Hypoplastic posterior arch of C-1 in children with Down syndrome: a double jeopardy.

Radiographs of 38 children with Down syndrome (aged 2-3 years) were retrospectively evaluated for hypoplasia of the posterior arch of the C-1 vertebra. The anteroposterior dimensions of the posterior arch of C-1 were measured and compared with those of an age- and sex-matched control group. The posterior arches of C-2 to C-5 were similarly evaluated. Radiographs demonstrated hypoplasia of the posterior arch of C-1 in 26%. This anomaly of C-1 was an isolated event; sagittal diameters of C-2 to C-5 were within normal limits compared with those of the control group and published standards. Because of the known, potentially devastating atlantoaxial dislocation in Down syndrome, the increased prevalence of a second C-1 anomaly causing preexistent narrowing of the vertebral canal is of clinical concern. A hypoplastic posterior arch of C-1 may amplify the risk of spinal cord damage following atlantoaxial subluxation, as the subsequently smaller vertebral canal has less room for dorsal movement of the odontoid process. This potential "double jeopardy" of C-1 vertebral abnormalities should be recognized and assessed on cervical spine screening radiographs in children with Down syndrome.