Which of the following defines a Chiari type I malformation on midsagittal MRI?

• A Downward displacement of the vermis, fourth ventricle and medulla into the cervical canal
• B Herniation of the cerebellar tonsils more than 5 mm below the foramen magnum ✓
• C Agenesis of the cerebellar vermis with a posterior fossa cyst
• D Small posterior fossa with upward herniation of the superior vermis

Explanation

Chiari I is defined as caudal descent of the peg-like cerebellar tonsils at least 5 mm below the foramen magnum, often associated with syringomyelia. Option A describes Chiari II, which includes displacement of the vermis, medulla and fourth ventricle and is associated with myelomeningocele. Option C describes Dandy-Walker malformation, a separate entity entirely.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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