Figure 2.

The concept and range of cerebellum/posterior fossa disorders in the “Dandy–Walker spectrum.”(A) Illustration of developing fourth ventricle/cerebellum and the impact of impaired egress of CSF. During normal development, the wall fourth ventricle normally thins, and the foramen of Magendie forms, in the midline. If the leptomeninges are abnormal, the cerebellum may be small and the outflow foramina of the fourth ventricle may not form. The fourth ventricle expands posteriorly (small black arrows) and superiorly, pushing the small cerebellar vermis (Cb) counterclockwise (large black arrow); the posterior fossa then enlarges. This combination of findings creates the classic Dandy–Walker malformation. Small black PF signifies pontine flexure, small black MF signifies mesencephalic flesure, large black P signifies prosencephalon, large black M signifies mesencephalon, large black R signifies rhombencephalon. (B) Classic Dandy–Walker malformation. Sagittal T2 weighted image shows the classic appearance with a small vermis (black arrow), rotated counterclockwise with abnormal foliation. The surrounding CSF spaces are markedly enlarged with abnormally enlarged posterior fossa and elevation of the tentorium cerebelli and the torcular Herophili. (C) Mega cisterna magna is a condition in which a collection of CSF in an enlarged cisterna magna (C) expands the posterior fossa but the midbrain and hindbrain are normal. It is seen in patients with gene mutations that, in siblings, cause the classic Dandy–Walker malformation. (D) Blake pouch cyst is a condition in which the ependymal wall of the fourth ventricle stretches out through the foramen of Magendie and causes enlargement of the foramen with mild rotation of the vermis (black arrow). It is considered by some to be an incidental finding and by others to be a mild form of the Dandy–Walker malformation.