Mega cisterna magna
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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:
- inflammation/infection: particularly cytomegalovirus
- chromosomal abnormalities: especially trisomy 18
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.
History and etymology
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:
- arachnoid cyst: can be difficult to distinguish from a mega cisterna magna
- epidermoid cyst: often shows a heterogeneous/dirty signal on FLAIR and restricted diffusion
- cerebellar atrophy / cerebellar hypoplasia
- Dandy-Walker malformation: vermis not intact
- Blake pouch cyst: usually hydrocephalus is present
- pilocytic astrocytoma: if very posterior and predominantly cystic; should contain a mural nodule
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