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.