Innervation of the Trapezius Muscle by the Intraoperative Motor Nerve Conduction Study

P019 Objectives: To demonstrate the presence of motor input from the spinal accessory and the branch of the cervical plexus using intraoperative motor nerve conduction study, in which motor action potential is measured by direct stimulation of the exposed nerve under direct vision during neck dissection. Methods: Twenty-four patients are studied on completion of the operative procedures. In each of these patients, the entire length of the spinal accessory nerve is preserved as well as the contributions from the upper cervical plexus that join the sternocleidomastoid, and some cervical plexus branches running to the trapezius independently. Compound muscle action potential (CMAP) are measured to each part of the trapezius muscle on stimulation of the spinal accessory, C2, C3, and C4 nerves. Results: With stimulation of the spinal nerve, evoked responses are obtained from all 24 patients in the upper, middle, and lower trapezius. C2 contributions are seen in 2 out of 24 patients, supplying all 3 parts of the muscle in 0. C3 contributions are seen in 11 out of 24 patients, supplying all 3 parts of the muscle in 8. C4 contributions are seen 20 out of 24 patients, supplying all 3 parts of the muscle in 16. Conclusion: The spinal accessory nerve provides the most important and consistent motor input to the trapezius muscle and reveals that C2, C3, and C4 provide motor input to the trapezius muscle but finds that they are either not consistently present or, when present, do not consistently innervate all 3 parts of the trapezius muscle.