Presentation
Referred for further assesment of hydrocephalus seen on CT.
Patient Data
There is an infravermian cyst that communicates with the fourth ventricle exerting a mild mass effect on the inferior part of the vermis which is of normal size (no vermian hypoplasia or rotation).
The cyst obstructs the CSF outflow at the level of the foramen of magendie causing upstream hydrocephalus involving the fourth ventricle and supratentorial ventricles (tetraventricular hydrocephalus).
The tegmento-vermian angle is normal (roughly zero degree).
The posterior fossa is not expanded.
The image illustrates the MRI findings in the setting of Blake pouch cyst.
Case Discussion
MRI findings are consistent with Blake pouch cyst.
The differential diagnosis includes:
Dandy-walker malformation: presents with vermis hypoplasia and enlarged posterior fossa
Dandy-walker variant: inferior vermian hypoplasia with abnormal widening of the tegmento-vermian angle
mega cisterna magna: do not cause hydrocephalus
arachnoid cyst: usually presents with occipital bone scalloping and mass effect against the posterior aspect of the vermis
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