Arachnoid cyst of the posterior fossa.

Arachnoid cysts of the posterior fossa are uncommon. Our case of a 49-year-old man presented with a 2 month history of headaches, nausea, and vertigo associated with walking instability. An MRI revealed a median well-circumscribed cystic lesion of the posterior fossa, with similar signal characteristics to CSF, and without connection to the fourth ventricle. This aspect suggested either arachnoid or hydatid cysts. Direct open surgery was performed allowing complete removal of the cyst wall, with a good outcome.