Re-Defining the MRI Reference Level for Cerebellar Tonsil, A Study of 225 Adolescents, Normal vs. Idiopathic Scoliosis

¹The Chinese University of Hong Kong
²The Hong Kong Polytechnic University

PURPOSE:
The association of Chiari-I malformation with idiopathic scoliosis has been reported with increasing frequency. However, the normal reference level of inferior displacement of cerebellar tonsils below the plane of the foramen magnum is still controversial as available references were based on studies of small number of adult MRI data. The aims of the present study were to redefine and compare the position of the cerebellar tonsil in adolescents with IS Vs normal controls and to correlate the results with severity of the scoliosis.

METHODS:
Whole spine MRI was performed in 135 patients with AIS with Cobb's angle less than 45 degrees, 29 patients with Cobb's angle more than 45 degrees, and 61 healthy adolescents aged 12 to 14. The standard reference line connecting the basion and opisthion (BO line) in the best selected MRI sagittal image was used. The perpendicular distance from the tip of tonsil to the BO line was defined as negative or positive values when the tonsils lied above or below the BO line respectively.

RESULTS:
In all the healthy controls, the tip of the cerebellar tonsil was found to be above the BO line, with a mean distance of -3.87 ± 2.95 mm. In the AIS groups, inferior displacement of cerebellar tonsil below the BO line was found in 4 of 135 patients with Cobb's angle less than 45 degrees (3.0%), and in 8 of 29 patients with Cobb's angle above 45 degrees (27.6%).

CONCLUSION:
This study has demonstrated that in adolescents, inferior displacement of the cerebellar tonsils below the BO line from standardized MRI measurements should be regarded as tonsilar herniation and that the currently used normal adult reference level of +5mm is not applicable in adolescents. This has important clinical etiological and prognostic significance since using the new definition, a significant number of patients with AIS would fall into the abnormal tonsillar herniation group.