Intracystic hematoma of middle fossa arachnoid cyst caused by rupture of internal carotid-posterior communicating artery aneurysm.

A 46-year-old woman was admitted with generalized convulsion and deep coma which occurred 3 weeks after sudden onset of severe headache and pyrexia. Initial computed tomography did not reveal any abnormal findings except for an arachnoid cyst in the right middle fossa. Three weeks later repeat computed tomography showed intracystic hematoma in the arachnoid cyst with uncal herniation. Angiography revealed a right internal carotid-posterior communicating artery aneurysm. The neck of the aneurysm was clipped successfully, but hemiparesis was persistent postoperatively. Angiography is required for investigation of intracystic hematoma of an arachnoid cyst, especially in the absence of head injury, to avoid delayed diagnosis of any ruptured aneurysm.