Paramesencephalic arachnoid cysts.

The term "paramesencephalic" is proposed to describe the location of a general class of arachnoid cysts observed in four patients. These cysts, which appear to arise within the subarachnoid space, are characterized by a lack of communication with the ventricular system. Usually, the arachnoid tissue is normal, and the cyst fluid resembles cerebrospinal fluid both macroscopically and microscopically. Hydrocephalus is variably accompanied by localizing signs. Problems may be encountered in the diagnosis of suprasellar (case 4), parapineal (case 3), incisural (case 1), and interpeduncular (cases 2 and 4) arachnoid cysts. Early surgical exploration is strongly recommended.