1) Excessive accumulation of cerebrospinal fluid within the cranium which may be a congenital or acquired disorder; hydrocephalus ex-vacuo refers to ventricular dilation that occurs as a result of brain substance loss from cerebral infarction and other conditions.

2) An abnormal increase in the amount of cerebrospinal fluid within the cranial cavity that is accompanied by expansion of the cerebral ventricles, enlargement of the skull and especially the forehead, and atrophy of the brain.

3) The abnormal buildup of cerebrospinal fluid in the ventricles of the brain.