Hydrocephalus comes from the terms hydro (water) and cephalus (head). In medicine, it's used to describe a condition in which the ventricles of the brain, which normally produce and contain cerebrospinal fluid (CSF), appear to swell. CSF normally protects and nourishes the brain, but as is often true, you can have too much of a good thing.

There are three basic reasons why this can happen:

1. Too much CSF production. This is rare, but can occur in rare forms of tumors like a choroid plexus papilloma.
2. Too little CSF reabsorption. CSF is usually absorbed into the veins near the top of the skull, and if the sites of CSF reabsorption are blocked by blood as in SAH or by inflammation as in meningitis, pressure builds up.
3. Blocked CSF flow. CSF usually flows from the ventricles through small openings called foramina. If these are blocked by a blood clot, tumor, or other obstruction, the CSF can't flow to the top of the head for reabsorption. The result is a build-up of pressure in the ventricles, with swelling as a result.

All of the above are caused by CSF build-up that increases the pressure in the brain. Another process is known as hydrocephalus-ex-vacuo ("water brain from space," a term that in English sounds like a weird 1950s sci-fi movie). This just means that the ventricles appear large, but it's because brain tissue has vanished, and CSF is filling up the empty space with no increased pressure.

Doctors can attempt to reduce hydrocephalus by decreasing CSF production with medications like acetazolamide, or by removing CSF with a neurosurgical shunt. This can be lifesaving in cases severely elevated intracranial pressure. Shunts can also be helpful in some cases of normal pressure hydrocephalus, a disorder that can lead to gait problems and dementia.