Lateral cervical spine x-ray demonstrates atlantoaxial instability (AAI) in Down syndrome. In this case, mild flexion of the neck demonstrates a widened atlanto-dental interval (solid bar) between the anterior tubercle of C1 (long arrow) and the odontoid process that exceeds the pathological threshold in the adult of 3 mm. The spinal canal (small arrow) is substantially diminished and results in spinal cord compression