A 24-year-old woman has occipital headache lasting seconds, triggered by coughing and straining. MRI shows peg-shaped, pointed cerebellar tonsils lying 8 mm below the foramen magnum with crowding at the craniocervical junction and a syrinx in the upper cervical cord. The most likely diagnosis is:

• A Dandy-Walker malformation
• B Chiari type II malformation
• C Chiari type I malformation ✓
• D Arachnoiditis of the posterior fossa

Explanation

Chiari I is defined by caudal displacement of peg-shaped tonsils more than 5 mm below the basion-opisthion line in an adult, presenting with Valsalva-induced occipital headache and associated syringomyelia. Chiari II is congenital, always linked to myelomeningocele, and involves downward displacement of the vermis, fourth ventricle and brainstem, not isolated tonsillar herniation.

Reference: Osborn's Brain, 2nd ed.

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Written and medically reviewed by the StethoPrep medical team.