A newborn is found to have a cystic posterior fossa mass on cranial ultrasound. MRI shows complete absence of the cerebellar vermis with the fourth ventricle communicating directly with a large posterior fossa cyst, and the posterior fossa itself is enlarged with upward displacement of the tentorium. Which congenital anomaly is this?

• A Mega cisterna magna
• B Blake pouch cyst
• C Joubert syndrome
• D Dandy-Walker malformation ✓

Explanation

Dandy-Walker malformation is defined by three key features on MRI: complete or partial absence of the cerebellar vermis, cystic dilatation of the fourth ventricle communicating with a large posterior fossa cyst, and enlargement of the posterior fossa with upward displacement of the torcular herophili and tentorium. Mega cisterna magna has a normal vermis and fourth ventricle. Blake pouch cyst shows inferior displacement of the vermis but the posterior fossa is not enlarged. Joubert syndrome shows the pathognomonic 'molar tooth sign' on axial MRI due to elongated superior cerebellar peduncles.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.