Correction of hindbrain herniation and anatomy of the vermis after in utero repair of myelomeningocele in sheep.

BACKGROUND/PURPOSE: In utero repair of myelomeningocele (MMC) in humans spares distal neurologic function, reverses the hindbrain herniation component of the Arnold-Chiari II malformation (ACM), and reduces the rate of postnatal shunt placement. The authors hypothesized that extravasation of cerebrospinal fluid (CSF) from the lumbar spinal cord results in herniation. This hypothesis was tested by assessing the impact of a spinal cord myelotomy on hindbrain anatomy in fetal sheep.

METHODS: A MMC lesion was created surgically in 34 fetal sheep at 75 days' gestation by excision of the L1-L5 lamina, the exposed dura, and surrounding tissues. A lumbar level myelotomy was performed in 28 of the 34 fetuses to open the central canal of the spinal cord to enhance egress of CSF through the MMC defect and potentially induce hindbrain herniation. At 102 days' gestation, a repair of the MMC lesion was performed in 14 fetuses with a myelotomy. Fetuses underwent autopsy at 102, 114, 120, or 140 days' gestation. Control animals underwent 2 unrelated fetal surgical procedures at approximately 70 and 110 days' gestation. The incidence of hindbrain herniation, ventricular size, biparietal diameter, brain weight, and brain anatomy were compared between the different animal groups.

RESULTS: After MMC creation, significant cerebellar tonsillar herniation was observed in 85% of fetuses that underwent creation of a myelotomy; none of the lambs without a myelotomy (n = 6) had hindbrain herniation. At autopsy, cerebellar tonsillar herniation was present at the time of MMC repair (102 days' gestation), 2 weeks after MMC repair, but was reversed 3 weeks post-MMC repair. At birth, tonsillar herniation was absent, and hindbrain anatomy was restored in 88% of the fetuses with a myelotomy that underwent fetal MMC repair. No significant differences in brain weight and ventricular size was observed between animals with and without MMC repair.

CONCLUSIONS: Adding a myelotomy to the sheep model of MMC leads to hindbrain herniation that is similar to that observed in the human ACM. These experiments support the hypothesis that leakage of CSF through the exposed central canal alters the normal CSF hydrodynamics, resulting in cerebellar tonsillar herniation. Fetal MMC repair reverses hindbrain herniation and restores gross anatomy of the vermis.