People tend to think that hydrocephalus means water on the brain, which the name implies, but it is actually cerebral spinal fluid in the brain. Cerebral spinal fluid (CSF) is a clear fluid that needs to be present in the brain, but when there is too much, pressure can be put on the brain. There is CSF surrounding the brain and spinal cord, along with in the four ventricles. Once it flows through the ventricles, it is supposed to be absorbed into the bloodstream. Often times there is a block in one of the ventricles, and it is not able to be absorbed into the bloodstream, it builds up in the brain. This puts pressure on the brain and can cause permanent brain damage (http://www.ninds.nih.gov).
There are two different kinds of hydrocephalus, congenital or acquired. Congenital hydrocephalus occurs before the baby is born. It can be due to genetic abnormalities or abnormalities in development. Acquired hydrocephalus occurs at birth or something after. It can occur because of injury or illness. In addition, there is communicating hydrocephalus and non-communicating hydrocephalus. Communicating hydrocephalus is present if there is a block somewhere after the CSF leaves the ventricles. It is communicating because the CSF is able to flow between the ventricles. Non-communicating hydrocephalus, which is also called obstructive hydrocephalus, occurs when there is a block in the ventricles and the CSF is unable to exit the ventricles properly. One of the most common causes of non-communicating hydrocephalus is when there is a block between the third and fourth ventricle. This is called aqueductal stenosis (http://nhfonline.org).
It is difficult to determine how many people have hydrocephalus because infants can be born with it, children can acquire it, and adults can acquire it. It is estimated that 0.6% of all pediatric hospital admissions are made up of hydrocephalus and that one out of 1,000 infants are born with hydrocephalus (http://www.ninds.nih.gov).