Giant cisterna magna, also called as mega cisterna magna, is a normal variation, and therefore, it is not a disease. It is found in about 1% of all scans done postnatally. The cisterna magna is the space between the inferior margin of the vermis and the posterior rim of the foramen magnum. The normal cisterna magna measures between 3 mm and 8 mm, and mega cisterna magna (MCM) is diagnosed when it reaches 10 mm or more.
Mega cisterna magna (MCM) should be differentiated from Dandy Walker Malformation (DWM). As opposed to DWM, in MCM, fourth ventricle is normal in size (it is not enlarged), cerebellar vermis is present and it is normal in size, and the posterior fossa is not enlarged.
Other differential diagnosis is posterior fossa arachnoid cyst. Arachnoid cyst may be asymptomatic or it may cause headaches.