Hydrocephalus is commonly referred to as "water on the brain." The so-called "water" is actually cerebrospinal fluid (CSF), a clear liquid that looks like water and is produced in the 4 ventricles (cavities) of the brain, connected by narrow pathways.

CSF is in constant production and absorption; it has a defined pathway from the lateral ventricles to the third ventricle and then into the 4th ventricle. Once the fluid is in the 4th ventricle, the CSF passes to the outside of the brain through three small holes near the base of the brain. It then circulates down and up the spinal column and over the outside of the brain. It provides nourishment to the brain, carries away debris, and protects the brain and spinal cord from injury. The CSF is then absorbed into the blood system where it is filtered and discarded by the body.

The body makes almost a pint of new CSF per day and the amount of CSF is perfectly balanced (not too much nor too little). Hydrocephalus occurs when there is an imbalance of production and/or absorption. With most types of hydrocephalus, the fluid gets trapped in the ventricles and can not complete its circulation, usually due to a blockage. The excess fluid causes the ventricles to expand.

As a result, pressure is placed on the brain at the skull, causing neurological problems. However, there are a couple of types in which there is an absorption problem (Benign External Hydrocephalus and Normal Pressure Hydrocephalus).