A mega cisterna magna is a controversial subject. In general, however, the term refers to non-pathological enlargement of the retrocerebellar CSF space, not associated with cerebellar abnormalities (normal cerebellar vermis and hemispheres).

Differential diagnosis includes Dandy-Walker malformations and variants, cerebellar hypoplasia, posterior fossa arachnoid cyst, and epidermoid cysts.

One of the supportive findings for the diagnosis of mega cisterna magna is the presence of internal lines crossing inside the cistern representing folds of the arachnoid membrane. These folds are virtually never seen in any similar lesion like a retrocerebellar arachnoid cyst or Dandy-Walker malformation.