Radiology Teaching Files > Case 33036251

previously visited BLAKE`S POUCH CYST
Contributed by: antonio aguiar, Radiologist, Real Hospital PortuguÍs De Pernambuco - Realimagem, Brazil.
Patient: 30 year old male
History: 30 year old male: , with headache, vertigo, nausea and visual blurring for six months.

Fig. 1: Contrast-enhance sagital T1-weighted image demonstrates a posterior fossa cyst in midline. There is compression of adjacent cerebellum and medula oblongata.

Fig. 2: Contrast-enhance sagital T1-weighted image shows a posterior fossa cyst with signal intensity similar to that of CSF, located inferior to a normally formed cerebellar vermis. The fourth ventricle is slightly displaced superiorly. Note the normal size of the posterior fossa.

Fig. 3: Contrast-enhance axial T1-weighted image demonstrates the symmetric, midline location of the CSF posterior fossa cyst. There is mass effect on the medial cerebellar hemispheres.

Fig. 4: Axial T2-weighted image shows the well marginated posterior fossa cyst , that promotes compression on the medial cerebelar hemisphere.

Fig. 5: Axial T2-weighted image: the lateral and third ventricles are enlarged due to hydrocephalus.

Fig. 6: Coronal T2-weighted image: symmetrical midline location of the posterior fossa cyst, associated with dilatation of the lateral and third ventricles (hydrocephalus). The posterior fossa is normal in size.

Fig. 7: Non-contast CT performed soon after surgery demonstrates exenteration of the posterior fossa cyst.

Retrocerebellar cyst formation, with signal intensity similar to that of cerebrospinal fluid (CSF), hypointense on T1 sequences and hyperintense on T2, without abnormal enhancement. Presence of mass effect with compression of adjacent cerebellum and medula oblongata.


The position of the dural structures (tentorium cerebelli, torcular herophili, falx cerebelli, straight and lateral sinus) and the size of The posterior fossa is normal in size.


There is dilatation of the lateral and third ventricles.



Diagnosis: Persistent Blake`s pouch.

Various classifications and descriptions of collections of cerebrospinal fluid (CSF) in the posterior fossa can be found in the literature. Familiarity with the normal embryologic development of the posterior fossa is helpful in understanding the pathogenesis, radiologic appearance and various terminologies used to describe cystic malformations of the posterior fossa.


Abnormal CSF collections within the posterior fossa are defined by the Dandy-Walker Complex (DWC) and Arachnoid Cyst (AC). The DWC includes the Dandy-Walker Malformation (DWM), the Dandy-Walker Variant (DWV) and the Mega-Cisterna Magna (MCM). Some features of these three abnormalities support the theory that DWM, DWV and MCM have the same embryologic origin and that they differ only in the severity of the insult and the anatomic predilection. Tortori-Donati et al added persistent Blake`s Pouch Cyst (BPC) as an independent entity within the Dandy-Walker Complex.


Dandy-Walker Malformation is defined by the characteristic triad: 1- complete or partial atresia of the vermis; 2- cystic dilatation of the fourth ventricle and 3- an enlarged posterior fossa. The report prevalence of Dandy-Walker Malformation is one per 25000 - 30000 births.


Dandy-Walker Variant describes that group in which the posterior fossa itself is normal in size, with varying degrees of vermian and cerebellar hypoplasia and with a prominent retrocerebellar space of CSF. This group may have some features of Dandy-Walker Malformation but are associated with a normal-sized posterior fossa. The tentorium is located in a normal position without torcular lambdoidal inversion, and the fourth ventricle is better formed and less expanded. Dandy-Walker Variant is more common than the true Dandy-Walker Malformation and represents approximately one-third of all posterior fossa malformations.


Mega-Cisterna Magna is characterized by expansion of the cisterna magna, which freely communicates with the ventricular system and with the perimedullary subarachnoid spaces, associated with morphologically normal vermis and cerebellar hemispheres. MCM is a fairly common condition, accounting for approximately 54% of cystlike posterior fossa malformation.


Arachnoid Cysts of the posterior fossa are benign CSF collections that develop within the layers of the arachnoid membrane. They do not communicate freely with the ventricular or subarachnoid spaces. Location in the posterior fossa is the second most common location after the temporal fossa (approximately one third of cases). They represent 1% of all intracranial space-occupying lesions. They can manifest at any age, but the vast majority are detected in the first 2 decades of life. Male patientes are more commonly affected than females.


Blake described a posterior evagination of the tela choroidea developing as a posterior expansion of the fourth ventricle in a normal 13 day old human embryo. This evagination represented a normal developmental stage and was associated with a normal vermis and had no communication with the perimedullary spaces.


The persistent Blake`s Pouch Cyst is a cystic malformation of the posterior fossa thought to derive from persistence and expansion of the normally transient Blake`pouch that arises from the area membranacea inferior (AMI) and that normally regresses during the fifth to eighth gestational weeks. According to this model, the persistent Blake`s pouch cyst is in communication with the fourth ventricle and separated from the subarachnoid space. BPC represents a posterior balloning of the superior medullary vellum into the cisterna magna.


As cerebellar developmental does not appear morphogenetically to involve the AMI, there is no malformation of the cerebellum proper. Although the BPC has been termed retrocerebellar arachnoid cyst, it is reasonable to consider it a separate entity from a true arachnoid cyst in that its membrane has a distinct ependymal componente and in that it is thought to have a distinct embryologic origin.


There is a marked variation in clinical features among individuals with abnormal cystic fluid collections in the posterior cranial fossa. Presentation is most often with symptoms and signs of hydrocephalus. Typically BPC becomes symptomatic early in life. Symptoms include headache, vomiting, double vision and visual blurring. In the abscence of associated supratentorial anomalies and uncontrolled or progressive hydrocephalus, patients with BPC have a good prognosis. The unusual feature of this present case is that the cystic malformation of posterior cranial fossa was not discovered until adulthood.


Pathologic and imaging features of BPC include posterior fossa cyst, widening of the valleculum, separation of the cerebellar hemispheres, and a constant association with hydrocephalus.


Magnetic Resonance Imaging (MRI) represents the diagnostic modality of choice for the diagnosis and differentiation of posterior fossa collections. It allows assessment of the aqueduct for patency, of the communication between the fluid collection and the ventricular system or the subarachnoid space and examination of the brain to search for associated anomalies.


The cysts are well marginated, have a smoth wall, and are nearly always unilocular and located in mdline. Their appearance on CT and MR images are identical to that of cerebrospinal fluid. The vermis, cerebellar hemispheres and brain stem can be compressed by the cystic due to mass effect but are morphologically normal (figures 1, 2, 3, 4, 5 and 6).


The treatment of choice in the present case was surgical ressection of the posterior fossa cyst (figure 7). The patient had good pos operative evolution.




1. Kollias SS, Ball WS, Prenger EC. Cystic Malformations of the Posterior Fossa: Differential Diagnosis Clarified through embryologic analysis. Radiographics 1993; 13: 1211 - 1231.


2. Calabro F, Arcuri T, Jinkins Jr. Blake`s Pouch Cyst: an entity within the Dandy-Walker continuum. Neuroradiology 2000; 42: 290 - 295.


3. Barkovich AJ. Pediatric Neuro-Imaging 3rd ed. Philadelphia, PA. Lippincott Williams and Wilkins. 2000.


4. Atlas SW. Magnetic Resonance Imaging of the Brain and Spine. 3rd ed. Philadelphia, PA. Lippincott Williams and Wilkins. 2002.


5. Atlas SW. Ressonância Magnética do cérebro e da Coluna Vertebral. 3ª ed. Rio de Janeiro, RJ. Livraria e editora Revinter Ltda. 2008.

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Additional Details:

Case Number: 33036251Last Updated: 11-21-2009
Anatomy: Cranium and Contents   Pathology: Congenital
Modality: CT, MRAccess Level: Readable by all users
Keywords: blake`s pouch cystACR: 15.1469

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