The fetal head is abnormal in most fetuses with meningomyeloceles. The posterior fossa is small, and the cerebellum is compressed against the occiput, obliterating the cisterna magna. During the second trimester, the small cerebellum is hypoechoic and curved around the cerebellar peduncles, with an appearance called the “banana” sign (Figure 5.1.4). Later in pregnancy, the cerebellum is larger and more echogenic, although it is still compressed against the occiput, and no fluid will be seen in the region of the cisterna magna.
Hydrocephalus, with dilation of the lateral ventricles, is present in the majority of cases of meningomyelocele and the Chiari II malformation.
During the second trimester, the cranium of most fetuses with meningomyeloceles has an abnormal contour termed the “lemon” sign. This is characterized by flattening or concavity of the frontal bones (Figure 5.1.5). By the third trimester, the cranium is more ossified and develops a normal oval configuration.
The spinal defect with meningoceles is typically smaller than with meningomyelocele, as is the dorsal sac. The sac of the meningocele is seen as a cystic lesion protruding posteriorly from a narrow opening in the spine (Figure 5.1.6). The posterior fossa appears normal, because the Chiari II malformation is not present.
5.2. Hemivertebra
Description and Clinical Features
A hemivertebra is an anomalous vertebra in which part or all of one side of the vertebral body and posterior elements is absent. The result is a kink or bend in the spine at the affected level, causing scoliosis. The anomaly can affect a single or multiple vertebrae. Affected vertebral bodies have a wedge shape. Associated anomalies of the skeletal system are common, most often involving the ribs and extremities. In addition, hemivertebra is a component of various syndromes with anomalies affecting many organ systems, including the cardiovascular, gastrointestinal, central nervous, and genitourinary systems. The prognosis depends on the degree of severity of the associated anomalies.
Sonography
On ultrasound, a hemivertebra is seen on a coronal view of the spine as a disruption of the normally paired posterior ossification centers. An extra posterior ossification center will be seen on one side, without a matching ossification center on the other (Figure 5.2.1). In addition, a kink or bend in the spine will be identified at that level. Hemivertebrae are often well visualized using three-dimensional (3D) ultrasound with skeletal settings (Figure 5.2.2).