Myelomeningocele and Related Neural Tube Defects



Myelomeningocele and Related Neural Tube Defects


Shenandoah Robinson and Alan R. Cohen




Embryology and Pathogenesis


Whether the primary pathogenesis is due to failure of closure of the bony elements, a disorder of neural induction, or reopening of a transiently closed posterior neuropore at 28 days’ gestation, is an unresolved issue.4 Risk of myelomeningocele in humans and experimental animals is increased with folic acid deficiency and exposure of the embryo to excessive retinoic acid or vitamin A.3 Approximately 70% of neural tube defects can be prevented by preconceptional folic acid supplementation of 0.4 mg per day.5,8 Although the condition is not a direct mendelian trait, mothers with one affected infant are at higher risk for another than the control population and should receive 4 mg per day of folic acid before and during subsequent pregnancies. This malformation involves all ethnic groups but is higher in some populations.



Specific Types


A meningocele is a cystic dilation of the meninges associated with spina bifida and a defect in the overlying skin. The spinal cord and nerve roots are normal in their structure and position in the spinal canal. Accordingly, infants with meningoceles typically do not show neurologic deficits as neonates (see Chapter 14).


A myelomeningocele is a more extensive lesion than a meningocele associated with abnormalities in the structure and position of the spinal cord. A myelomeningocele results from failure of primary neurulation. The neural tube does not fuse dorsally, leaving an open neural placode, similar to an open book. The majority of myelomeningoceles are located in the low thoracolumbar spine or more distally. Affected infants typically show neurologic deficits below the level of the lesion. Emerging evidence suggests that in addition to failure of neural tube closure, a primary disruption of brain development may be involved.2,4


Spina bifida occulta refers to a nonfusion of one or more of the posterior arches of the spine, usually in the lumbosacral region. A frequent abnormality found at L5 in 30% of adults, the lesion is of neurologic consequence only when associated with underlying abnormalities such as tethering of the spinal cord. This condition comprises low-lying conus below L2/L3, best diagnosed by MRI. It may require prophylactic surgery as vertebral growth provides traction on the tethered spinal cord. Significant associated abnormalities usually are signaled by the presence of a neurocutaneous signature such as a hemangioma, a patch of abnormal hair, a dimple, or a lipoma in the lumbosacral area. Underlying lesions can include diastematomyelia (split cord malformation), spinal lipomas, dermoids, and dermal sinuses.



Clinical Expression


The extent of neural tissue involvement in the myelomeningocele determines the severity of the deficit in motor and sensory function involving the lower extremities and the presence of involvement of bladder and bowel functions (Table 65-1). Many infants with lesions at the L5 to S1 level ultimately will ambulate, with or without short leg braces. Although the ability to predict later function from the level of early neurologic function has been shown to be only modestly accurate,7 long-term outcomes of patients with myelomeningocele are favorable. In a longitudinal 25-year study of 220 consecutive patients with myelomeningocele, 78% had hydrocephalus treated with ventricular peritoneal shunts.9 Sixty-three percent required one or more shunt revisions. A tethered cord ultimately developed in 40 patients (20%). Over a 25-year follow-up period, there were 5 deaths (2%), and 18 patients (9%) with severe neurologic morbidity. Ninety-seven percent of survivors with an initial lesion below L2 were functioning independently, attending regular schools, and achieving “social continence.” In a separate report studying 50 teenagers with myelomeningocele, 21 patients were dependent on wheelchairs. Overall, this group described diminished health-related quality of life, specifically regarding emotional well-being, self-esteem, and peer relations.6 More recent studies of infants born in the 2000s suggest the need for permanent shunt insertion has declined to 50% to 60%.


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Jun 6, 2017 | Posted by in PEDIATRICS | Comments Off on Myelomeningocele and Related Neural Tube Defects

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