Dandy-Walker malformation must be differentiated from a subarachnoid cyst in the posterior fossa because both are associated with a large cisterna magna.
Patients with Dandy-Walker malformation have an elevated tentorium, while patients with megacisterna magna and posterior fossa arachnoid cysts in the region of the cisterna magna have a normally placed tentorium. Megacisterna magna refers to a primary bone abnormality that leads to a large cisterna magna (Figure 118.1) and, therefore, central nervous system structures within the posterior fossa are normal.

Figure 118.1.— Schematic representation of a megacisterna magna. All neurological structures are within normal limits. There is no compression of the cerebellar vermis or hemispheres.