Cerebellar tonsillar position (illustration)
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Cerebellar tonsillar position is the vertical distance (green arrows) from the tip of the cerebellar tonsils to a line drawn between the anterior and posterior rims of the foramen magnum (McRae line - dotted blue line). 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.