Spina bifida results from abnormal development of the neural tube, with failure of the posterior vertebral arches to close and form the bony sheath that surrounds the spinal cord. Often the defect extends through the skin, in which case the maternal serum α-fetoprotein level is typically elevated. When the meninges are exposed or protrude through the spina bifida defect, the abnormality is termed a meningocele. When nerve roots or spinal cord are also exposed or protrude through the spina bifida defect, the abnormality is a meningomyelocele. A dorsal sac is present when these protruding tissues extend posteriorly through the defect in the spine and skin. The spinal defect can occur at any level but most commonly affects the lower lumbar and/or sacral spine.

Meningomyeloceles are almost always accompanied by the Chiari II malformation in the fetal head, characterized by obstructive hydrocephalus, a small posterior fossa, effacement of the cistern magna, an enlarged foramen magnum, and downward herniation of the cerebellar tonsils and vermis. Ventriculomegaly is often present in utero, and obstructive hydrocephalus usually requires shunt placement after birth. The posterior fossa is virtually always abnormal in utero, with the cerebellum compressed against the occiput in a small posterior fossa.

The incidence of spina bifida with meningomyelocele is approximately 1 to 4 per 1,000 births, but the incidence varies with geographic locations and race. Folic acid deficiency in early pregnancy has been identified as a risk factor. Most cases of spina bifida with meningomyelocele can be detected by screening pregnant women for elevated levels of maternal serum α-fetoprotein.

The prognosis for meningomyelocele depends on the level of the spinal defect. The higher the level, the greater the neurologic deficit in the lower body. In addition, many affected children have intellectual impairment, especially if there is ventriculomegaly at birth.

With meningoceles, the spinal defect is typically small, and the prognosis is better than for meningomyeloceles, because neurologic deficits are uncommon and the Chiari II malformation of the posterior fossa is not present. These lesions are not usually detected by screening tests, because meningoceles are often covered by skin, and the maternal serum α-fetoprotein levels may be normal.

When spina bifida with meningomyelocele is present, sonographic abnormalities are usually visible in both the fetal head and the spine. On a longitudinal view of the spine at the affected level, the posterior ossification centers are typically disrupted and a cystic mass, representing the dorsal sac, may be seen protruding posteriorly at the site of the defective posterior arch (Figure 7.1.1). On a transverse view, the posterior ossification centers are splayed apart and diverge. The dorsal sac, when present, protrudes posteriorly and usually contains fluid and solid material, representing nerves and meninges (Figure 7.1.2). In the absence of a dorsal sac, the defect appears concave with amniotic fluid extending into the region of the spinal canal (Figure 7.1.3).

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 7.1.4). Later in pregnancy, the cerebellum is larger and no longer has a banana shape, but it is still compressed against the occiput and no fluid will be seen in the cisterna magna.

Ventriculomegaly, characterized by dilation of the lateral ventricles, is present in the majority of cases of meningomyelocele and the Chiari II malformation prior to birth (Figure 7.1.5).

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 7.1.6). By the mid to latter part of the third trimester, the cranium is more ossified and develops a normal oval configuration.