A 4-month-old boy has progressive macrocephaly with prominent occiput, downward gaze paresis, and truncal ataxia. MRI reveals severe hypoplasia of the cerebellar vermis, a large posterior fossa cyst continuous with the fourth ventricle, and an abnormally high position of the tentorium and torcula. What is the most likely diagnosis?

• A Chiari II malformation
• B Arachnoid cyst of the posterior fossa
• C Joubert syndrome
• D Dandy-Walker malformation ✓

Explanation

The triad of cerebellar vermis agenesis, cystic dilatation of the fourth ventricle filling the enlarged posterior fossa, and elevation of the tentorium and torcula defines the Dandy-Walker malformation. An isolated arachnoid cyst compresses rather than replaces the vermis and does not elevate the torcula. Joubert syndrome shows the molar tooth sign with a normal sized posterior fossa, and Chiari II is associated with a small posterior fossa.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.