OBJECTIVE: Extracranial-intracranial (EC-IC) bypass has been well described in chronic vaso-occlusive cerebrovascular diseases, including both moyamoya disease (MMD) and atherosclerotic disease (AD). This study aimed to compare factors associated with bypass occlusion between these two diseases. METHODS: An institutional database of 357 patients with intracranial bypass procedures performed between August 2002 and May 2022 was retrospectively reviewed. Patients with MMD and AD were selected for study. Baseline characteristics, surgical technique, and flow-related measurements were compared in relation to the outcome of bypass occlusion. RESULTS: A total of 232 patients met inclusion criteria (AD, n = 108
MMD, n = 124). The average age and sex differed significantly between groups (AD 57.2 years, 56.5% male
MMD 36.6 years, 31.5% male
p <
0.002). The modified Rankin Scale scores at surgery and at follow-up were higher in the AD group (p = 0.004 and p <
0.002, respectively), showing a slightly worse baseline functional status, and higher rates of stroke were observed in the AD group by last follow-up (p = 0.005). Patients with AD also were more likely to require an interpositional graft (p <
0.002). At last follow-up, rates of occlusion did not differ between AD and MMD groups (25.2% vs 25.4%, respectively). Of occluded bypasses, the AD group had more occlusions within 1 week compared to MMD (51.9% vs 35.5%, p = 0.176), although the difference was not significant. In patients with more than 1 year of follow-up and in those with more than 2 years of follow-up, MMD tended to have higher rates of occlusion (31.2% vs 26.1% [p = 0.558], and 26.4% vs 20.7% [p = 0.564]). Flow measurements did not differ between AD and MMD groups, but in subgroup analyses of patients with AD and those with MMD, both bypass flow and cut flow index predicted occlusion in both groups. CONCLUSIONS: Despite different disease etiologies treated with bypass, rates of occlusion at last follow-up did not vary between groups, although short-term follow-up would suggest earlier bypass failure in AD, and extended follow-up trended toward higher occlusion rates in MMD. Additionally, patients with AD were more likely to have further occurrences of stroke by last follow-up. Importantly, the bypass flow and cut flow index at the time of surgery predicted occlusion in both AD and MMD.