Hydrocephalus: a condition of excessive cerebrospinal fluid (CSF) in the ventricles, or cavities, of the brain. Hydrocephalus may cause increased intracranial pressure inside the skull and progressive enlargement of the head (in children), cognitive impairment and eventually if progressive, stupor and coma. Hydrocephalus is usually due to blockage of CSF outflow in the ventricles or in the subarachnoid space over the brain. It may occur after a brain infection (meningitis), brain hemorrhage or from a tumor compressing the ventricular system. Treatment is often by placing of a ventriculo-peritoneal (VP) shunt or in some case by a 3^rd ventriculostomy.