This case demonstrates typical radiologic features of a Chiari I malformation, wich is the most common variant of the Chiari malformations. 

The tonsillar herniation is assessed measuring its distance from the line that connects the inner margins of the foramen magnum (basion to opisthion). Above 5 or 6 mm is considered a Chiari I, values between 3 to 6 mm are indeterminate, and needs to be correlated with symptoms and presence of syrinx. 

Cervical cord syrinx is present in ~35% (range 20-56%) of patients with Chiari I. 

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