Hydrocephalus

Hydrocephalus, which means literally, "water on the brain," is a condition in which excessive fluid collects inside the skull. The fluid is a naturally produced liquid that normally is found in the brain. Accumulation of excessive amounts of the fluid may build pressure to levels that cause brain damage and subsequent disability.

The brain rests within the natural bony vault of the cranium. There it is protected by the skull and by layers of fibrous material that help to stabilize it and to contain the fluid that surrounds it. The brain itself is a very soft, gelatinous material that requires substantial protection. Three layers of connective tissue line the skull and surround the brain. The pia mater (which means literally "tender mother") lies directly on the brain, following its contours and continuing along the spinal cord as it descends through the spine. The second layer is the arachnoid (like a spider's web), a very thin, fibrous membrane without blood vessels. It, too, lies close to the brain but does not follow its every bump and wrinkle. The space between the pia mater and the arachnoid, called the subarachnoid space, contains the arteries and veins that circulate blood to the brain and the cerebrospinal fluid that bathes the nervous tissues. The outermost layer, the dura mater ("hard mother") is a twolayered, leathery, tough membrane that adheres closely to the inside of the skull. The inner layer is contoured to the brain to support it. The outer layer lies against the cranium and continues into the spinal canal.

The fluid that bathes the brain and spinal cord—cerebrospinal fluid or CSF—is manufactured and secreted in the brain by a structure called the choroid plexus. Cerebrospinal fluid is a colorless, clear fluid that contains oxygen, some proteins, and glucose (a form of sugar). Normally the fluid will circulate through the cranium and down the spinal column. It will be absorbed by special structures called villi in the arachnoid tissue or it will drain from one of several outlets. Excessive fluid accumulates because the brain is manufacturing too much CSF or the drainage routes are blocked and the fluid cannot drain properly.

The capacity of the ventricles in the brain and the space around the spinal cord is approximately 0.5 c (125 ml). The choroid plexus manufactures from 2-3 c (500-750 ml) of fluid each day. The pressure of the CSF within the nervous system, therefore, is related to the rate of manufacture versus the rate of drainage of the fluid. Fluid pressure can be measured by inserting a needle between two of the lumbar vertebrae into the spinal canal. The needle is connected to a meter that indicates the fluid pressure.

The choroid plexus is composed of specialized cells that line the ventricles of the brain. The ventricles are four small, naturally formed cavities in the brain that act as reservoirs for CSF. Overproduction of fluid or its failure to drain can enlarge the ventricles and press the brain against the bony vault of the skull.

Newborn babies who have hydrocephalus often will develop grossly swollen heads. The bones of the skull have not fused and the pressure of the fluid inside the skull can expand the disconnected bony plates.

The two types of hydrocephalus are called communicating and noncommunicating. Communicating hydro-cephalus is caused by overproduction of fluid by the choroid plexus. The fluid, which overwhelms the absorption capacity of the arachnoid, collects inside the ventricles as well as outside the brain. This is the most common form of hydrocephalus occurring in adults and is the result of injury or infection such as encephalitis. At the onset of the condition the patient will become clumsy in walking and appear tired. Other signs will develop indicating a brain injury. To diagnose communicating hydrocephalus the physician will review the patient's recent history to determine whether an infection or head injury has occurred. In addition, such diagnostic measures as a magnetic resonance image (MRI) of the skull can reveal the presence of excess fluid. This condition is readily treatable.

Noncommunicating hydrocephalus is the most common form of the condition in childhood. Usually it will be diagnosed immediately after birth, when signs such as a swollen cranium are seen. Here the problem lies in a narrowing of a drainage aqueduct which inhibits passage of the CSF out of the cranium. The ventricles enlarge greatly and the fluid pressure begins to push the brain against the skull. In this case a drain can be implanted in the skull to drain the fluid into a vein to relieve the pressure.

This form of hydrocephalus also is associated with a congenital condition called meningomyelocele. A newborn with this condition is born with the spinal cord and its superficial coverings exposed. The spinal canal, the opening through which the spinal cord passes, has not fused, so the cord can protrude through the open side. Almost always, the surgical repair of the meningomyelocele will result in hydrocephalus, which will in turn require surgical correction.

This form of hydrocephalus also can occur in an adult and generally is the result of the formation of a tumor that blocks the drainage area.

All forms of hydrocephalus can be treated surgically, so it is important that diagnosis be made as soon as possible after the condition develops. With excessive fluid pressure inside the skull brain damage can occur, leading to various forms of disability. That can be avoided if treatment is timely.