Cervical-spine instability in children with Down syndrome (trisomy 21).

Eighty-five children with Down syndrome, between sixteen months and eighteen years old, were evaluated for instability of the cervical spine at the atlanto-axial joint. The mean atlas-odontoid process interval was three millimeters in flexion and two millimeters in extension. Ten patients (12 per cent) exhibited abnormal intervals (4.5 millimeters or more) during either flexion or extension. The configuration of the odontoid process was considered normal in eighty patients and abnormal in another five patients (6 per cent). The correlation between the thickness of the interval and the degree of ligament laxity was statistically significant, as was the correlation between ligament laxity and age. Of the ten patients with an increased atlas-odontoid process interval, neurological deficit (hyperreflexia and clonus) developed in only one after a one-year follow-up.