Mega Cisterna Magna and Retrocerebellar Arachnoid Cysts

Abstract

There is debate as to whether mega cisterna magna (MCM) arises from a pathologic insult; recent evidence suggests that it may be on the mildest end of the Dandy-Walker continuum. However, it can be considered an asymptomatic normal variation in which the posterior fossa appears mildly enlarged on imaging, typically with prominence of the subarachnoid space (SAS) posterior to the cerebellum (retrocerebellar space). This prominence of the cerebellar-medullary cistern communicates freely with the SAS and does not result in CSF obstruction or hydrocephalus because the foramina of Magendie and Luschka are patent, by definition. The development of the cerebellar hemispheres and vermis are normal. The tentorium may appear slightly higher than normal but has a normal insertion. MCM is thought to result from evagination of the tela choroidea’s posterior membranous area, possibly caused by delayed completion of the foramen of Magendie. Larger MCMs may simulate and be very difficult to distinguish from arachnoid cysts (which have a thin surrounding membrane) because of mild compression of the cerebellar vermis or hemispheres. They may have a split falx/dural fold, which can be seen in over half of the cases traversing the MCM and can distinguish MCM from arachnoid cysts.