Management of Myelomeningocele and Related Disorders of the Newborn

Introduction

Open spina bifida aperta, or open myelomeningocele (MMC), is a midline vertebral defect that may occur anywhere along the spinal column, resulting in exposure of the contents of the spinal column to the outside environment. It is the most common nonlethal congenital birth defect of the central nervous system, occurring in approximately 3 to 4 per 10,000 live births in the United States annually. Improved treatments have helped decrease mortality, but long-term morbidity exists secondary to medical concerns including hydrocephalus, Chiari II malformations, spinal cord tethering, neurogenic bowel and bladder, and orthopedic abnormalities. Consequently, children with spina bifida require complex multispecialty care to help maximize their quality of life and prevent serious long-term sequelae.

Pathophysiology

Open neural tube defects (NTDs) are caused by a spontaneous failure of neurulation occurring within the first 3 to 4 weeks of gestation. There is likely a multifactorial etiology based in both genetic predispositions and environmental influences. Up to 16% of NTDs may be associated with a chromosomal anomaly ( Table 54.1 ). Genes that seem to be associated with NTD include the retention and metabolism of folate and vitamin B ₁₂ , the methylation cycle and transulfuration, glucose transport and metabolism, oxidative stress, retinoid metabolism, transcription factors, and DNA repair. Folate (vitamin B ₆ ) has been shown to play a particularly important role. In the 1980s, folate use was shown to play a critical role in a methylation cycle. Because the effects on the methylation cycle occur before a pregnancy is detected, public health efforts were initiated to introduce folate fortification for the general public. Although this reduced some degree of the incidence of NTDs between the years 1984 and 1994, the incidence has remained 0.7 to 0.8 per 1000 live births, with some regional variation, since 2004. ^, In the United States, this ranges between 0.3 and 1.43 per 1000 live births.

Spina bifida occulta encompasses a variety of etiologies that result from a malformation of the midline dorsal neural, mesenchymal, and cutaneous ectodermal structures during embryogenesis. Unlike spina bifida aperta, the dorsal dysraphism is skin covered and does not require immediate intervention at the time of delivery. Although there is a wide array of etiologies ( Table 54.2 ), if they cause clinical symptoms it is typically due to impairment through tethering of the spinal cord, neuronal compression, or myelodysplasia. Symptoms may develop over time due to tethering, which can cause progressive symptoms through tension on the spinal cord, leading to vascular abnormalities. However, this clinical course can vary drastically. Progressive symptoms are sometimes responsive to intervention, although some patients have baseline neuronal deficits due to the aberrant formation of the neural placode during development. Even after the initial surgical repair, progressive symptoms of tethering may recur in a delayed fashion. For the purposes of this chapter, we will primarily describe spina bifida aperta, as it is most often dealt with during the acute perinatal period.

Spinal development occurs during three basic embryologic stages: gastrulation, primary neurulation, and secondary neurulation. Gastrulation occurs during the second or third week of development and involves the differentiation of the embryonic disc into the ectoderm, mesoderm, and endoderm. During primary neurulation, which occurs during the third and fourth week of development, the notochord and overlying ectoderm then form the neural plate. This then folds to form the neural tube and closes in a bidirectional, zipper-like manner. Secondary neurulation, which occurs during weeks 5 to 6 of development, thus further differentiates the neuronal tissue and undergoes cavitation to form the conus medullaris and filum through retrogressive differentiation. Typically, spina bifida aperta occurs when the caudal end of the notochord incompletely fuses and leads to a persistent and exposed neural placode.

Clinical Features

Clinical manifestations of open spina bifida occur due to the initial primary failure of neuronal closure and aberrant nervous tissue formation as well as secondary insults to the exposed nervous tissue. The original neural placode can have relatively normal neuronal tissue. However, throughout gestation and exposure to the amniotic fluid, the exposed nervous tissue may become hemorrhagic and die. Spina bifida aperta likely has associated brain malformations and hydrocephalus due to the effects of ongoing exposure of the placode and loss of cerebrospinal fluid (CSF).

Chiari II malformations compose the majority of brain anomalies associated with MMCs. Ongoing development with a surrounding small posterior fossa as well as ongoing CSF loss caudally leads to herniation of the cerebellum through the foramen magnum. This is often associated with distortion of the midbrain, or tectal beaking, in 65% of infants. The medulla can additionally be elongated and kinked at the spino-medullary junction for 70% of infants. Other brain anomalies include underdevelopment of the corpus callosum in up to 50% of infants. This suggests that some disruption of neuronal migration typically occurs in the second trimester. Hydrocephalus can develop throughout the course of gestation from obstruction of the CSF flow. Several hypotheses have been proposed, but most likely this occurs due to obstruction of the fourth ventricle through herniation and crowding of the cerebellum due to ongoing rostral CSF loss through the defect.

Neurologic deficits can affect the innervation of the bladder, leading to neurogenic bladder. Abnormalities in storage and emptying of urine can lead to high pressure and cause secondary deterioration in renal function. Renal deterioration can be seen in patients with noncompliant bladders with high intravesical pressures. Neurogenic bowel can also be seen and can complicate urologic function as well. The majority of infants will require some degree of support to optimize bowel and bladder function. Congenital and acquired orthopedic anomalies result largely from muscular imbalance, paralysis, and decreased sensation. Both hip dislocation and foot deformities should be primarily assessed early in life.

Diagnosis