MRI of a 26-year-old woman with occipital headaches shows peg-like cerebellar tonsils extending 7 mm below the foramen magnum with crowding at the craniocervical junction and a normal-sized posterior fossa. The fourth ventricle and vermis are normally positioned. What is the most likely diagnosis?

• A Chiari II malformation
• B Intracranial hypotension
• C Dandy-Walker malformation
• D Chiari I malformation ✓

Explanation

Tonsillar descent exceeding 5 mm below the foramen magnum in an adult, with a normal posterior fossa, normally positioned fourth ventricle and no vermian or brainstem malformation, defines Chiari I malformation. Chiari II features a small posterior fossa with caudal displacement of the vermis, brainstem and fourth ventricle and is associated with myelomeningocele. Dandy-Walker malformation shows vermian hypoplasia with a cystic fourth ventricle, which is absent here.

Reference: Osborn's Brain, 2nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.