A 16-week fetal ultrasound shows a posterior fossa cyst, absence of normal cerebellar vermis, and splaying of cerebellar hemispheres. The posterior fossa is markedly enlarged. This triad is MOST consistent with:

• A Arachnoid cyst of the posterior fossa
• B Mega cisterna magna
• C Blake's pouch cyst
• D Dandy-Walker malformation (classic) ✓

Explanation

Classic Dandy-Walker malformation (DWM) is characterised by: (1) complete or partial agenesis of the cerebellar vermis; (2) cystic dilation of the 4th ventricle communicating with a large posterior fossa cyst; and (3) upward displacement of the tentorium, transverse sinuses, and torcula (torcular-lambdoid inversion). Hydrocephalus occurs in ~75%. Mega cisterna magna has a normal vermis and normal 4th ventricle. Arachnoid cyst does not communicate with the 4th ventricle. Blake's pouch cyst has an intact vermis with 4th ventricle outlet obstruction.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.