Distribution of cerebellar tonsil position: implications for understanding Chiari malformation

Clinical article

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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.


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.


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).


Cerebellar tonsil position follows an essentially normal distribution and varies significantly by age. This finding has implications for advancing our understanding of CM.

Abbreviation used in this paper:CM-I = Chiari malformation Type I.

Article Information

Address correspondence to: Cormac O. Maher, M.D., Department of Neurosurgery, University of Michigan, 1500 E. Medical Center Dr., Room 3552 Taubman Center, Ann Arbor, MI 48109-5338. email: cmaher@umich.edu.

Please include this information when citing this paper: published online June 14, 2013; DOI: 10.3171/2013.5.JNS121825.

© AANS, except where prohibited by US copyright law.



  • View in gallery

    Magnetic resonance images depicting examples of cerebellar tonsil morphology: pegged (A), intermediate (B), or rounded (C).

  • View in gallery

    Graphs illustrating the mean lowest tonsil position according to the distance from the foramen magnum (mm) by patient age (yrs). Error bars represent the standard error for a calculated mean for this population by decade. Upper: The mean lowest tonsil position by age in 2400 patients. Lower: The mean lowest tonsil position by age in 2378 patients. Twenty-two patients with a clinical diagnosis of CM were excluded from the original 2400 patients.

  • View in gallery

    Bar graphs representing distributions of the mean lowest tonsil position measurement with respect to the foramen magnum for the following age groups: 0–10 years (A), 11–20 years (B), 21–30 years (C), 31–40 years (D), 41–50 years (E), 51–60 years (F), 61–70 years (G), and ≥ 71 years (H). Negative numbers correspond to a more rostral tonsil location with respect to the foramen magnum.



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