Dislocation of the cerebellar tonsils, downward out of the skull; usually measured in mm (millimeters) or cm (centimeters).

• What originally distinguished a tonsillar ectopia from a Chiari Malformation rested solely on the size of the herniation.
• Since 3-5% of the population has some degree of tonsillar ectopia (without symptoms), a tonsillar ectopia was defined as being < 5mm and a CM was > 5mm. (The proper position of the tonsils should actually be 3mm above the foramen magnum.)
• As studies have shown that size IS NOT the biggest factor in evaluating symptomology, they have moved away from this size based standard approach to diagnosing Chiari.