Cyst of the third ventricle as an unusual cause of acquired hydrocephalus.

Cysts of the third ventricle are rare congenital suprasellar malformations of arachnoidal, endodermal, or neuroepithelial origin. Depending on their size and location, they can cause space-occupying intracranial lesions and hydrocephalus occlusus by obstruction of the aqueduct or foramen of Monro. They can be missed on routine computer tomography and magnetic resonance imaging. A 2-year-old boy presented signs of intracranial pressure. Initial magnetic resonance imaging revealed a triventricular internal hydrocephalus with no visible etiologic lesion. An extended investigation with the constructive interference in steady state-technique showed a mobile cystic cerebrospinal fluid-intense lesion within the third ventricle, causing transient occlusion of the foramen of Monro. A suprasellar cyst of the third ventricle is an important differential diagnosis in apparently "idiopathic" internal hydrocephalus. In such cases, magnetic resonance imaging using the constructive interference in steady state-technique with a slice thickness of 1 mm is the method of choice for detecting intraventricular cysts. Neurosurgical fenestration and/or resection of the cyst by neuroendoscopy can resolve cerebrospinal fluid circulation disturbances and seems to be superior to a shunt.