Contributor: Thomas J. Cummings
Hydrocephalus refers to excessive accumulation of CSF within the intracranial space resulting from an imbalance of CSF formation and absorption.  There are three basic types of hydrocephalus. Communicating hydrocephalus occurs when the site of obstruction is within the subarachnoid space; thus there is circulation of CSF out of the ventricles but blockage of the arachnoid villi prevents absorption. Non-communicating or obstructive hydrocephalus [hydrocephalus - obstructive] occurs when the obstruction is within the ventricle and prevents the circulation of fluid out of the ventricles. Hydrocephalus ex-vacuo is compensatory ventricular dilatation and accumulation of CSF resulting from cerebral atrophy as occurs in neurodegenerative diseases such as Alzheimer disease.  Normal-pressure hydrocephalus is a chronic condition in which CSF pressure is within the physiological range, yet a pressure gradient between the ventricles and the brain results in ventricular enlargement and white matter wasting. Normal pressure hydrocephalus is characterized by a classic clinical triad in adults of dementia, incontinence, and gait disturbance. There are many causes of hydrocephalus including congenital lesions such as Arnold-Chiari malformation, Dandy-Walker syndrome (atresia of the foramina of Luschka and foramen of Magendie), spina bifida and myelomeningocele, encephalocele, and aqueductal obstruction, inflammatory lesions such as leptomeningitis, neoplasms including posterior fossa tumors and choroid plexus tumors, head trauma, intracranial hemorrhage, and obstruction of a functioning ventricular shunt.  X-linked hydrocephalus is a rare inheritable form of aqueductal stenosis that occurs only in males. Hydrocephalus can result in ophthalmic, neurological, and endocrine manifeatations if not corrected. Surgical intervention with placement of a ventricular shunt is usually required  to drain CSF and normalize the CSF pressure.