The brain and spinal cord are always surrounded by a clear fluid, known as cerebral spinal fluid, or CSF. CSF plays three important roles. It transports nutrients to cells and carries away wastes, much like blood. It cushions the brain against blows. And it serves as a barrier, preventing certain chemicals in the blood from reaching brain cells. Sometimes, however, a patient can have too much CSF in all or part of the brain---a condition doctors call hydrocephalus. Hydrocephalus is a medical emergency. It puts pressure on brain tissues, which can damage or destroy cells in all parts of the brain, triggering a huge range of neurological symptoms. Left untreated, it can be fatal. But it is a treatable condition. The National Institutes of Health reports that "some people recover almost completely after treatment and have a good quality of life." There are several known causes of hydrocephalus, though in some cases, the cause is still unknown.
Traumatic Brain Injury
CSF circulates through the brain and spinal cord on a regular pathway. It accumulates in four small chambers in the brain, known as ventricles, and circulates through narrow channels between them. These channels, however, can become blocked if brain tissue swells, closing the small pathways. This happens often in the case of a traumatic brain injury, such as a blow to the head, which can cause the brain to swell, squeezing the passageways closed and damming the flow of CSF. Because this is a temporary condition, doctors will often to insert a temporary device that drains fluid outside the patient's body, known as an extra-ventricular device, or EVD.
Brain Hemorrhage
Bleeding in the brain can have a similar effect. When a blood vessel bursts in the brain, as happens in a hemorrhagic stroke, blood begins to pool in part of the brain. Because the brain is encased inside the skull, which can't grow to accommodate the extra fluid, this puts pressure on brain tissue and can squeeze the passageways closed. This blocks the flow of CSF, meaning that too much fluid can accumulate in part of the brain. Doctors will often solve this problem by inserting an EVD into the patient's brain.
Developmental Disorders
Some developmental disorders, present from birth, can cause hydrocephalus. Spina bifida, a condition in which a child is born with the spine not fully closed over the spinal cord, is one of several conditions that cause an overproduction of CSF. The Spina Bifida Association reports that "about 80 percent of people with spina bifida have hydrocephalus that requires treatment." To compensate, surgeons will insert a permanent drainage system, known as a shunt, which drains CSF from the brain into another cavity in the body where it can be harmlessly reabsorbed.
Brain Tumors
Some brain tumors can trigger the body to overproduce CSF. Treatment in these cases can be a judgment call for doctors. If the tumor can be quickly removed, an EVD may be sufficient to protect the brain from the pressure of too much fluid until the tumor is removed. If removal of the tumor is going to be a long process, a shunt may make the most sense.
Unknown Causes
Some cases of hydrocephalus are never fully explained. According to the Mayo Clinic, in many cases the cause is unknown. It may be present from birth in some children, with no obvious cause. Hydrocephalus can also be a complication of premature birth. In such cases, a permanent shunt can help the child to live a nearly normal life. Shunts, however, are prone to infection and require regular medical monitoring. They also don't grow with the child. It is typical for children to have a shunt replaced every few years---a risky neurosurgical operation. According to the Hydrocephalus Association, "Fifty percent of shunted individuals require a revising operation within two years."
