Cisterna magna enlargement beyond 1.0 cm with ventriculomegaly and cerebellar hemispheric crowding is most consistent with which condition?

• A. Dandy-Walker malformation
• B. Acrania
• C. Hydranencephaly
• D. Arachnoid cyst

Answer: (A)

This pattern points to a posterior fossa malformation where the cerebellar vermis is underdeveloped and the fourth ventricle dilates, creating a cystic enlargement of the posterior fossa. The enlarged cisterna magna (>1.0 cm) with ventriculomegaly and crowding of the cerebellar hemispheres is classic for Dandy-Walker malformation, which features vermian hypoplasia or agenesis, a dilated fourth ventricle, and posterior fossa enlargement that can extend to hydrocephalus. Acrania would show absence of the calvarium, hydranencephaly involves near-total loss of cerebral hemispheres, and an arachnoid cyst is a discrete cystic lesion that does not produce the same vermian hypoplasia and fourth-ventricle dilation pattern.