The natural growth and rupture risk of intracranial aneurysms located in different segments of internal carotid artery: a multicenter cohort study.

Aneurysm location is strongly related to the risk of aneurysm growth and rupture. Due to the unique anatomic features of internal carotid artery (ICA), unruptured intracranial aneurysms (UIAs) located in different segments of ICA may exhibit distinct instability risks. However, the natural instability risk of UIAs across ICA segments has not been extensively studied. This study included patients with ICA UIAs from two prospective, multicenter Chinese cohorts. All patients underwent regular follow-up for at least four years. The primary outcome was aneurysm instability (rupture, growth, and morphological change). ICA segments were classified per Bouthillier's. C7 bifurcation (Bifur), posterior communicating artery (Pcom), and others. The 4-year instability risk was analyzed by segment. Cox regression identified factors associated with instability. This study included 779 patients. During 3153 aneurysm-years of follow-up, aneurysm instability occurred in 21/29 (incidence rate [IR]: 25.94%; 95%CI, 15.63-36.26%) UIAs in the C7-Pcom, 18/62 (IR: 10.64%; 95%CI, 6.34-14.94%) UIAs in the C7-Bifur, 28/688 (IR: 0.97%; 95%CI, 0.61-1.32%) UIAs in other segments of ICAs. Multivariable analysis showed that UIAs located in the C7-Pcom(hazard ratio, 9.76; 95%CI, 5.39-17.67) and in the C7-Bifur (hazard ratio, 15.79; 95%CI, 8.54-29.19) were associated with instability of ICA UIAs. UIAs in the C7-Bifur/Pcom have substantially higher instability risks than those in other ICA segments. The precise anatomic location should be prioritized in risk evaluation.