Cerebellar tonsillar position (illustration)

Case contributed by Assoc Prof Frank Gaillard

The cerebellar tonsillar position is the distance (yellow line) is measured from the tip of the cerebellar tonsils to a line drawn between the anterior and posterior rims of the foramen magnum (McRae line - blue line) at right angles.

Normal is for the tonsils to be above the foramen magnum although it is fairly frequent to descend a few millimeters without constituting a Chiari 1 malformation. 

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