Diagnosis of Spina Bifida and Other Dysraphisms in the Fetus

Fetal Head

On two-dimensional ultrasound, the views of interest for initial screening and diagnosis of spina bifida and other neural tube defects are the transventricular and the transcerebellar views ( Figure 12-1 A, B ).

On the transventricular view, the secondary ventriculomegaly often associated with spina bifida and the frontal bone scalloping (‘lemon sign’) ( Figure 12-1C ) are diagnosed with the Arnold–Chiari II malformation (‘banana sign’) recognized on the transcerebellar view ( Figure 12-1D ). The degree of descent of the cerebellum may be significant, and due to shadowing from the occipital bones, the cerebellar abnormality may only be seen with a transvaginal (TV) scan ( Figure 12-2 ).

It is only with three-dimensional ultrasound that the prenatal assessment of cranial bones, in the normal and abnormal fetus, has become reproducible and adequate ( Figure 12-3 ). This imaging modality allows:

• detailed evaluation of the fetal brain;

• neat characterization of cranial bones, sutures and fontanelles.

In this way, both delayed ossification or premature closure of the fontanelles and/or sealing of the sutures (synostosis) has become readily recognizable in utero. This modality can therefore be advantageously employed to also characterize the bony component in encephalocoeles.

Fetal Spine

The sonographic assessment of the fetal spine depends on the visualization of the ossification centres of the fetal vertebral bodies in the coronal, sagittal and transverse planes.

Each vertebral body has three ossification centres: one in the body and one at the base of each transverse process ( Figure 12-4 ).

Ossification of the vertebral body (also known as the centrum) begins at the thorocolumbar junction and progresses in cephalic and caudad directions.

There may be at least three initial centres of ossification for the neural arch with ossification progressing cranially and caudally from each. Within the lumbar region ossification progresses in a caudad direction, possibly more quickly in females.

Within the neural arches of the lumbosacral region ossification progresses caudally at a rate of one vertebral level every 2 to 3 weeks after 16 weeks ( Figure 12-5 A and B ; Table 12-2 ).

The importance of recognizing this normal evolution is that too early a scan may not be able to exclude a neural tube abnormality in fetuses at high risk for a neural tube defect. The technique for examination must take into account fetal gestational age as discussed and fetal position.

Axial View.

The three ossification centres will appear as three dots of bone. The two posterior elements should converge towards each other to form an arch. However the normal arching of the posterior ossification centres is far better appreciated in the prone fetus with its back to the transducer ( Figure 12-6 ). This view also allows for assessment of the integrity of the overlying skin.

Pitfall 1.

The fetus lying decubitus will appear to have posterior elements that are in parallel to each other ( Figure 12-7 ).

Pitfall 2.

Splaying of the posterior elements in the lumbosacral region will be produced by angling too obliquely when attempting to image axially ( Figure 12-8 ). This effect may be exaggerated by side lobe artefact producing an apparent meningocoele.

Coronal View.

In this view the three ossification centres give a three line appearance. The inter-pedicular distance is essentially constant throughout the spine ( Figure 12-9 ). This view also allows the level of a defect to be determined as the 12th rib will be seen.

Pitfall 3.

There is a normal physiological widening in the upper spine before the cranio-occipital junction and at the lumbosacral spine just prior to the normal tapering towards the sacrum ( Figure 12-10 A and B ).

Sagittal View.

This view demonstrates two bony structures: the ossification centre of the vertebral body and also the junction of the posterior ossification centres. The all-important integrity of the overlying skin covering will be visualized.

This view will also demonstrate the normal hypo-echoic spinal cord tapering to the level of the upper sacrum and its continuity with the echogenic cauda equina ( Figure 12-11 ).

Pitfall 4.

A true sagittal view of the lower sacrum may only demonstrate the ossification centre of the body due to an insufficiently ossified neural arch.

Pitfall 5.

Splaying in the thoracic region will be mimicked by imaging the ribs and not the ossification centre of the transverse process.

Delayed Ossification of the Fetal Spine.

The sacrum may show incomplete ossification prior to 25 weeks' gestational age ( Figure 12-12 ).

3D Fetal Spinal Ultrasound.

Three-dimensional imaging is able to provide the sonologist with even more self-explanatory images of the fetal spine and rib-cage. In most cases a transabdominal approach suffices, but in obese women and in those in which a breech fetal lie is obscuring a more direct approach to the spine, transvaginal scanning is able to provide dramatic images of spinal anatomy, especially if three-dimensional ultrasound is used.

The main difference between two and three-dimensional ultrasound is that with three-dimensional ultrasound a volume dataset of the whole spine is acquired. After acquisition, the operator can choose to navigate through this volume, applying in post-processing a series of rendering modalities to highlight the skin contour, the bony skeleton and to render the coronal plane. This, in turn, determines a higher degree of accuracy in reaching the final diagnosis and in its characterization both from a diagnostic and a prognostic standpoint ( Figure 12-13 ).

Iniencephaly

Iniencephaly is included here as it has features of both a cranial and a spinal lesion.

• This is a rare abnormality with a prevalence of 0.1–10:10,000 births. It has a pronounced female predilection F9: M1.

• This is a rare dysraphic malformation which has three major primary defects: a deficient occiput and inion combined with rachischisis of the cervical and thoracic spine with retroflexion of the head.

• The exact aetiology is unknown, but seasonal and yearly variations exist. Other contributing factors include low parity and low socioeconomic status.

The diagnostic criteria for iniencephaly are:

• A variably sized deficit of the occipital bones resulting in an enlarged foramen magnum ( Figure 12-14 ).

  

• Partial or total absence of several cervical and thoracic vertebrae with incomplete closure of the vertebral arches and/or bodies. Any vertebrae that are visible are often abnormally fused to each other.

• Significant shortening of the spinal column due to marked lordosis and hyperextension of the malformed cervical–thoracic spine.

• The profound fixed hyperextension and lack of a neck will result in an upward-turned face and mandibular skin directly in continuity with the skin of the chest ( Figures 12-15 and 12-16 ) and the fetal occiput will be adjacent to the thoracic spine ( Figure 12-16 ).

  
  

There are almost always (84% of cases) additional abnormalities in this invariably fatal defect.

Open Spinal Dysraphisms

Spinal cord malformations are collectively referred to as spinal dysraphisms. They are generated from defects occurring in the early embryologic stages of gastrulation (weeks 2–3), primary neurulation (weeks 3–4), and secondary neurulation (weeks 5–6).

Spina Bifida

Incidence: 1 in 1000 at birth. There is a higher prevalence in Whites and in Hispanics than in African-Americans and Asians. The birth prevalence of spina bifida is slowly decreasing due to the increased prenatal detection rate.