Cerebellar tonsillar position

Last revised by Joshua Yap on 26 Mar 2023

The cerebellar tonsillar position is the perpendicular distance from the tip of the cerebellar tonsils to a line drawn between the anterior margin (basion) and posterior margin (opisthion) of the foramen magnum (McRae line).

As is to be expected, values used vary somewhat from author to author 1,2:

Some authors advocate a simpler rule 3:

To make matters worse, the "normal" position of the cerebellar tonsils varies with age. In neonates, the tonsils are located just below the foramen magnum and descend further during childhood, reaching their lowest point somewhere between 5 and 15 years of age. As the individual ages, the tonsils usually ascend coming to rest at the level of the foramen magnum 4. As such, although 5 mm descent in an adult should be viewed with suspicion, in a child it is most likely normal 4.

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