Extra fluid on the brain is a condition known as hydrocephalus. The fluid is actually cerebral spinal fluid, or CSF, that surrounds the brain and spinal cord. When there is an excess of CSF in the brain spaces, or ventricles, this causes the ventricles to widen and can lead to excessive pressure on brain tissue. Hydrocephalus has several different causes, including acquired or congenital causes that arise from issues with obstruction, absorption or overproduction of CSF.
Congential Versus Acquired Hydrocephalus
The four ventricles in the brain are connected by narrow passages. CSF, which is produced in the brain, normally flows through the ventricles and exits at the base of the brain, then covers the surface of the brain and spinal cord, where it is reabsorbed in the bloodstream. Hydrocephalus is caused by a disruption of this process and can be caused by congenital factors -- those present at birth -- that can include genetic influences or events that occurred during the development of the fetus. Acquired hydrocephalus occurs during birth itself or after birth. Acquired hydrocephalus can be caused by injury or disease. Both types of hydrocephalus result from either an obstruction, poor absorption or overproduction of CSF.
The most common cause of hydrocephalus is some type of obstruction in the normal flow of the CSF. The obstruction can occur between the ventricles or in the spaces around the brain. A common cause is a condition called “aqueductal stenosis,” the result of a narrowing in a brain structure known as the aqueduct of Sylvius, which is a very small passage between the third and fourth ventricles in the middle of the brain. This narrowing causes an obstruction of CSF flow and can lead to hydrocephalus. This type of hydrocephalus is often called "noncommunicating hydrocephalus," because the flow of CSF is obstructed between the ventricles. Additional causes of noncommunicating hydrocephalus are tumors, brain hemorrhage or other congenital malformations in the brain.
Hydrocephalus is sometimes caused by a problem with the ability of the blood vessels to absorb the CSF. For example, communicating hydrocephalus is a type of hydrocephalus that occurs when there is no obstruction in the flow of CSF between the brain ventricles, but the brain has a problem absorbing the fluid. Absorption problems most often result from inflammation of the brain tissues that can be caused by disease or brain injury. Normal pressure hydrocephalus can occur after a brain injury or may be the result of congenital malformations and result in enlarged ventricles and poor absorption of CSF. A condition called "hydrocephalus ex vacuo" can occur with the brain atrophy that results from conditions like Alzheimer disease.
Overproduction of CSF
A rare cause of hydrocephalus occurs when the brain mechanisms that are responsible for producing CSF produce it at a rate faster than the CSF can be absorbed. The most common condition responsible for this phenomenon is called choroid plexus papilloma. The choroid plexus, located within the brain ventricles, are tissues of the brain that produce CSF; in this condition, they are affected by a slow-growing tumor. This leads to overproduction of CSF and can cause hydrocephalus.