Background: It is well-known that arachnoid cysts of the middle fossa are associated often with chronic subdural hematomas and/or intracystic hemorrhages. However, reports of an arachnoid cyst associated with an aneurysm are rare.
Case description: A 45-year-old man was admitted with headache due to intracystic hemorrhage in the sylvian fissure, associated with a saccular aneurysm at the bifurcation of the internal carotid artery. The aneurysm ruptured into the arachnoid cyst, without causing a subarachnoid hemorrhage. Subsequently, the neck of the aneurysm was clipped successfully. The patient's postoperative course was uneventful, and there was no vasospasm.
Conclusions: This is the first reported case of an intracystic hematoma due to a ruptured aneurysm. We discuss the possible etiology of the association between the aneurysm and the arachnoid cyst, and the formation mechanism for the intracystic hematoma that did not result in a subarachnoid hemorrhage.