A case of brief psychosis associated with an arachnoid cyst.

Little is known about the arachnoid cyst and there have been few reports of it accompanying psychiatric disturbances. A 57-year-old patient was admitted due to sudden headache, auditory hallucination, and delusion of persecution. An arachnoid cyst was found in the anteromedial aspect of middle cranial fossa on his magnetic resonance image. This was his first psychotic episode and he was also negative to other clinical evaluations including endocrine system. His psychotic symptoms were suspected to be induced by the arachnoid cyst and it was well controlled by low-dose risperidone administration. He left hospital free from psychotic symptoms on 14th hospital day.