Electrophysiological testing of spinal accessory nerve in suspected cases of nerve transection

Introduction: In this study we sought to determine whether standard electrophysiological testing of the spinal accessory nerve (SAN) accurately identifies patients who would benefit from surgical repair. Methods: Sixteen consecutive patients sent for surgical evaluation of unilateral SAN injury were studied clinically and electrophysiologically. Results: All patients demonstrated a low-amplitude SAN compound muscle action potential (CMAP) that required a higher stimulus intensity to obtain it than on the unaffected side. Upper trapezius needle electromyography showed dense fibrillation potentials in 16 of 16 nerves, with voluntary motor unit potentials (MUPs) in 5 of 16. Intraoperatively, 12 of 16 nerves were transected; 4 of 16 had neuromas across which there was no nerve action potential. Patients underwent direct repair (6 of 16) or interpositional nerve grafting (10 of 16). Fourteen of 15 patients seen postoperatively had improvement in pain, muscle bulk, and range of motion. Conclusions: Surgical exploration of the SAN is warranted in patients with clinical signs of severe injury, even when electrophysiological testing shows low-amplitude CMAPs and/or residual MUPs.