Fetal Surgery for Open Neural Tube Defects

Rationale for Fetal Repair

Although there is large spectrum of abnormalities observed in children with myelomeningoceles, it is useful to separate the neurologic deficits into two groups: primary and secondary. The primary neurologic deficits are those directly caused by the arrested development of the neural placode, which usually occurs in the lumbosacral region. Because neural tube closure occurs during the third and fourth weeks of gestation, the spinal cord in this region is very immature at the stage when a myelomeningocele develops. Although the structure of the spinal cord is severely disrupted at the involved level, it is unknown whether the placode is capable of further development. The functional neurologic level is either at the same level as the vertebral anomaly, or actually higher than the vertebral level, resulting in worse neurologic function, in more than 80% of patients with open forms of spina bifida.

The secondary neurologic deficits in patients with spina bifida include delayed loss of motor function, worsening bowel and bladder control, and other structural anomalies such as scoliosis. Other associated conditions, such as a Chiari II malformation, spinal cord syrinx, tethered spinal cord, and hydrocephalus, can all worsen neurologic function. Magnetic resonance imaging (MRI) studies of most myelomeningocele lesions following repair show a dysplastic spinal cord terminating in the overlying soft tissues at the site of the repaired defect. For this reason, virtually all patients with myelomeningocele have a tethered spinal cord by radiologic criteria. However, it is not clear why some patients with a myelomeningocele have either minor or no symptoms of a tethered spinal cord.

The theoretical advantage of fetal repair for myelomeningoceles is that the neural tube is covered and protected many months before the expected delivery date. The basis for expecting improved neurologic function is that restoration of the dysplastic neural placode within the spinal canal isolates it from the amniotic fluid and prevents ongoing injury. ^, Mueli and others surgically created a spinal-cord lesion in fetal sheep at 75 days of gestation that simulated a spina bifida lesion. After delivery at term, the gross and microscopic appearance of the exposed spinal cord resembled a human spina bifida lesion, and the animals were incontinent and had loss of sensation and motor function below the lesion level. One group of animals with surgically created spina bifida lesions were treated using a myocutaneous flap at 100 days of gestation, and these animals had near-normal motor function and normal bowel and bladder control after delivery at full term. Although provocative, these large animal experiments clearly rely on a model system that has distinct differences with the human disease.

Timing for Fetal Surgery

If closure of an open neural tube defect reduces secondary injury occurring to the placode, then surgical intervention should be performed as early as possible. In practice, surgical timing is determined by diagnosis and technical limitations of the actual procedure. Most myelomeningoceles are detected during the second trimester, either during an investigation of a positive maternal screening test, or during a routine ultrasound study. The quality of current ultrasonography allows detection of most fetuses with myelomeningoceles by the midportion of the second trimester. From a practical viewpoint, this means that a diagnosis is made between 18 and 22 weeks of gestation. Taking into consideration current obstetrical practice, it is unlikely that detection of fetuses with spina bifida will occur any earlier unless new, more sensitive screening tests are discovered.

Preoperative fetal imaging studies usually begin with a detailed ultrasonogram ( Fig. 73.1A ). This study is able to determine some anatomic features with great precision. These include size of the overlying sac, the level of the defect, the position of the cerebellar tonsils, and the presence of lower-extremity deformities. Limitations include difficulty determining some associated brain anomalies, other intraspinal anomalies, and at times, the exact dysraphic level. Most patients being considered for fetal surgery will undergo a fetal MRI study ( Fig. 73.1B ). Because of motion, images in sagittal, axial, and coronal planes are obtained randomly by repeatedly imaging the fetus over time. The preferred MRI technique is a single-shot, fast spin-echo T2-weighted sequence. There is some evidence that MRI may improve the ability to detect coexisting spinal and brain anomalies that may not be apparent on ultrasound studies. ^,