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KEY POINTS

KEY POINTS

  1. Sonographic evaluation of the fetal vertebral column is considered an integral part of the routine second-trimester ultrasound examination.

  2. Starting early in pregnancy it is possible, in selected cases, to ascertain some vertebral anomalies. Still, some of these anomalies can remain undiagnosed even later.

  3. 3D ultrasound is increasingly used and has become instrumental in imaging the vertebral column in the three orthogonal planes using the x-ray or maximum mode rendering.

  4. MRI may at times be necessary to complete the workup.

INTRODUCTION

The assessment of the fetal vertebral column by trans­abdominal ultrasound (US) is an essential part of the second-trimester anatomical scan. The possibility of evaluating the spine at as early as 13 weeks of gestation by transvaginal high-resolution US has greatly improved its diagnostic capabilities and enabled the diagnosis of some of the common vertebral anomalies at this stage of pregnancy. Because not all spinal anomalies can be diagnosed during the late first or early second trimester, transabdominal US continues to be the “everyday” tool in the diagnosis of spinal and spinal cord malformations.1,2 One example of this is the tethered cord that may be visible, in some but not all cases, from the mid-second trimester as the conus medullaris becomes more clearly defined.

The introduction of three-dimensional (3D) US with minimum intensity projection, or radiograph mode, contributes to projecting and locating the exact level of the malformation.3–5 The additional value of this technology is still controversial, and the projection of a 3D acquisition of the spine is not always reassuring, as it may miss small lesions in the lower spine.6

Magnetic resonance imaging (MRI) has been proposed as an additional tool in the diagnosis and counseling of patients with spinal cord anomalies, particularly regarding the evaluation of the spinal canal and its contents. So far, this technology has not been widely used, but initial reports appear promising.7–10

DEVELOPMENT OF THE SPINE AND SPINAL CORD

Vertebral development starts during gastrulation (week 2–3) with the formation of the notochord; further developmental events include primary and secondary neurulation.11,12 Each vertebral body has two primary ossification centers, one dorsal and one ventral. These centers fuse to form the centrum, which creates three primary ossification loci by the end of embryonic development; one develops in the vertebral body and the other two on each half of the vertebral arch.11 According to US studies, by 16 postmenstrual weeks, S1 and S2 ossification centers are almost all visualized in coronal planes, but S3 is present in only about half of these fetuses; the nucleus of S4 develops slightly later and is observed in all fetuses at 21 weeks.13 The posterior arch synchondrosis is not ossified in fetuses.

Closure of the neural tube seems to begin separately at several different levels. Progressive ...

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