Ex Vacuo hydrocephalus also refers to an enlargement of cerebral ventricles and subarachnoid spaces, and is usually due to brain atrophy (as it occurs in dementias), post-traumatic brain injuries and even in some psychiatric disorders, such as schizophrenia.
As opposed to hydrocephalus, this is a compensatory enlargement of the CSF-spaces in response to brain parenchyma loss - it is not the result of increased CSF pressure.
Delayed ventricular enlargement months to years after TBI may instead be due to atrophy (hydrocephalus ex vacuo) secondary to diffuse axonal injury, and may not represent true hydrocephalus. It may not be possible to accurately differentiate these two conditions, and the decision to shunt may, therefore, be difficult (similar to the dilemma in patients with NPH vs. atrophy).
Features that favor hydrocephalus include:
dilatation of the temporal horns
lack of dilatation of parahippocampal fissures
increased frontal horn radius
acute ventricular angles
periventricular interstitial edema from the transependymal flow
intraventricular flow void from CSF movement on MRI
widening of the third ventricular recesses: midsagittal plane
upward displacement of corpus callosum: midsagittal plane
depression of the posterior fornix: midsagittal plane
decreased mamillopontine distance: midsagittal plane
narrow callosal angle
cingulate sulcus sign