Improvement in Hindbrain Herniation Demonstrated by Serial Fetal Magnetic Resonance Imaging Following Fetal Surgery for Myelomeningocele

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Abstract
Myelomeningocele is the most common severe birth defect involving the central nervous system, occurring with an incidence of 4.5 per 10,000 live births.1 The fetal prevalence is undoubtedly higher, since myelomeningocele is frequently detected by screening amniocentesis and ultrasound, and parents often elect to terminate the pregnancy. In addition to spinal cord dysfunction, children with spina bifida almost invariably have an associated Chiari II hindbrain malformation, consisting of a small posterior fossa and downward displacement of the cerebellar vermis below the foramen magnum into the cervical spinal canal with elongation of the brainstem and obliteration of the fourth ventricle.2 Approximately 20% of myelodysplastic children develop symptoms of hindbrain, cranial nerve, and spinal cord compression, usually before age 3 months. This is the principal cause of death in this population.3,4 In addition, hindbrain herniation with obstruction of the outflow of cerebrospinal fluid (CSF) from the fourth ventricle is believed to be the cause of hydrocephalus, which is present in 85% of individuals with myelomeningocele.5 In the past, it was believed that the hindbrain herniation that accompanies myelomeningocele was part of an overall cerebrospinal dysgenesis, but there is experimental2 and clinical6-8 evidence that both hindbrain herniation and hydrocephalus are acquired early in fetal life and progress in severity before birth.

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