Intracranial Changes after Fetal Spina Bifida Repair by 2D and 3D Slice-to-Volume MRI Reconstruction: Implications for the Etiology of Hydrocephalus

BACKGROUD AND PURPOSE: Open spinal dysraphism is associated with hindbrain herniation and ventriculomegaly. Prenatal surgery improves outcomes by protecting neural tissue and altering CSF dynamics. In our experience, fetal MRI, particularly with 3D slice-to-volume reconstruction (SVR), complements prenatal ultrasound in its diagnostic and prognostic accuracy. This study aims to document prenatal intracranial changes on MRI after fetal spina bifida repair, and compare the accuracy and operator confidence of 2D versus 3D SVR and volumetry-based measurements. MATERIALS AND METHODS: We conducted a retrospective review of fetal MRIs in 132 consecutive pregnancies undergoing prenatal spina bifida repair. MRIs were performed at 3 time points: preoperatively, 1 week postoperatively, and in the third trimester of pregnancy. We calculated changes in maximal ventricular width and posterior fossa parameters. For each MRI, operators recorded their confidence in the accuracy of their measurements using a 5-point Likert scale, comparing 2D versus 3D SVR. RESULTS: Hindbrain herniation reversed in 86% (113/132) within 1 week after surgery and in nearly all cases by the third trimester (61/62). Maximal ventricular width increased over that time, but this did not always parallel the reversal of the hindbrain herniation. The ventricular dimensions were all significantly different in fetuses requiring postnatal CSF diversion surgery compared with those who did not. Operators reported significantly higher confidence in their measurements taken from 3D SVR images compared with their 2D equivalents (average Likert score 0.38 ± 0.12). CONCLUSIONS: Our findings support the hypothesis that reversal of hindbrain herniation and ventricular enlargement can occur independently, suggesting the etiology is not purely obstructive in nature. Furthermore, 3D SVR enhances measurement confidence.