Cerebellar tonsillar position (illustration)

Case contributed by Frank Gaillard
Diagnosis not applicable

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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