Presentation
Vomiting and vertigo.
Patient Data
The posterior fossa shows a well-defined large cyst following the CSF signal intensity and shows a connection to the 4th ventricle which looks dilated, the cyst causes elevation of the Tentorium on the right side with the normal position of the torcula and left tentorium. This cyst has mass effect on the adjacent structures and has caused obstructive hydrocephalus, which is already treated by two ventriculoperitoneal shunts.
The posterior fossa is of normal size with a normal vermis.
Case Discussion
The patient presented with vertigo and vomiting after placement of the second VP shunt and was referred for MRI to exclude a trapped ventricle.
Review of previous scans and surgical history showed that the patient has already had her first VP shunt placement surgery probably a year ago as she had hydrocephalus due to a posterior fossa cystic lesion as per her previous reports.
As in the case description, the posterior fossa cyst represents a persistent Blake pouch cyst.
Blake pouches are a developmental structure that is formed temporarily early during fetal life and regresses later leaving a central aperture that gives rise to the foramen of Magendie. Failure of the pouch to regress will result in a persistent Blake pouch cyst.
Persistent Blake pouch cyst is one of the few cystic structures that develop in the posterior fossa including the arachnoid cyst and mega cisterna magna. It is also considered on the Dandy-Walker continuum.
Blake pouch cysts are differentiated from mega cisterna magna by the fact that mega cisterna magna will never cause hydrocephalus while a persistent Blake pouch cyst will cause hydrocephalus. While an arachnoid cyst can cause hydrocephalus, it does not usually show communication with the 4th ventricle when the Blake pouch always shows a connection to the 4th ventricle.