Blake's pouch cyst
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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 4,5.
It is a rare and underdiagnosed entity 4.
It can present:
in childhood or adulthood with signs of hydrocephalus (usually headache, vomiting, blurred or double vision)
later in life or be even asymptomatic, probably due to adequate CSF flow through lateral foramina of Luschka
Blake's pouch, also known as the rudimental fourth ventricular tela choroidea, is a normal transient structure that regresses during embryological development, usually by 12 weeks gestation, when it starts fenestrating to form the foramen of Magendie (which forms up to the fourth month of gestation).
Persistent Blake's pouch cysts occur due to failed perforation of the foramen of Magendie. As the foramina of Luschka open later than the foramen of Magendie during the embryologic development, this non-perforation of the foramen of Magendie causes enlargement of the ventricular system until the foramina of Luschka open and establishes a precarious compensation via CSF flowing into the cisterns 4.
Blake's pouch cysts used to be classified as part of the Dandy-Walker continuum, however, this entity is not related to cerebellar or fourth ventricle malformations 4,5.
MRI is the imaging modality of choice due to its capacity to differentiate this entity from other posterior fossa malformations. Features include:
infravermian cyst that communicates with fourth ventricle
cyst is smooth with thin walls that can be visualized on thin sagittal T2 images
it can impress on medial side of cerebellar tonsils due to size
cyst does not communicate with the cisterna magna posteriorly
upward displacement of the vermis
no vermian hypoplasia or rotation
usually hydrocephalus involving the fourth ventricle and supratentorial ventricles (tetraventricular hydrocephalus)
choroid plexus can extend from the fourth ventricle into the superior portion of the cyst, which is essentially a ventricular diverticulum
Treatment and prognosis
Preferable treatment of hydrocephalus is endoscopic third ventriculostomy. The prognosis is good after treatment.
History and etymology
It is named after Joseph A Blake (1864-1937), an American physician who studied the physiology related to the foramen of Magendie 3.
It was first described as an independent entity within the Dandy-Walker complex in 1996 by Paolo Tortori-Donati et al 3,4.
It needs to be distinguished from other causes of enlarged retrocerebellar "CSF" spaces:
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- 4. Cornips EM, Overvliet GM, Weber JW et-al. The clinical spectrum of Blake's pouch cyst: report of six illustrative cases. Childs Nerv Syst. 2010;26 (8): 1057-64. doi:10.1007/s00381-010-1085-2 - Free text at pubmed - Pubmed citation
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