Questioning the rationale and conduct of the management of myelomeningocele study.

Surgical intervention in fetal spina bifida developed from the belief that amniotic fluid damages the spinal cord in utero and low spinal pressure from failure of neural tube closure causes hindbrain herniation leading to hydrocephalus after birth for many infants with open spinal lesions. Intrauterine intervention is undergoing a randomised human trial known by the acronym MOMS. It is hoped that randomisation and long-term follow up will demonstrate whether benefits to the infant may result from closure of the vertebral defect before birth. It is argued here that the premise upon which the pathogenesis of neural injury in human spina bifida used to launch this study is mistaken. This has implications for the conduct and conclusions of the trial. It is proposed that fetal surgery improves central nervous system outcome by improving cerebrospinal fluid flow at the foramen magnum. Successful intervention results in a more normal development of both neural and skeletal components of the neuraxis. Closure of the defect is required before signs of hindbrain herniation and ventriculomegaly are evident on ultrasound imaging as these are indicators of the presence of fetal hydrocephalus.