[A spontaneous bilateral subdural effusion in an 11-year-old boy with middle cranial fossa arachnoid cyst].

An 11-year-old boy gradually developed headache, vomiting and diplopia over a period of 1 month. Repeated examinations of head CT scan revealed an arachnoid cyst in the right middle cranial fossa and bilateral subdural effusion of enlarging size. Papilledema was absent on admission, but it became evident after 1 week, and lumbar puncture disclosed very high pressure (800 mmH2O) of the cerebrospinal fluid. Fenestration of the cyst to the basal cisterms quickly alleviated his symptoms of intracranial hypertension as well as the bilateral subdural effusion on CT. Macroscopically, there was a small tear on the wall of the arachnoid cyst, and it probably served as a communication valve with the subdural space. Since he had no history of head trauma in the past few months, the reason of the tear formation was unclear. Intracranial arachnoid cyst is a relatively common congenital malformation of usually benign and non-pathogenic nature. However, it may occasionally cause non-traumatic subdural effusion and intracranial hypertension.