The prevalence of congenital C1 arch anomalies

To determine the prevalence, radiographic features and reporting rate of, and the association between the congenital anterior and posterior C1 arch anomalies.

The computed tomography (CT) images of the cervical spines of all patients over 18 years who had CT examinations in our hospital during the study period were reviewed to evaluate for congenital anomalies of the anterior and posterior C1 arches. Radiology reports of the corresponding CT examinations were reviewed to determine the reporting rate of these defects.

Of 3273 subjects, 185 (5.65%) had congenital atlas anomalies: 169 isolated posterior (5.16%), 15 combined anterior and posterior (bipartite, 0.46%), and one isolated anterior (0.031%) arch defects. Females had a higher prevalence than males (7.46 versus 4.72%, P = 0.0013). Eighty-three cases (44.9%) of C1 arch anomalies were not reported. The Currarino type A, B, C and E posterior arch defects accounted for 81.6, 8.1, 1.1, and 0.5% of all arch anomalies while type D was not observed. Fifteen patients (0.46%) had combined anterior and posterior arch anomalies (bipartite atlas) versus only one with an isolated anterior C1 defect, indicating a significant association between the anterior and posterior arch defects (P < 0.0001).

Although some types of congenital C1 arch anomalies are rare, type A defects are relatively common radiological findings that are unreported approximately 45% of the time. Based on the significant association between the anterior and posterior arch defects, we propose possible mechanisms for the formation of the bipartite atlas.