Reproducibility in the measurement of atlanto-occipital instability in children with Down syndrome

Study Design. This study compared the reproducibility of three techniques used to measure translation between the occiput and C1 in children with Down syndrome. Objectives. Intraobserver and interobserver variability were computed to determine if there is a reliable way to measure occiput-C1 instability. Summary of Background Data. No studies have been performed comparing measurement techniques in children with Down syndrome. Methods. Powers ratios, basion-axial intervals, and translational anteroposterior motion as described by Wiesel and Rothman were calculated for 60 pairs of flexion-extension lateral cervical radiographs from children with Down syndrome. Calculations were made on two occasions by four reviewers. Results. Mean differences between measurements for two observers were 0.14 for the Powers ratio, 1.3 mm for the Wiesel technique, and 1.8 mm for the basion- axial interval. The 95th percentiles of the differences between measurements for two observers were 0.38 for Powers ratios, 3.5 mm for the Wiesel technique, and 5.3 mm for the basion-axial interval. Conclusions. Measurement of atlanto-occipital translation by any of these methods is not reproducible. Although the technique by Wiesel and Rothman is the easiest to apply, confirmation of instability with magnetic resonance imaging should guide management.