Mega cisterna magna

Last revised by Dr Yuranga Weerakkody on 16 Aug 2022

Mega cisterna magna refers to a normal variant characterized by a truly focal enlargement of the CSF-filled subarachnoid space in the inferior and posterior portions of the posterior cranial fossa. It is an incidental finding on neuroimaging, and no imaging follow up is necessary. 

A mega cisterna magna is thought to occur in ~1% of all brains imaged postnatally. It constitutes 54% of all cystic posterior fossa malformations 4.

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

In children, it has also been identified in association with autism spectrum disorder 9,10

However, when a mega cisterna magna occurs as an isolated finding and the ventricles are normal it should be considered a variant of normal with no prognostic significance. 

There are no specific symptoms related to this condition. 

Some authors have proposed that mega cisterna magna is a result of a delayed Blake pouch fenestration; when fenestration does not occur, it results in a Blake pouch cyst 6

On antenatal ultrasound, mega cisterna magna refers to an enlarged retrocerebellar CSF space:

  • usually >10 mm (some consider up to 12 mm within normal limits)
  • septa may be seen within a mega cisterna magna, which are thought to be Blake pouch vestigial remnants 3
  • the vermis should be closely evaluated to exclude Dandy-Walker continuum abnormalities

Typically seen as prominent retrocerebellar cerebrospinal fluid (CSF) appearing space with a normal vermis, normal 4th ventricle, and normal cerebellar hemispheres. An enlarged cisterna magna usually measures >10 mm on midsagittal images. An enlarged posterior fossa can sometimes be present 6

The term was coined by the Belgian neurosurgeon Richard Gonsette (1929-2014) 8 in 1962, in patients with cerebellar atrophy 7

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

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Cases and figures

  • Case 1: T2
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  • Case 2: T2
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  • Case 3: on antenatal ultrasound
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  • Case 4
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  • Case 5
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  • Case 6
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  • Case 7
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  • Case 8
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