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Health A-Z

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Harvard Medical School

What Is It?

Hydrocephalus, also known as "water on the brain," is a condition in which there is extra cerebrospinal fluid around the brain and spinal cord. Cerebrospinal fluid acts as a cushion for the brain and spinal cord, supplies nutrients, and takes away waste products.

Hydrocephalus can be present at birth (congenital) or can develop later (acquired).

  • Congenital hydrocephalus is present at birth, although it may not be detected until later in life. It forms when the brain and surrounding structures develop abnormally. The exact cause is usually unknown, but contributing factors may include genetics and certain infections during pregnancy.

  • Acquired hydrocephalus results from injuries or illnesses that occur at birth or later, including infections in the brain and spinal column (meningitis), bleeding (hemorrhage) of blood vessels in the brain, severe head injury, brain tumors or cysts. Hydrocephalus also can occur when there is no known injury or illness to cause it.

Hydrocephalus can be classified according to its cause:

  • Obstructive (non-communicating) hydrocephalus is caused by a blockage in the system of cavities (ventricles) in the brain. The blockage prevents the cerebrospinal fluid from flowing (or "communicating") with the area that surrounds the brain and spinal cord (subarachnoid space), as it normally should. Blockages can be present at birth or can occur later. One of the most common types is aqueductal stenosis, which occurs because of narrowing of the aqueduct of Sylvius, a channel that connects two of the ventricles together.

  • Non-obstructive (communicating) hydrocephalus results from problems with cerebrospinal fluid being produced or absorbed. One of the most common causes is bleeding into the subarachnoid space. Communicating hydrocephalus can be present at birth or can occur later.

Another type of hydrocephalus, called normal pressure hydrocephalus is an acquired type of communicating hydrocephalus in which the ventricles are enlarged but not under high pressure. Normal pressure hydrocephalus is seen in older adults. It may be the result of injury or illness, but in the majority of cases the cause is unknown.

Premature infants born before 34 weeks or weighing less than 4 pounds have a higher risk of blood vessels bleeding in the brain. Severe bleeding can lead to acquired hydrocephalus, communicating or non-communicating, depending on the site and extent of the bleeding.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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