We studied the radiographs of thirty-two patients with Down syndrome for
evidence of atlanto-axial instability. One of the patients had instability
in 1970 and seven had it in 1983. The interval between the atlas and the
odontoid process in the patients who demonstrated motion at that interval
radiographically averaged 2.78 millimeters in 1970 and 6.93 millimeters in
1983 (p less than 0.005). Four patients whose radiographs showed
atlanto-axial motion in 1970 lost that motion by 1983, and in seven
patients who did not show atlanto-axial instability in 1970 it developed by
1983. Atlanto-axial instability was more likely to develop in boys who were
more than ten years old. Accessory upper-cervical ossicles became evident
in three patients, none of whom had atlanto-axial motion. However, one of
these three patients had an abnormally wide atlanto-axial interval.