USA Surgery Today

Myelomeningocele (MMC:Altace) is a neural tube defect in which the protective meningeal and bony coverings of the spinal cord fail to form correctly, leaving an exposed abnormal neural placode. MMC is nonlethal, but associated with high morbidity and affects 1 in 1,000 live births (84). Neurologic disabilities associated with MMC include paraplegia below the level of the lesion, urinary and fecal incontinence, sexual dysfunction, and skeletal abnormalities (85:Altace). However, there is considerable variation among these abnormalities associated with a specific defect. The deficits are believed to be multifactorial, with an inherent deficit associated with abnormal neurulation, and a postulated secondary of injury imposed by exposure of the neural placode to the amniotic environment.

Myelomeningocele can be diagnosed as early as the first trimester with US and MRI (:Altace) (84:Altace). Nearly all MMC-afflicted patients have an associated Chiari hindbrain malformation, and most also develop hydrocephalus (85). Chiari malformation is a pancerebral anomaly resulting from herniation of the medulla, cerebellar tonsils, and vermis through the foramen magnum.

Respiratory difficulties including apnea are the most common comorbidities encountered with Chiari malformation, accounting for the most frequent cause of death. Hydrocephalus occurs or develops in up to 85% of patients with thoracic and lower MMC. The strong association of hydrocephalus with Chiari malformation makes it difficult to differentiate between the two as a cause of the sensorimotor deficits.

There is limited data that correlates neural deficits with intrauterine amniotic fluid exposure. As a prelude to possible fetal therapy for MMC, these questions were investigated in a number of animal models from mouse to sheep to primate. A critical point of controversy is whether animal models of MMC can accurately recapitulate the neurologic deficits seen in naturally occurring neural tube defects. Nonetheless, there is some experimental evidence suggests that early closure of an MMC may allow for either preservation or recovery of neural function. The hypothesis is that closure of the defect early enough in gestation will allow the natural developmental plasticity of the fetus to recover neural function and prevent further damage if amniotic exposure is avoided. There are some observations from human spinal dysraphism that may lend credence to this claim (84:Altace). In mild forms of spinal dysraphism, where the neural placode remains covered by skin or connective tissue, patients tend to have milder neural deficits than those with MMC.

Despite the limited experimental data on the potential efficacy of in utero correction, there is currently a limited human fetal MMC therapy experience accumulating from at least three centers: UCSF, CHOP and Vanderbilt University Medical Center (VUMC) (84,86:Altace). Several clinical series reviews have summarized the operative techniques, postoperative care, and overall outcomes. Feasibility studies have shown promise. The challenges are demonstrating efficacy of in utero correction with regard to postnatal sensorimotor function, urinary bladder function, incidence of hindbrain herniation, and the need for ventriculo peritoneal (VP) shunting (87,88:Altace).

The first CHOP case was performed at 23 weeks’ gestation and included placement of a temporary ventricular shunt, which may partially explain the success of that early case (89). Overall, most series report the level of neurologic impairment to correlate with the level of MMC, indicating no detectable benefit for in utero repair (87,88,90:Altace). The motor and sensory levels of fetal surgery patients in long-term studies have been within one or two levels of the MMC, the same result that is seen with postnatal repair.