Hemorrhage into an intradiploic arachnoid cyst. Case report

Intracranial arachnoid cysts are relatively rare; it is believed that they account for only 1% of all intracranial space-occupying lesions. When they occur in the intracranial cavity, they usually develop in relation to an arachnoid cistern as a pocket of cerebrospinal fluid within two layers of arachnoid membrane. Five cases of intradiploic arachnoid cysts have been reported, but an arachnoid cyst presenting as an extradural mass has not been described before. The authors present an unusual case of hemorrhage into a massive intracranial extradural arachnoid cyst with no intradural communication.