275 Absence of Hindbrain Herniation in Myelomeningocele Myeloschisis: A Closer Look at Patients Who Progress to Hindbrain Herniation in the Prenatal and Postnatal Period

INTRODUCTION: Prenatal repair is the gold standard for appropriate patients with fetal myelomeningocele/myeloschisis. Hindbrain herniation is a prerequisite for fetal repair; however, there is a subset of patients without herniation on initial imaging and then progress to herniation on subsequent prenatal/postnatal imaging. METHODS: From July 2016 to July 2022, patients seen at the Center of Fetal Diagnosis and Treatment for myelomeningocele/myeloschisis were evaluated for hindbrain herniation. The osseous lesion level was defined as the bony defect level on prenatal ultrasound. Prenatal sac volume was measured on MRI. Prenatal ventricular size was determined on US. The fronto-occipital horn ratio was measured on the first postnatal US. RESULTS: Of 422 patients identified with a prenatal diagnosis of myelomeningocele/myeloschisis, 73 were identified who initially had negative hindbrain herniation; of these, 9 and 11 patients progressed to herniation on subsequent prenatal and postnatal imaging, respectively; 32 remained without herniation throughout the prenatal/postnatal period. Sac volume was significant (p = 0.04) for those whose hindbrain herniation was detected on subsequent prenatal visits but not for those diagnosed in the postnatal period (p = 0.10). The majority of patients with hindbrain herniation progression had L1-2 lesions. A subset of patients ultimately underwent permanent CSF diversion (no herniation: 21.8%, prenatal herniation: 33.3%, postnatal herniation: 36.3%). CONCLUSIONS: Patients who initially exhibit no hindbrain herniation and then ultimately progress to herniation in subsequent prenatal or postnatal evaluation are more likely to have higher levels of osseous defect. Sac volume was determined as significant for delayed prenatal progression of hindbrain herniation. Further long-term outcomes are necessary to better characterize this unique cohort of patients.