Specific Imaging Findings

The hallmark of Dandy-Walker malformation (DWM) is vermian hypoplasia with verticalization (counter-clockwise rotation) of the vermis; as a consequence, the vermis lies behind the quadrigeminal plate. The foliation of the vermis is rudimentary and sometimes dysplastic. The cerebellar hemispheres are displaced bilaterally but are not significantly hypoplastic. The brain-stem is generally not affected. The fourth ventricle is markedly enlarged and bulges posteriorly, forming a large pseudocyst; in some cases, it extends upward through a congenital dehiscence of the tentorium, thereby occupying a space between the occipital lobes. Inferiorly, the cyst may bulge into the foramen magnum, whereas laterally, the extension of the fourth ventricle is limited by the reflection of the pia mater over the posterior surface of the cerebellar hemispheres. The entire posterior fossa is enlarged. Supratentorial hydrocephalus is frequently associated but does not technically form a constituting element of the malformation.

Pertinent Clinical Information

Patients with DWM often present with macrocephaly in the neonatal period. Infants may elicit medical attention because of hydrocephalus, developmental delay, or ataxia. Apnea and seizures are seen frequently, whereas developmental delay and mental retardation are highly variable. Mortality is significant in patients with severe obstructive hydrocephalus or multiple associated congenital anomalies. Incidental DWM in asymptomatic adults has been reported.