Presentation
Investigation for hydrocephalus
Patient Data
Infravermian cyst that communicates with the fourth ventricle exerts a mild mass effect on the inferior part of the vermis.
No vermian hypoplasia is seen.
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 posterior fossa is not expanded.
Case Discussion
Blake's pouch cyst is a cystic appearing structure that represents posterior ballooning of the inferior medullary velum into the cisterna magna, below and posterior to the vermis, that communicates with an open fourth ventricle. It is caused by a failure of the regression of Blake's pouch secondary to the non-perforation of the foramen of Magendie.
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