Diffusion-weighted imaging of the fetal brainstem and cerebellum before and after prenatal myelomeningocele repair.

BACKGROUND

Myelomeningocele (MMC) is associated with hindbrain herniation (HBH) and brainstem compression as part of Chiari II. Prenatal MMC repair can reverse HBH, but its effect on brainstem microstructure remains unclear.

OBJECTIVE

This study aims to evaluate fetal brainstem and cerebellar diffusion characteristics before and after prenatal MMC repair using Diffusion-weighted imaging (DWI), and to compare these changes according to HBH reversal.

METHODS

Retrospective study of fetuses who underwent a prenatal MMC repair at a single institution. Fetal brain MRI at referral and 6 weeks postoperatively was performed using a 1.5 T scanner including axial DWI. HBH was graded (0-3) based on cerebellar tonsil position relative to the foramen magnum. Regions of interest were placed in the pons, cerebellar vermis, and hemispheres to obtain apparent diffusion coefficient (ADC) values (×10-3 mm2/s). ADC values were compared by HBH grade and by HBH reversal status using Mann-Whitney U and Kruskal-Wallis tests.

RESULTS

Fifty-nine fetuses were included (50 fetoscopic, 9 open repair). Before surgery, all had HBH (grade 2 = 17%; grade 3 = 83%) with no ADC difference between grades. At 6 weeks, HBH resolved in 78% of fetuses, partially reversed in 12%, and persisted in 10%. ADC values were significantly higher in the pons, vermis, and cerebellar hemispheres of fetuses with persistent severe HBH compared with those with partial or complete reversal (p ≤ 0.04). Fetuses without HBH reversal showed the highest ADC values, consistent with greater microstructural disruption.

CONCLUSION

Persistent HBH after prenatal MMC repair is associated with increased ADC in the brainstem and cerebellum, suggesting ongoing microstructural alteration likely related to chronic compression. DWI may provide a candidate imaging marker associated with the degree of hindbrain herniation reversal after fetal surgery and may help refine postoperative imaging assessment.