Distribution of cerebellar tonsil position: implications for understanding Chiari malformation.

OBJECT: Prior attempts to define normal cerebellar tonsil position have been limited by small numbers of patients precluding analysis of normal distribution by age group. The authors' objective in the present study was to analyze cerebellar tonsil location in every age range.
METHODS: Two thousand four hundred patients were randomly selected from a database of 62,533 consecutive patients undergoing MRI and were organized into 8 age groups. Magnetic resonance images were directly examined for tonsil location, morphology, and other features. Patients with a history or imaging findings of posterior fossa abnormalities unrelated to Chiari malformation (CM) were excluded from analysis. The caudal extent of the cerebellar tonsils was measured at the midsagittal and lowest parasagittal positions.
RESULTS: The mean tonsil height decreased slightly with advancing age into young adulthood and increased with advancing age in the adult age range. An increasing age in the adult age range was associated with a decreased likelihood of a tonsil position 5 mm or more below the foramen magnum (p = 0.0004). In general, the lowest tonsil position in each age group was normally distributed. Patients with pegged morphology were more likely to have a tonsil location at least 5 mm below the foramen magnum (85%), as compared with those having intermediate (38%) or rounded (2%) morphology (p < 0.0001). Female sex was associated with a lower mean tonsil position (p < 0.0001). Patients with a lower tonsil position also tended to have an asymmetrical tonsil position, usually lower on the right (p < 0.0001).
CONCLUSIONS: Cerebellar tonsil position follows an essentially normal distribution and varies significantly by age. This finding has implications for advancing our understanding of CM.