PURPOSE

To characterize natural history and early changes of craniovertebral junction stenosis in achondroplasia correlating with clinical and radiological outcome.

METHODS

Retrospective measures on craniovertebral junction were performed blindly, on sagittal T2-weighted images, in 21 patients with achondroplasia referred from 2008 to 2020. Clinical and polysomnography data were retrospectively collected. Each patient was paired for age and gender with four controls. Wilcoxon means comparison or Student's t-tests were applied.

RESULTS

Twenty-one patients (11 females, from 0.1 to 39 years of age) were analyzed and paired with 84 controls. A craniovertebral junction stenosis was found in 11/21 patients (52.4%), all before the age of 2 years. Despite a significant reduction of the foramen magnum diameter (mean ± SD: patients 13.6 ± 6.2 mm, controls 28.5 ± 4.7 mm, p < .001), craniovertebral junction stenosis resulted from the narrowing of C2 dens-opisthion antero-posterior diameter (8.7 ± 3.9 mm vs 24.6 ± 5.1 mm, p < .001). Other significant changes were opisthion anterior placement (-0.4 ± 2.8 mm vs 9.4 ± 2.3 mm, p < .001), posterior tilt of C2 (46.2 ± 13.7° vs 31.6 ± 7.9°, p < .001) and of C1 (15.1 ± 4.3° vs 11.9 ± 5.0°, p = 0.01), and dens thickening (9.4 ± 2.2 mm vs 8.5 ± 2.1 mm, p = 0.03), allowing to define three distinguishable early craniovertebral junction patterns in achondroplasia. All children with C2-opisthion antero-posterior diameter of more than 6 mm had a better clinical and radiological outcome.

CONCLUSION

Craniovertebral junction in achondroplasia results from narrowing between C2 dens and opisthion related to anterior placement of opisthion, thickening of C2 dens, and posterior tilt of C1-C2. A threshold of 6 mm for dens-opisthion sagittal diameter seems to correlate with clinical and radiological outcome.