Extent of Vascular and Fiber Damage at Varying Degrees of Stretch of Hypoxic Neonatal Brachial Plexus

Introduction: Neonatal brachial plexus palsy (NBPP) is defined as an injury to brachial plexus (BP) nerves due to over-stretching during complex birthing scenarios. Despite advancements in obstetric care, NBPP has an incidence of 1-4 cases per 1000 live births. Neonatal hypoxia can also occur as a result of obstetric complications, such as disrupted blood supply to the fetus due to umbilical cord compression, and can significantly impact the tissues and other organ functions. Characterizing the extent of vascular damage of hypoxic neonatal BP at varying degrees of stretch can further the understanding of the injury thresholds of NBPP.

Materials and

Methods: All procedures were approved by the Institutional Animal Care and Use Committee. Fifteen neonatal piglets (3-5 days old) were anesthetized and exposed to FiO2 of 7% for 1 hour to induce hypoxia and re-perfused to FiO2 of 21%. Using an axillary approach the BP was exposed and BP nerves were stretched at a rate of 500 mm/min to predetermined low- ( < 15%) and high- (>15%) strains. Post-stretch, BP nerves were harvested and OCT-embedded. Ten-µm-thick serial longitudinal sections were stained with Hematoxylin-Eosin (H&E) and neurofilament (NF). Using the Olympus BX53 motorized microscope, H&E stained slides were imaged at 10x magnification along the nerve length. Each stitched H&E image was split along its nerve length into regions of interest (ROIs) using a custom MATLAB script. NF-stained slides were imaged at 10X along nerve length as well. An independent-blinded observer scored each ROI for vascular damage on a scale of 0-2 (0-no damage, 1-torn vessel, 2-scattered blood cells) (Fig. 1A), and fiber damage on a scale of 0-4 (0-no to minimal damage, 1-straightening, 2-increased spacing, and 3 and 4-torn to scattered fiber disruption, respectively) (Fig. 1B).

Results, Conclusions, and Discussions: Based on our modified scoring system, preliminary results show that vascular damage and fiber damage increased with increasing stretch injury (Fig. 2A, 3), and similar degrees of vascular damage were observed in the central and peripheral regions of the nerve (Fig. 2B). This ongoing study is the first to characterize structural changes of hypoxic neonatal BP nerves with respect to varying degrees of stretch. Determining the depth of vascular damage of the BP by analyzing the three regions presented in conjunction with the overall vascular and fiber damage will provide a more detailed characterization of the injury. Due to the anatomical similarities between neonatal piglets and neonatal human BP, this study will help further the understanding of injury thresholds of NBPP.

Acknowledgements (Optional): This project was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health R15HD093024; R01HD104910-01A1 and NSF CAREER grant Award #1752513.