Hydrocephalus is an abnormal expansion of cavities (ventricles) within the brain caused by the accumulation of cerebrospinal fluid.

Hydrocephalus is the result of an imbalance between the formation and drainage of cerebrospinal fluid (CSF). Approximately 500 milliliters (about a pint) of CSF is formed within the brain each day, by structures called choroid plexus, with epidermal cells lining chambers called ventricles. Once formed, CSF usually circulates among all the ventricles before it is absorbed and returned to the circulatory system. The normal adult volume of circulating CSF is 150 ml, so that the CSF turn-over rate is more than three times per day. Production is independent of absorption, and reduced absorption causes CSF to accumulate within the ventricles.

Reduced absorption most often occurs when one or more passages connecting the ventricles become blocked, preventing movement of CSF to its drainage sites in the subarachnoid space just inside the skull. This...