Introduction Posterior fossa arteriovenous malformations (AVMs) are considered higher risk, reported by some to be due to the enclosed space and highly eloquent functional areas contained therein, despite often smaller hematoma volumes after rupture. Associated aneurysms engender a higher risk of poor outcome as well. We suggest that posterior fossa malformations are also more prone to associated aneurysms, specifically feeder vessel aneurysms, leading to a higher risk of hemorrhage and poor outcome.

Methods A retrospective review ofAVMs evaluated at our institution was performed with attention paid to locationand presence of aneurysms, designated as a feeder vessel aneurysm, intranidalaneurysm, or both. The hemorrhage status and origin of the hemorrhage was alsoreviewed.

Results A total of 570 AVMs wereanalyzed, with a total of 162 associated aneurysms identified (284 ruptured).90 AVMs were located in the posterior fossa, including cerebellar and brainstemlocations. 34 of those malformations (37.8%) had aneurysms: 29 with at leastone feeder vessel aneurysm, 3 with intranidal aneurysms, and 2 malformationswith both. Of the 480 supratentorial AVMs, 126 (26.2%) harbored aneurysms: 82with feeder aneurysms, 37 with intranidal and 7 with both. The overallincidence of aneurysms was appreciably higher in posterior fossa AVMs. Thisassociation was most pronounced for feeder aneurysms, which were evident in34.4% of infratentorial AVMs compared to 18.5% of supratentorial malformations(P<0.01). Intranidal aneurysms were slightly more common supratentorially(9.2% versus 5.5%).

When evaluating for rupturesite, 18 hemorrhages in the posterior fossa could be attributed to a feederaneurysm, yielding at least a 58% rupture rate for those aneurysms, andaccountable for 30% of ruptures amongst infratentorial AVMs. In comparison, 17supratentorial hemorrhages could be attributed to a feeder aneurysm, yielding a19% aneurysm rupture rate, and accounting for 7.6% of total supratentorial AVMhemorrhages (p<0.01). This amounts to an odds ratio (OR) of 5.86 for infraversus supratentorial hemorrhages from feeder aneurysms.

Conclusion Posterior fossa AVMs aremore prone to developing associated aneurysms, specifically feeder vesselaneurysms. Feeder vessel aneurysms are also more likely to be the source ofhemorrhage in the posterior fossa, contributing to the increased morbidity andmortality of these malformations. These high risk features may be appropriatetargets for initial and prompt control by embolization or surgery due to theirelevated threat.

Disclosures J. Orning:None. S. Amin-Hanjani: None. Y. Hamade: None. X. Du: None. V. Aletich: None. F.Charbel: None. A. Alaraj: None.