Evaluation of Fetal First and Second Cervical Vertebrae: Normal or Abnormal?

ABSTRACT

OBJECTIVES: To use 3-dimensional sonographic volumes to evaluate the variable appearance of the normal fetal cervical spine and craniocervical junction, which if unrecognized may lead to misdiagnosis of malalignment at the first and second cervical vertebrae (C1 and C2). METHODS: Three-dimensional sonographic volumes of the fetal cervical spine were obtained from 24 fetuses at gestational ages between 12 weeks 6 days and 35 weeks 1 day. The volumes were reviewed on 4-dimensional software, and the vertebral level was determined by labeling the first rib-bearing vertebra as the first thoracic vertebra. The ossification centers of the cervical spine and occipital condyles were then labeled accordingly and evaluated for alignment and structure by rotating the volumes in oblique planes. The appearance on multiplanar images was assessed for possible perceived anomalies, including malalignment, particularly at the C1 and C2 levels. Evidence of head rotation was correlated with the presence of possible malalignment at C1-C2. Head rotation was identified in the axial plane by measuring the angle of the anteroposterior axis of C1 to the anteroposterior axis of C2. RESULTS: Of the 24 fetuses, 16 had adequate quality to assess the entire cervical spine and craniocervical junction. All 16 cases showed an osseous component of C1 that did not align directly with C2 on some of the multiplanar images when the volumes were rotated, which could lead to suspected diagnosis of spinal malalignment or a segmental abnormality, as occurred in 2 clinical cases in our practice. All 16 cases showed at least some degree of head rotation, ranging from 2° to 36°, which may possibly explain the apparent malalignment. The lateral offset from C1 to C2 ranged from 0.0 to 3.3 mm. CONCLUSIONS: The normal C1 and C2 ossification centers may appear to be malaligned due to normal offsetting (lateral displacement) of C1 on C2. An understanding of the normal development of the cervical spine is important in assessing spinal anatomy.