What Is an Open Neural Tube Defect and What Are Its Physiologic Effects?

Open neural tube defects (NTDs) are a group of congenital defects that occur due to failure to close the neural tube. This leads to the exposure of neural tissue to the amniotic fluid with subsequent neurodegeneration due to the persistent exposure. This exposure leads to neuronal cell death and loss of axonal connections. The most common form of open NTDs is a MMC in which the spinal cord is exposed and may be covered by the meningeal sac. This most commonly occurs in thoracolumbar, lumbar, and lumbosacral areas. The open defect leads to abnormal neurologic function at and below the level of the lesion with subsequent sensory and motor deficits such as neurogenic bowel and bladder dysfunction. More severe forms of open NTD are incompatible with life. Spinal dysraphism, such as spina bifida and tethered cord, refers to a NTD in which normal skin covers the underline of the lesion.

What Is the Incidence of NTDs and What Are the Causes?

NTDs have an approximate worldwide incidence of 1 in every 1,000 pregnancies. Although the incidence of most open NTDs is higher in females, MMC defects have an equal incidence in males and females. The causes of NTDs are multifactorial. Most of the cases are sporadic. A genetic predisposition is present in some cases with the involvement of genes that regulate folate metabolism. Environmental issues such as low folate levels or the presence of diabetes or prenatal exposure to valproic acid prenatally increases the incidence. Although some markers such as maternal alpha fetoprotein can be useful as diagnostic screening, pre-natal ultrasound during the first trimester is the gold standard to confirm the diagnosis.

What Anomalies Are Associated with NTDs

NTDs are frequently associated with other abnormalities such as skeletal, genitourinary, and gastrointestinal abnormalities. The most common associated defect with prenatal progression of a MMC is the development of a Chiari type II malformation with hindbrain herniation and hydrocephalus. This prenatal progression to a Chiari type II malformation occurs in 75% of cases and requires a VP shunt in most cases. Neurogenic bladder is also significant with 83% of patients requiring intermittent catheterization. Postnatal orthopedic repairs may also be necessary for congenital taloequinovarus (CTEV). Given the multiple medical problems, these patients require a multidisciplinary approach to management.

What Public Health Intervention Has Been Important for the Prevention of NTDs?

A worldwide primary prevention campaign has been implemented with the supplemental administration of folate to reduce the incidence of NTD. Women planning a pregnancy receive 0.4 mg of folic acid per day and women with a high risk of NTD receive 4–5 mg a day. The underlying mechanism of the preventive effect of folic acid is related to DNA replication to facilitate cell proliferation, as well as DNA methylation involving gene expression. It is estimated that even with appropriate pre-gestational administration NTDs still occur in 0.7–0.8 cases per 1,000 pregnancies.

What Are the Surgical Options for the Repair of Open NTDs?

There are three approaches to surgical management: open fetal surgery, endoscopic/fetoscopic surgery, and postnatal repair of the NTD. Parents should receive detailed information about the pros and cons of the different approaches based on the best evidence to facilitate the decision-making process. The open fetal surgery involves a transverse (Pfannenstiel) or vertical incision with the performance of a hysterotomy with fetal exposure of the defect and closure of the defect under direct visualization. The fetoscopic repair requires the exteriorization of the uterus without the performance of a hysterotomy. Instead two ports are inserted into the uterus and the amniotic fluid is partially replaced by insufflated CO₂. The postnatal repair occurs in the first 24 hours after birth to decrease the risks of infection into the central nervous system. Postnatal care requires frequent monitoring with head ultrasounds to detect worsening hydrocephalus given the high frequency of these patients requiring cerebrospinal shunting.

What Are Important Considerations During the Preanesthetic Evaluation for Patients Undergoing Prenatal Repair of an NTD?

Patient selection is critical given the potential maternal risks and the potential benefits of the procedure to the fetus. The first consideration is for maternal safety such that serious and uncompensated comorbidities preclude the performance of a fetal procedure. During the physical examination, emphasis must include the evaluation of the airway and the examination of the spine.

The description of the NTD and the surgical fetal planning has evolved. Therefore, it is important to review the studies preoperatively including fetal ultrasonography and fetal MRI. Specific points of utility to the anesthesiologist include the extension of the NTD defect, the dynamic evaluation of the motor function of the lower extremities, any associated fetal anomalies, the estimated fetal weight, the presence of a Chiari II malformation, the presence and severity of hydrocephalus, and the placenta location. Any important associated fetal anomalies or abnormal karyotype precludes fetal intervention. Another important consideration during the preoperative evaluation is a multidisciplinary approach that includes members of the fetal maternal team, a neonatologist, a neurosurgeon, a pediatric anesthesiologist with experience in fetal surgery, a pediatric surgeon, operating room nurses, and a social worker.