Mega cisterna magna

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

Epidemiology

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

Associations

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.

Clinical presentation

There are no specific symptoms related to this condition. 

Pathology

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

Radiographic features

Ultrasound

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)
• septa may be seen within a mega cisterna magna, which are thought to be Blake pouch vestigial remnants ³
• vermis should be closely evaluated to exclude Dandy-Walker continuum abnormalities

CT/MRI

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

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 retrocerebellar CSF space:

• arachnoid cyst
• epidermoid cyst: often shows a heterogeneous/dirty signal on FLAIR and restricted diffusion
• cerebellar atrophy / cerebellar hypoplasia
• Dandy-Walker malformation: vermis not intact
• Blake's pouch cyst: usually a hydrocephalus is present 
• pilocytic astrocytoma: if very posterior and predominantly cystic

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