Evolution of Posterior Fossa Anatomy Following Prenatal Myelomeningocele Repair

ABSTRACT Background and Purpose Chiari II deformation in open neural tube defect (ONTD) includes hindbrain herniation (HBH) and posterior fossa remodeling. While HBH reversal after prenatal ONTD repair is well described, associated changes in cerebellar morphology, torcular position, and bony configuration remain poorly characterized. This study evaluated longitudinal changes in posterior fossa anatomy following fetal ONTD repair and their associations with HBH reversal and evolving ventriculomegaly. Methods We retrospectively reviewed 112 fetuses who underwent prenatal ONTD repair at a single center (2011–2023), each with fetal MRI before surgery (median 22.9 19.1–25.4 weeks) and 6 weeks after surgery (31.4 28.6–34.0 weeks). Two independent examiners assessed cerebellar and brainstem position, occipital bone outpouching, clivus concavity, torcular position, and preservation of supratentorial subarachnoid space. HBH reversal was defined as cerebellar ascent above the foramen magnum. Severe ventriculomegaly was defined as a mean atrial width ≥15 mm. Associations were tested using Chi‐square or McNemar tests. Results At referral, 17.9% had severe ventriculomegaly and 79.5% had lesions below L2. After surgery, clivus concavity (97.3% vs 84.1%, p <0.01) and subarachnoid space preservation (82.1% vs 3.6%, p <0.01) increased significantly, while occipital outpouching and low torcular position showed no change. Complete HBH reversal occurred in 62.5%. Subarachnoid space preservation was more frequent with HBH reversal (98% vs 63.3%, p <0.01). Severe ventriculomegaly at postoperative MRI (58.9%) was not associated with posterior fossa abnormalities. Conclusions Prenatal ONTD repair improves HBH and supratentorial CSF space preservation but has limited impact on deeper posterior fossa remodeling, which appears only partially reversible.