1119 Myelomeningocele affects about 1 of every 2000 ba- bies born in the world. The spinal cord malformation leads to neurologic injury below the lesion, with paraple- gia, urinary and fecal incontinence, sexual dysfunction, and skeletal deformations. Nearly all children born with myelomeningocele demonstrate symptomatic hindbrain herniation, which is the principal cause of death in these children. 1 Fourteen percent of children with myelo- meningocele die in the first 5 years of life, and 73% of the deaths are attributable to hindbrain herniation. 1 Hindbrain herniation is defined as displacement of the inferior vermis, the pons, and the medulla oblongata into the cervical canal, as well as elongation of the fourth ventricle into the cervical canal. This central nervous sys- tem malformation is independent of the level of the spinal lesion. Hydrocephalus necessitating ventricular decompression to preserve brain function develops in >90% of affected children. The pathogenesis of the hindbrain herniation is not yet completely clear but is hypothesized to be caused by pressure imbalance result- ing from loss of cerebrospinal fluid (CSF) through the spinal lesion. 2 Clinical results indicate that hindbrain herniation may be improved after in utero myelomeningocele repair. 3-5 However, there has been no large animal model of hind- brain herniation. The aim of this study was to create a reliable model of hindbrain herniation and to study the impact of different techniques of fetal surgical myelo- meningocele repair on the development of hindbrain herniation. Material and methods A surgical open-canal myelomeningocele-like spinal cord defect was created in fetal lambs at 75 days’ gesta- tion (term, 147 days), modifying previously described techniques (Fig 1). 6 Briefly, the lumbar region of the fetus was exposed through a hysterotomy. An oval skin ex- cision over the lumbar portion of the spine and excision of soft tissue and the paraspinal muscles were performed. Subsequently, a laminectomy (first through fourth lum- bar vertebrae) was performed, and the dura was excised by means of loupe magnification. The central canal was then incised longitudinally in the midline of the spinal cord to allow CSF to leak into the amniotic fluid, as is seen in spontaneous myelomeningoceles. Two thirds of the fetuses underwent myelomeningo- cele repair at 100 days’ gestation, whereas every third fetus had no repair. Two techniques for repair were used. One was a standard neurosurgical repair performed by both a pediatric neurosurgeon and a pediatric plastic sur- geon experienced in myelomeningocele repairs postna- tally in human patients. 7 The open neural placode and its overlying fascia were separated by means of sharp dissec- tion to create a cord-dural plane. The placode was then reformed into a tube with 6-0 nylon sutures, and the dural sheath was re-formed around it. The paraspinal From the Division of Pediatric Surgery, Department of Surgery, and The Fetal Treatment Center, University of California at San Francisco. Presented at the Twentieth Annual Meeting of the Society for Maternal- Fetal Medicine, Miami Beach, Florida, January 31–February 5, 2000. Reprint requests: Bettina W. Paek, MD, The Fetal Treatment Center, University of California at San Francisco, San Francisco, CA 94143- 0570. Copyright © 2000 by Mosby, Inc. 0002-9378/2000 $12.00 + 0 6/6/108867 doi:10.1067/mob.2000.108867 Hindbrain herniation develops in surgically created myelomeningocele but is absent after repair in fetal lambs Bettina W. Paek, MD, Diana L. Farmer, MD, C. Corbett Wilkinson, MD, Craig T. Albanese, MD, Warrick Peacock, MD, Michael R. Harrison, MD, and Russell W. Jennings, MD San Francisco, California OBJECTIVE: The purpose of our study was to determine whether prenatal repair of myelomeningocele pre- vents or reverses hindbrain herniation in the sheep model. STUDY DESIGN: A myelomeningocele was surgically created in fetal sheep. One group was repaired later in utero; the others were delivered without repair.After delivery, lambs were assessed for the presence of hind- brain herniation. RESULTS: In all lambs that had not undergone repair of the myelomeningocele, severe hindbrain herniation developed, whereas the brains of all lambs that had undergone fetal repair were normal. CONCLUSION: Prenatal repair of myelomeningocele prevents or reverses development of hindbrain hernia- tion in the fetal lamb model. (Am J Obstet Gynecol 2000;183:1119-23.) Key words: Myelomeningocele, hindbrain herniation, fetal surgery, sheep