Case Discussion
Arachnoid cysts are non-neoplastic, extra-axial cysts arising from the arachnoid and containing cerebrospinal fluid. Most are congenital and typically occur in the middle cranial fossa. Usually they are asymptomatic but sometimes may cause mass effect. In these cases, surgery is curative. Microscopically, they are lined by a layer of mature arachnoid cells, but the lining is delicate and may be lost or not apparent. The lining cells stain for EMA.
