Posterior atlanto-occipital membrane bulging preceding hindbrain herniation in open spinal dysraphism: a case report.

INTRODUCTION: Hindbrain herniation (HBH) is a feature of Chiari II malformation in fetuses with open spinal dysraphism (OSD) and constitutes an eligibility criterion for prenatal repair in many fetal surgery protocols. Bulging of the posterior atlanto-occipital membrane (PAOM) is frequently observed in association with vermian descent; however, its significance in the absence of HBH remains unclear. CASE PRESENTATION: We report two fetuses with OSD who initially showed no HBH on fetal MRI but presented with isolated CSF-filled PAOM bulging. In both cases, repeat MRI within two weeks demonstrated interval development of HBH to C1, thereby fulfilling criteria for prenatal repair. Case 1 presented at 22.3 weeks with L4 myelomeningocele, normal ventricles, and preserved motor function. Prenatal repair was performed at 26.0 weeks. Case 2 presented at 20.3 weeks with L5 myelomeningocele, normal ventricles, and intact motor function. Prenatal repair was performed at 25.3 weeks. In both fetuses, postoperative MRI demonstrated reversal of HBH, whereas PAOM bulging persisted. CONCLUSION: PAOM bulging may be associated with the subsequent development of HBH, but this observation requires confirmation in larger longitudinal cohorts. In fetuses with OSD and absent HBH at initial assessment, the presence of PAOM bulging may warrant consideration of short-interval follow-up MRI while prenatal repair remains an option.