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Blake pouch cyst

Blake 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 open fourth ventricle.

Clinical presentation

It may be asymptomatic or it can present

  • in children or in adulthood with signs of hydrocephalus (usually headache, vomiting, blurred or double vision)
  • late in life or be even asymptomatic, probably due to adequate CSF flow through lateral foramen of Luschka

Pathology

It is a normal transient structure during embryological development but regresses  usually in a period of 5 to 12 weeks of gestation when it starts fenestrating to form the foramen of Magendie (which forms in up to 4th month of gestation).

Persistent Blake pouch cysts are thought to be due to a failed perforation of foramen of Magendie. Some authors class this malformation as part of the Dandy-Walker continuum.

As it arises from area membranacea posterior it does not involve cerebellum or 4th ventricle malformation, which both arise from area membranacea anterior.

Radiographic features

Features include:

  • infravermian cyst that communicates with fourth ventricle
    • cyst is smooth with thin walls
    • it can impress on medial side of cerebellar tonsils due to size 
    • cyst does not communicate with the cisterna magna posteriorly
  • no vermian hypoplasia or rotation
  • usually hydrocephalus of 4th and supratentorial ventricles
  • elevation of the tentorium but usually normally positioned torcular
  • choroid plexus can extend from 4th ventricle into superior portion of cyst

Treatment and prognosis

Preferable treatment of hydrocephalus is endoscopic third ventriculostomy. Prognosis is good.

Differential diagnosis

It needs to be distinguished from other causes of enlarged retro cerebellar "CSF" space and other entities of the Dandy-Walker continuum.

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