MRI of an infant with macrocephaly shows a large posterior fossa cyst that communicates with the fourth ventricle, hypoplasia of the cerebellar vermis, and upward displacement of the tentorium. Which malformation is described?

• A Lissencephaly
• B Arnold-Chiari type I malformation
• C Holoprosencephaly
• D Dandy-Walker malformation ✓

Explanation

The diagnostic triad of Dandy-Walker malformation is agenesis or hypoplasia of the cerebellar vermis, cystic dilation of the fourth ventricle filling an enlarged posterior fossa, and elevation of the tentorium and torcula. Obstructive hydrocephalus develops in most patients and requires shunting. In Arnold-Chiari type I the posterior fossa is small rather than enlarged, and holoprosencephaly involves failure of forebrain cleavage, not a posterior fossa cyst.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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