Subarachnoid hemorrhage due to ruptured cerebral aneurysm at the distal part of anterior inferior cerebellar artery-posterior inferior cerebellar artery variant after γ-knife irradiation

Aneurysms in the distal portion of the anterior inferior cerebellar artery (AICA) are rare, but AICA ANs occurring after gamma-knife surgery (GKS) for vestibular schwannomas are even rarer. Vessels wherein the ipsilateral AICA bleeds into the posterior inferior cerebellar artery (PICA) because of PICA dysplasia on one side are known as AICA-PICA variants. Nevertheless, AICA-PICA variant aneurysms (APANs) are rare and originate from the side wall of the parent artery, whereas none originate from the branching point. In this study, we report a case of subarachnoid hemorrhage of a 76-year-old woman because of the rupture of an AICA-PICA aneurysm 7 years after GKS for right vestibular schwannoma. Head computed tomography (CT) showed a subarachnoid hemorrhage with a cerebral aneurysm at the bifurcation where PICA branched off from AICA, termed AICA-PICA bifurcation. The parent artery was subsequently occluded by endovascular treatment. The patient was transferred to a rehabilitation hospital on day 56 with Glasgow Coma Scale 14 (E: 4, V: 4, M: 6), mRS2, and no obvious cerebral vasospasms. Although radiation-induced AN is an extremely rare complication, long-term follow-up imaging monitoring for its occurrence is recommended.