MRI scan showing Mega cisterna magna

A mega-cisterna magna, is a controversial entity among experts. In general however, the term is applied to non-pathological prominance (usually exceeding 10 mm in antenatal imaging) of the retro-cerebellar CSF space and not associated with cerebellar abnormalities. There is a normal vermis and normal cerebellar hemispheres.


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


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

  • infarction
  • inflammation and infection (e.g. Cytomegalovirus)
  • chromosomal abnormalities (e.g. trisomy 18)

Radiographic features on CT

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


The term was first coined by Gonsetteet al in 1968, in patients with cerebellar atrophy.

Dandy-Walker malformation: (A, B)The fourth ventricle opens into a large posterior fossa cyst. There is associated hydrocephalus. (C) The cerebellum is hypoplastic & a thin rim of cerebellar tissue is seen forming the wall of the posterior fossa cyst (arrow). The vein of Galen, straight sinus, & venous confluence are elevated above the level of the lambdoid suture.

Differential diagnosis

Differential is among other causes of an enlarged retro-cerebellar CSF space, including:

  • arachnoid cyst
  • epidermoid cyst
  • cerebellar atrophy/cerebellar hypoplasia
  • Dandy-Walker malformation