Hydrocephalus

The brain floats in a fluid called cerebrospinal fluid, and inside the brain there are connected cavities, the cerebral ventricles, through which that fluid circulates. Hydrocephalus is the buildup of that fluid inside the ventricles when it causes symptoms.

There are three ways to end up there, and it is worth telling them apart because they are not the same. The flow may become obstructed somewhere along the route. The fluid may be produced normally and circulate normally, but fail to be absorbed into the venous system. Or too much of it may be produced. It is a closed circuit with fluid coming in constantly: if the outlet closes, pressure rises.

Knowing where it comes from and where it goes helps make sense of it. It is produced mainly in the choroid plexus, a tissue that sits inside the ventricles themselves. And it is absorbed at the arachnoid granulations, extensions of the layers that wrap around the brain, which push into the dural venous sinuses, the channels through which blood leaves the skull.