MRI of a 9-month-old with macrocephaly and developmental delay shows a large midline posterior fossa cyst continuous with the fourth ventricle, hypoplasia of the inferior cerebellar vermis, and an enlarged posterior fossa with upward displacement of the tentorium. What is the diagnosis?

• A Arnold-Chiari malformation type II
• B Dandy-Walker malformation ✓
• C Aqueductal stenosis
• D Arachnoid cyst of the posterior fossa

Explanation

The triad of vermian hypoplasia, cystic dilation of the fourth ventricle filling an enlarged posterior fossa, and torcular-lambdoid inversion (elevated tentorium and transverse sinuses) defines Dandy-Walker malformation. Hydrocephalus develops in most affected children and may require shunting. In Chiari II the posterior fossa is small with downward herniation, classically associated with myelomeningocele. An arachnoid cyst compresses but does not communicate with the fourth ventricle and does not produce vermian agenesis.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.