verification Study) and standardized incidence ratios versus the general population were computed using Poisson methods. Ten subarachnoid hemorrhage events occurred (1.31 per 1,000 patient-years; 95% confidence interval, 0.63-2.41), with most arising from untreated or de novo aneurysms; only 1 case originated from a previously clipped site, demonstrating durable protection at treated sites. Subarachnoid hemorrhage incidence was substantially lower than that reported in natural-history cohorts of conservatively managed unruptured intracranial aneurysms. In contrast, age-adjusted all-stroke incidence remained approximately 2-fold higher than in the general population (standardized incidence ratio 2.11; 95% confidence interval, 1.50-2.90; p < 0.001), driven predominantly by ischemic events. These findings indicate that microsurgical clipping effectively reduces the risk of subarachnoid hemorrhage from treated aneurysms, whereas overall cerebrovascular risk remains elevated, reflecting persistent systemic vascular vulnerability rather than failure of local treatment. Long-term follow-up incorporating appropriate imaging surveillance and intensive management of modifiable vascular risk factors is therefore essential to optimize outcomes in this patient population.