Letter: Detailed Analysis of Hydrocephalus and Hindbrain Herniation After Prenatal and Postnatal Myelomeningocele Closure: Report From a Single Institution.

Abstract

To the Editor: In their important article, Flanders et al1 present outcomes of their recent experience with fetal surgery versus postnatal surgery to close the placode in patients with myelomeningocele. Their fetal surgery center was one of the 3 centers that participated in the landmark randomized control trial of outcomes of fetal surgery vs postnatal, the Management of Myelomeningocele Study (MOMS), which was stopped for efficacy in 2010 with a demonstration that fetal surgery reduced the absolute risk of cerebrospinal fluid (CSF) diversion.2 Reporting on their post-MOMS experience, Flanders et al1 confirmed that fetal surgery reduced the absolute risk of CSF diversion to treat hydrocephalus and added that in some cases, fetal surgery reversed fetal hindbrain herniation, a component of the Chiari II malformation. This reversal was associated with the absence of hydrocephalus. The “unified theory” of McLone et al3,4 posits that all brain anomalies in myelomeningocele, including hydrocephalus, are a consequence of fetal CSF venting through the open myelomeningocele, which changes vectors of fetal brain growth and causes the Chiari II malformation. Since fetal surgery prevents or reduces CSF venting, the finding that the reversal of hindbrain herniation is associated with the reduction of hydrocephalus is consistent with the “unified theory.” Absence of fetal hindbrain herniation is currently an exclusion criterion for fetal surgery,1,2 based on the reasoning that the risk of developing hydrocephalus would be less.5 In Flanders et al,1 26% of their postnatal comparison group (16 patients) had absent hindbrain herniation and were therefore not eligible for fetal surgery. To our knowledge, however, CSF diversion frequency in patients with no fetal hindbrain herniation has not been studied. We think it is possible that Flanders et al1 have the data at hand to do this. There could be 2 useful implications, depending on findings. First, if CSF diversion is necessary in a substantial percentage of those who had no fetal hindbrain herniation on prenatal magnetic resonance imaging (MRI), then the requirement of the presence of hindbrain herniation for fetal surgery could be reconsidered. Second, if patients who had no fetal hindbrain herniation and no postnatal hindbrain herniation had a lower risk of CSF diversion, then it would further support the “unified theory” of McLone et al.3,4 A final comment: If enough patients were available to provide statistical power, a more quantitative method for describing hindbrain herniation might provide more detailed risk stratification. For example, the position of the obex relative to the foramen magnum could be measured in millimeters rather than simply noting the presence or absence of hindbrain herniation. Funding This study did not receive any funding or financial support. Disclosures The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.