Congenital arachnoid cysts.

Three cases of congenital arachnoid cysts are discussed. They all presented with hydrocephalus. The benign nature and the importance of early surgical intervention are emphasized.More severe hydrocephalus results from the posterior fossa and quadrigeminal plate cysts because of pressure on the aqueduct and the fourth ventricle. Therefore, every effort must be made to demonstrate the patency of these structures during air studies. In cases of failure, positive contrast study is indicated. Indentation and anterior displacement of the posterior portion of the third ventricle, and elevation of the suprapineal recess are important clues for the diagnosis of the cysts of the quadrigeminal plate and for infratentorial cysts with supratentorial extension.