Our case (CL1173) compared to prior CT from 9 months earlier (CL1025):
"Isolation" or "entrapment" of the 4th ventricle is a complication in shunted patients. Associated brainstem compression can cause ataxia, nystagmus, dysarthria, pyramidal weakness, cranial nerve palsies, eye movement disorders, etc.
See this article for more information:
Sometimes you will be asked to compare to prior exams - please always use the most recent, which may be an US or an MRI.
This can be very challenging. Use cross-referencing in PACS to make sure you measure similar portions of the ventricle.
And don't just eyeball; measure, measure, measure!