[A case of a ruptured pseudoaneurysm arising from the distal part of the posterior inferior cerebellar artery].

We report a rare case of a pseudoaneurysm arising from the distal part of the posterior inferior cerebellar artery (PICA) and which was associated with intraventricular hemorrhages. A 48-year-old female had sudden onset of severe headache and vomiting, and was admitted in a semicomatous state to our hospital. Initial CT scan showed ventricular hematomas resulting in acute hydrocephalus, but no obvious parenchymal or subarachnoid hemorrhage. Ventricular drainage was carried out. Subsequently, cerebral angiography was performed, but neither, an aneurysm nor other vascular abnormality was detected. 50 days after admission, MRI revealed a homogenous and well-circumscribed hypointense mass (12 mm in diameter) in the right cerebellar tonsil, suggesting the presence of hemosiderin. 60 days after the onset, the second angiography showed an aneurysmal shadow arising from the distal part of the PICA. An aneurysmal clipping was performed via the suboccipital approach. A thrombosed aneurysm was identified in the right tonsil. At the time of the operation, we confirmed that the part of the PICA around the aneurysm was intact with neither branching nor discoloration. Histological examination revealed that elastic fiber was not identified in the aneurysmal wall, and the aneurysm was diagnosed as a pseudoaneurysm. Histories of trauma or infection, which may have induced the cerebral pseudoaneurysm, were not obtained by interview.