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What measurement is used?
A line is drawn on the midline sagittal plane across the inner margins foramen magnum (ophisthion to basion), and measuring the inferior most part of the tonsils. Definitions as to exact measurements vary, and depend on desired sensitivity and specificity for the sign. The easiest measurements to remember are: A) above foramen magnum = normal; B) 1 - 5mm = benign tonsillar ectopia; C) >5mm = Chiari 1 malformation.