Definition of Normal pressure hydrocephalus (NPH):

Normal pressure hydrocephalus (NPH) is a rise in cerebrospinal fluid (CSF) in the brain that affects brain function.

Causes, incidence, and risk factors:

Normal pressure hydrocephalus (NPH) is a form of hydrocephalus, also known as "water on the brain," which means there is too much fluid pressing on the brain.

NPH can occur without a known cause, or it may be caused by any condition that blocks the flow of cerebrospinal fluid (CSF). The fluid-filled chambers (ventricles) of the brain enlarge to fit the increased volume of CSF. They press down on and damage or destroy brain tissue.

Risk factors include:

• Bleeding from a blood vessel or aneurysm in the brain (subarachnoid hemorrhage)
• Closed head injury
• Meningitis or similar infections
• Surgery on the brain (craniotomy)

CSF is produced in normal amounts in these conditions, but it is prevented from being reabsorbed normally.

NPH is thought to account for about 5% of all dementias.