Hydrocephalus ex vacuo, also known as compensatory enlargement of the CSF spaces, is a term used to describe the increase in the volume of CSF, characterised on images as an enlargement of cerebral ventricles and subarachnoid spaces, caused by encephalic volume loss.
It can be classified as a communicating hydrocephalus without obstruction to CSF absorption, although we usually do not refer to them as a hydrocephalus. Please refer to the article on hydrocephalus for a specific discussion about this classification.
It is usually seen in asymptomatic elderly people as a result of ageing brain with related volume loss, as well in pathological conditions that promote brain shrinkage:
- related to generalised brain degeneration (e.g. Alzheimer disease and leukodystrophies)
- related to encephalomalacia due focal damage (e.g. stroke and traumatic injuries)
Accurately distinguishing between hydrocephalus and compensatory enlargement of the CSF spaces can be challenging in some cases. Please refer to the article on hydrocephalus versus atrophy for a specific discussion of that differentiation.