Associate Professor University of Utah Salt Lake City, Utah, United States
Disclosure(s):
Mark Alexander Mahan, MD, FAANS: AxoGen Inc.: Consultant (Ongoing), Research Grant (Ongoing); Boston Scientific: Grant/Research Support (Ongoing); joimax: Consultant (Ongoing); NIH: Research Grant (Ongoing); Renerva: Consultant (Ongoing), Ownership Interest (Ongoing)
Introduction: Neuropathic pain (NP) is a consequence of pathologic neuroinflammation, neural circuitry or threshold remodeling. Putative mechanisms from experimental injury models lack clinical correlates. There has been no systematic examination of chronic NP in pathophysiologic neuroma-in-continuity injuries, particularly the correlation of NP to cellular composition of neuroma, dorsal root ganglion (DRG), and spinal cord.
Methods: C57BL/6J mice underwent rapid-stretch nerve rupture injury which heal in-continuity. Behavioral assays (n=11) of von Frey, Dixon Up-Down method (DUDM), and guarding assessed post-injury pain. Sixteen weeks post-injury, sciatic nerve, DRG, and spinal cord were harvested from injured and contralateral sides (n=14). Immunofluorescent staining of neuronal, architectural, inflammatory, and pain neuropeptides assessed for correlated changes. Naïve, age-matched C57BL/6J mice served as behavioral (n=8) and histologic (n=9) controls.
Results: At 16 weeks, von Frey revealed increased paw withdrawal frequency for injured and non-injured sides compared to control (p < .0001). No difference between sides was observed (p=.92), consistent with bilateral hypersensitivity. The DUDM revealed decreased sensitivity threshold for both injured and non-injured sides (p < .0001), without difference between sides (p=.98). Animals demonstrated increased guarding behavior (p < .001); indicative of spontaneous pain. Neuroma immunofluorescence demonstrated disorganized unmyelinated fibers. Entropic architectural remodeling also presented - consistent with clinical specimens. Notably, the neuroma microenvironment was hallmarked by inflammatory foamy macrophages, adaptive immune cells, and significant hypercellularity (p < .01). DRG ipsilateral to the injury demonstrated reduced neuronal staining with pronounced bilateral inflammatory cell presence. Spinal cord histology revealed bilateral axonal loss, in the dorsal and ventral columns (p < .05).
Conclusion : Unilateral severe nerve injury manifests in profound and unresolving inflammation in the nerve at long-term follow-up. Unusually, the effect is bilateral, with inflammation and neuronal loss present in the DRG and spinal cord of injured and non-injured sides. The mechanisms underpinning NP development in neuroma-in-continuity lesions remain unexplored. Linkage of inflammatory perpetuation and neuronal loss appears conducive for NP pathogenesis.
