An aneurysm rupturing into a middle cranial fossa arachnoid cyst presenting as an intracystic hemorrhage.

We report a case of an aneurysm rupturing into an arachnoid cyst presenting as a confined intracystic hemorrhage. A 37-year-old man with a history of sporadic headaches for several years complained of a headache of several days' duration. Brain computed tomography and magnetic resonance imaging revealed a cystic lesion in the left side of the middle cranial fossa that included a 3-cm-diameter hematoma in the Sylvian fissure without extracystic hemorrhage. Left carotid artery angiography showed an internal carotid artery-posterior communicating artery (IC-PC) aneurysm and an anterior choroidal artery aneurysm. The cyst wall was excised, and the aneurysms were clipped. The bleeding point was the spot at which the IC-PC aneurysm entered the arachnoid cyst. Right oculomotor nerve palsy developed after surgery, but resolved after 3 months. To the best of our knowledge, this is the only fourth report to date in the literature of a confined aneurysm rupturing into an arachnoid cyst.