Retrograde regeneration following neurotmesis of the ulnar nerve

A 41-year-old woman experienced a gunshot wound to the forearm with neurotmesis of the ulnar nerve. Surgery 9 months later revealed a neuroma-in-continuity in the midforearm. Intraoperative nerve stimulation failed to elicit direct nerve responses or motor responses from the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles. However, neurotonic discharges in response to mechanical irritation of the neuroma were recorded in the FDI, but not the ADM. Surprisingly, after resecting the ulnar nerve distal to the neuroma, neurotonic discharges were still elicited in the FDI following perturbation of the neuroma. Moreover, neurotonic discharges were elicited during ulnar nerve resection 2 cm proximal to the neuroma. No anastomoses or anomalous branches were noted. The findings suggest that regenerating fibers did not reach the FDI through the distal nerve segment. Rather, we speculate that nerve fibers regenerating at random, or impeded by scar tissue, contacted the proximal nerve portion, at which point growth became polarized in a retrograde direction. Retrograde regeneration may have proceeded to a branch point in the forearm (possibly an undetected anomalous branch or fibrous adhesion), where growth of regenerating fibers extended outward into surrounding damaged tissue planes before redirecting distally to reach the FDI.