Hydrocephalus is a condition where there is an accumulation of excess CSF (cerebrospinal fluid) in the brain, and with that, an increase of intracranial pressure. Treatment is (usually) via a shunt, which displaces excess CFS into a different part of the body (normally the abdominal cavity) to be absorbed.

In Evanna's case, the buildup is due to the chiari blocking the normal flow of the CFS.