Late prenatal development of hindbrain herniation in open spina bifida.

Introduction: We describe four cases of open spina bifida initially presumed to be "closed" defects in which serial prenatal ultrasound examinations documented the occurrence of sac rupture and development of Chiari II malformation later in the pregnancy.

Case Presentation: In each case, the ultrasound examinations starting from 19-24 weeks' gestation, showed spinal cystic lesions containing neural elements, but no evidence of Chiari II malformation. The lack of hindbrain herniation rendered them ineligible for prenatal surgical repair; but follow-up imaging at 24-32 weeks' gestation, demonstrated the cystic lesions no longer visible or visible but decompressed, with the additional development of de novo hindbrain herniation.

Conclusion: We conclude by recommending serial ultrasound surveillance of presumed "closed" spina bifida as these may be in fact open defects, with confirmatory diagnosis only possible later in pregnancy. This detection is important because it may change counseling since prenatal repair or planned delivery may be warranted.