(E-167) Extent of Impaired Axons at Varying Degrees of Stretch of Neonatal Brachial Plexus

Introduction: Brachial plexus (BP) is an intricate network of nerves responsible for providing motor and sensory innervation to the upper extremities [1]. Neonatal brachial plexus palsy (NBPP) is a common BP injury in infants, described as overstretching of BP during complicated birthing scenarios [2, 3]. Despite obstetric care advancements, NBPP has a worldwide incidence of 1-4 per 1000 live births [3, 4]. Surgical intervention remains controversial since timing and type of surgery rely on understanding extent of axon impairment observed after injury [3]. The objective of this study was to characterize the extent of axon impairment at varying degrees of stretch of neonatal BP using a neonatal porcine model.


Materials and Methods: Seventeen neonatal piglets (3-5 days old) were anesthetized and BP was exposed using an axillary approach per approved procedures by Institutional Animal Care and Use Committee. Biomechanical Studies: A custom-built mechanical testing device was used to stretch BP nerves at a rate of 500 mm/min to predetermined low (< 10%) and high ( >10%) strains. To measure actual strain, black acrylic paint markers were placed along nerve length and a stereo camera (ZED Mini) was positioned above. Load and displacement data were acquired at a sampling rate of 1000 Hz. Images were captured at 100 frames/s. Histology: Three hours post-stretch, BP nerves were harvested and OCT-embedded. Ten-µm-thick serial longitudinal sections were stained with Hematoxylin-Eosin (H&E), neurofilament (NF), and beta-amyloid precursor protein (βAPP). Then imaged at 10X along nerve length using Leica DMI 4000B microscope. Each image was scored by an independent-blinded observer for vascular on a scale of 0-2 (0-no damage, 1-torn vessel, 2-scattered blood cells) 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). Extent of fiber damage was measured at thickest fascicle by binarizing image and measuring 100x100 pixel region of interest. βAPP accumulation was measured as percent area of positive βAPP staining over background.


Results, Conclusions, and Discussions: Using our modified scoring systems, vascular damage (Fig. 1A) and fiber damage (Fig. 1B) increased with increasing stretch, while extent of fiber disruption decreased with increasing stretch (Fig. 1C, D) and βAPP accumulation increased with increasing stretch (Fig. 1E, F). Previous studies have reported injury thresholds in adult rat nerve roots [5] and adult guinea pig optic nerve [6]; however, nerve roots lack epineurium and perineurium connective tissues and optic nerve is a peripheral nerve but not a plexus. The BP is anatomically complex, and no such injury thresholds exist. Determining injury thresholds for distinct terminal nerve branches as they extend from various nerve root levels will give insight into understanding the injury mechanism of NBPP. This study is the first to characterize structural changes of neonatal BP with respect to varying degrees of stretch. Understanding extent of axon impairment after stretch may aid in the understanding of injury thresholds of NBPP because neonatal piglet and neonatal human BP share anatomical similarities [7]. Future functional studies may further contribute to the understanding of NBPP injury mechanism.


Acknowledgements (Optional): This project was supported 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.


References (Optional): [1] Abid, A. Orthopaedics & Traumatology: Surgery & Research 102:125-132 (2016). [2] Mehlman, C.T. The Pediatric Upper Extremity 589-605 (2015). [3] Abzug, J.M. Orthopedics 33: 430-435 (2010). [4] Johnson, E.O. Injury 44(3), 293-298 (2013). [5] Singh, A. Journal of Neurotrauma 26: 627-640 (2009) [6] Bain, A.C. Journal of Neurotrauma 18: 499-511 (2001). [7] Hanna, A.S. J Anat 240: 172-181 (2022).