Historically, there have been changes to the definition of Chiari based solely on the size of the cerebellar tonsillar ectopia (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 tonsils should be 3mm above the foramen magnum (the hole at the bottom of the skull).

• As studies have shown that size IS NOT the biggest factor in evaluating symptomology (even in the development of a syrinx), they have moved away from this size-based standard.