A mega-cisterna magna, is a controversial subject. In general however, the term refers to non-pathological enlargement of the retro-cerebellar CSF space and not associated with cerebellar abnormalities (normal cerebellar vermis and hemispheres).
Differential diagnosis include Dandy Walker malformations and variants, Cerebellar hypoplasia, posterior fossa arachnoid cyst, epidermoid cysts.
One of the supportive findings for the diagnosis of mega cistern magna is the presence of internal lines crossing inside the cistern representing folds of the arachnoid membrane. These folds virtually are not seen in any similar lesion like retrocerebellar arachnoid cyst and dandy – walker malformation.