Spontaneous fluctuation in the size of a midline posterior fossa arachnoid cyst.

Arachnoid cysts of the posterior fossa are rather uncommon compared with their supratentorial counterparts. Spontaneous disappearance of middle cranial fossa arachnoid cysts has been reported but there are none in the English literature on the spontaneous fluctuation in size of a posterior fossa arachnoid cyst. We present a 41-year-old male, with a midline posterior fossa arachnoid cyst, which appeared to lessen in size spontaneously with complete disappearance of symptoms and then enlarged with a worsening in neurological status.