Effect of upright position on tonsillar level in adolescent idiopathic scoliosis.

PURPOSE: The aim of this study was to investigate the effect of an upright position on cerebellar tonsillar level in patients with adolescent idiopathic scoliosis (AIS).
METHODS: Twenty-five patients with clinically diagnosed AIS and 18 normal controls were examined in both supine and upright positions using 0.25T MRI. The position of the inferior cerebellar tonsil tip relative to a reference line connecting the basion to the opisthion (BO line) was measured in millimetres.
RESULTS: None of the 18 normal control subjects had cerebellar tonsillar descent below the BO line in either supine or the upright position. Forty-eight percent of AIS patients had tonsillar descent in the upright position, compared to 28 % in the supine position. In the upright position, cerebellar tonsillar position was lower in AIS patients than in normal subjects (mean -0.7 ± 1.5 vs. +2.1 ± 1.7, p < 0.00001). AIS patients also had a large degree of tonsillar excursion between upright and supine positions compared to normal subjects (mean -1.9 ± 2.3 vs. -0.1 ± 0.2, p < 0.00001).
CONCLUSIONS: When considering the theoretical likelihood that a low tonsillar position may affect spinal cord function, one should bear in mind that tonsillar descent in AIS is significantly greater in the upright position. KEY POINTS: • AIS patients exhibited greater cerebellar tonsillar descent in upright than supine position. • Cerebellar tonsillar position was lower in AIS patients than normal subjects. • AIS patients exhibited greater tonsillar excursion between supine and upright positions.