De novo aneurysm formation and untreated aneurysm growth after microsurgical clipping: a 20-year prospective cohort study.

OBJECTIVE

This prospective study determined incidence rates and risk factors for de novo aneurysm formation and growth of untreated aneurysms after microsurgical clipping of asymptomatic unruptured intracranial aneurysms (UIAs) during extended follow-up.

METHODS

From 2003 to 2025, 930 patients (mean age 62.8 [SD 10.5] years, 70.4% female) who underwent clipping of asymptomatic anterior circulation UIAs were prospectively followed with serial MR angiography (MRA). Primary endpoints were de novo aneurysm formation and untreated aneurysm growth. Imaging consisted of MRA at 12-month intervals for patients without untreated aneurysms and 6-month intervals for those with untreated aneurysms. De novo aneurysms were defined as newly detected saccular lesions ≥ 2 mm. Aneurysm growth was defined as morphological change or a ≥ 1-mm diameter increase. Statistical analysis included Kaplan-Meier survival analysis and multivariable Cox regression. Risk factors included age, sex, hypertension, smoking history, family history of subarachnoid hemorrhage (SAH), multiplicity, and untreated aneurysm size.

RESULTS

Of 930 patients, 896 (96.3%) underwent at least one MRA follow-up beyond 1 year for a mean of 8.31 (SD 4.46) years, totaling 7117.7 patient-years. De novo aneurysm formation occurred in 37 patients (0.52% per year, 95% CI 0.37%-0.72%), with a cumulative incidence of 1.0% at 5 years, 5.6% at 10 years, and 8.9% at 15 years. Of 160 untreated aneurysms in 148 patients, 30 grew during 1212.1 aneurysm-years (2.48% per year per aneurysm, 95% CI 1.67%-3.53%), with a cumulative incidence of 9.4% at 5 years, 27.4% at 10 years, and 35.1% at 15 years. Neither de novo formation nor aneurysm growth showed significant associations with risk factors. Twenty-seven patients underwent additional intervention (annual rate 0.38%). SAH occurred in 10 patients (1.31 per 1000 patient-years): 1 from local recurrence, 5 from untreated aneurysms, and 4 from de novo aneurysms.

CONCLUSIONS

Both de novo aneurysm formation (0.52% per year) and untreated aneurysm growth (2.48% per year per aneurysm) occurred at meaningful rates after clipping of asymptomatic UIAs. The 15-year cumulative risks of 8.9% for de novo formation and 35.1% for growth underscore the necessity of lifelong surveillance. Annual MRA surveillance appears reasonable for all postclipping patients, including those with untreated aneurysms. Given the relatively low absolute annual growth rate and the lack of evidence that more frequent imaging prevents rupture, the decision to intensify surveillance should be individualized rather than applied uniformly.