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Mega cisterna magna

A mega cisterna magna refers to adult patients with significantly enlarged CSF retrocerebellar cisterns in the posterior fossa with normal cerebellar morphology. Controversy remains about whether mega cisterna magna is a normal anatomical variant or due to volume loss of the cerebellum

Mega cisterna magna has also been referred to as Blake's pouch or retrocerebellar arachnoid pouch


A mega cisterna magna is thought to occur in ~1% of all brains imaged postnatally. 


Especially if noted antenatally, a mega cisterna magna has been associated with:

  • infarction
  • inflammation/infection: particularly cytomegalovirus
  • chromosomal abnormalities: especially trisomy 18

However, when a mega cisterna magna occurs as an isolated finding with normal ventricles the prognosis is good.

Radiographic features


Typically seen as prominent retrocerebellar cerebrospinal fluid (CSF) appearing space with a normal vermis and normal cerebellar hemispheres


In antenates, mega cisterna magna refers to an enlarged retrocerebellar CSF space

  • usually >10 mm in antenatal imaging (some consider up to 12 mm within normal limits)
  • vermis should be closely evaluated to exclude Dandy-Walker continuum abnormalities

History and etymology

The term was first coined by Gonsette et al in 1962, in patients with cerebellar atrophy. 

Differential diagnosis

Mega cisterna magna needs to be distinguished from other causes of an enlarged retro-cerebellar CSF space:

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