ObjectPatients with posterior fossa arteriovenous malformations (AVMs) are more likely to present with hemorrhage than those with supratentorial AVMs. Observed patients subject to the AVM natural history should be informed of the individualized effects of AVM characteristics on the clinical course following a new, first-time hemorrhage. The authors hypothesize that the debilitating effects of first-time bleeding from an AVM in a previously intact patient with an unruptured AVM are more pronounced when AVMs are located in the posterior fossa.MethodsThe University of California, San Francisco prospective registry of brain AVMs was searched for patients with a ruptured AVM who had a pre-hemorrhage modified Rankin Scale (mRS) score of 0 and a post-hemorrhage mRS score obtained within 2 days of the hemorrhagic event. A total of 154 patients met the inclusion criteria for this study. Immediate post-hemorrhage presentation mRS scores were dichotomized into nonsevere outcome (mRS ≤ 3) and severe outcome (mRS > 3). There were 77 patients in each group. Univariate and multivariate logistic regression analyses using severe outcome as the binary response were run. The authors also performed a logistic regression analysis to measure the effects of hematoma volume and AVM location on severe outcome.ResultsPosterior fossa location was a significant predictor of severe outcome (OR 2.60, 95% CI 1.20-5.67, p = 0.016) and the results were strengthened in a multivariate model (OR 4.96, 95% CI 1.73-14.17, p = 0.003). Eloquent location (OR 3.47, 95% CI 1.37-8.80, p = 0.009) and associated arterial aneurysms (OR 2.58, 95% CI 1.09, 6.10; p = 0.031) were also significant predictors of poor outcome. Hematoma volume for patients with a posterior fossa AVM was 10.1 ± 10.1 cm(3) compared with 25.6 ±28.0 cm(3) in supratentorial locations (p = 0.003). A logistic analysis (based on imputed hemorrhage volume values) found that posterior fossa location was a significant predictor of severe outcome (OR 8.03, 95% CI 1.20-53.77, p = 0.033) and ...