Redefining the magnetic resonance imaging reference level for the cerebellar tonsil: A study of 170 adolescents with normal versus idiopathic scoliosis

Study Design. Magnetic resonance imaging (MRI) was used to compare quantitatively the position of the cerebellar tonsil in neurologically normal adolescents with that in idiopathic scoliosis (AIS) patients and age-matched healthy controls. Objectives. To redefine the normal reference level of the cerebellar tonsil in healthy adolescents and to compare the differences in tonsillar level with AIS of different severities. Summary of Background Data. Asymptomatic Chiari I malformation has been reported not infrequently in children and adolescents with AIS. The definition of tonsillar herniation and its variations with sex, age, and curve severities have not been clarified. Methods. Sagittal MRI of hindbrain was performed on 170 adolescents, including 117 patients with AIS and 53 age-matched healthy controls. Results. The cerebellar tonsillar tip in healthy patients was found at an average of 2.8 mm above the basionopisthion reference line (BO line) (range, 0-10.5 mm), in contrast to the finding that 17.9% of the AIS patients had tonsillar tip below the BO line. The position of tonsil in the AIS group was lower than that in the healthy controls (P< 0.05), and the differences also increased with the severity of the curve (P = 0.027). Conclusions. Using the current MRI reference standards, the incidence of tonsillar herniation could be significantly underestimated. According to our experience, any inferior displacement of a tonsil below the BO line in adolescents should be regarded as abnormal. Scoliosis could be an important manifestation of subclinical tonsillar herniation.