Innate healing in the fetal sheep model of myelomeningocele: A standardized defect grading system

Background/purpose: The fetal sheep model of myelomeningocele (MMC) is well-established. While the variability of innate fetal healing of the defect at the time of the repair operation has been acknowledged, it remains poorly described. We characterized the healing within the fetal sheep MMC model and present a standardized defect grading system.

Methods: Forty-three fetuses underwent surgical MMC creation at a gestational age of 75 days and repair at 100 days. Defects were graded based on percentage of exposed spinal cord and degree of scarring. A scale was developed and validated by interrater reliability testing.

Results: The phenotypic appearance of the defect at the time of the repair operation was highly variable. The defect length ranged from 3.0 to 5.5cm. Three spinal cords remained fully exposed across their length; 18 were completely covered in fibrinous exudate. Twenty-two fetuses demonstrated varying degrees of open spinal cord. Interrater reliability revealed consistent grades 91% of the time.

Conclusion: Despite consistent defect creation, the fetal MMC defect has a wide spectrum of appearance at the time of the second surgery. This study describes the innate fetal healing within the model and presents a feasible and valid grading scale to ensure scientific rigor within the model.