[An arachnoid cyst in the left lateral ventricle].

Generally arachnoid cysts are congenital in origin and found over the cerebral convexity, in the major fissures, and in the folded portions of the brain wherever the arachnoid membrane extends. Including other central nervous system (CNS) cysts, the location of these cysts has implications as to their origin, for examples, intracerebral cysts generally are ependymal cysts, while extracerebral cysts are mostly arachnoid cysts. An 8-month-old boy was admitted to our department because of enlarged head and developmental retardation. Computerized tomographic scan (CT) revealed remarkable hydrocephalus and a relatively high-dense, round lesion between the lateral ventricles. During surgery the cyst was found to arise from the left ventricular floor. Histologically the cyst wall was arachnoid membrane.