Hydrocephalus is an active distension of the ventricular system of the brain resulting from inadequate passage of CSF from its point of production within the cerebral ventricles to its point of absorption into the systemic circulation. The phenotype is Hydrocephalus (HP_0000238, a Human Phenotype Ontology term). Also known as: Hydrocephaly, Nonsyndromal hydrocephalus, Too much cerebrospinal fluid in the brain.