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

OBJECTIVE: The purpose of study was to demonstrate the presence of motor input from the spinal accessory and the branches of the upper cervical plexus. METHOD: Twenty-four patients were studied during modified radical neck dissection. The entire length of the spinal accessory nerve, the contributions from the upper cervical plexus and some cervical plexus branches to run to the trapezius independently were preserved in each of these patients. Compound muscle action potentials were measured to each part of the trapezius muscle on stimulation of the spinal accessory, C2, C3, and C4 nerves. RESULTS: Spinal motor nerve evoked responses were obtained from all 24 patients in the upper, middle, and lower trapezius. C2 contributions were seen in 2 out of 24 patients, but were in no patient supplying all three parts of the muscle. C3 contributions were seen in 11 out of 24 patients, but C3 nerve supplied all three parts of the muscle in 8. C4 contributions were seen in 20 out of 24 patients, supplying all three parts of the muscle in 16. CONCLUSION: This study demonstrated that the spinal accessory nerve provided the most important and consistent motor input to the trapezius muscle. Although C2, C3, and C4 provided motor input to the trapezius muscle, they were not consistently present and if present, did not consistently innervate all three parts of the trapezius. Compared with other studies, it was interesting to note that C4 gave more consistent motor input to the trapezius than other cervical branches.

Function of Trapezius Muscle after Various Types of Neck Dissection

Radical neck dissection(RND) has been a standard treatment for the head and neck cancer with metastasis to the cervical lymphatics. The sacrifice of the spinal accessory nerve innervating trapezius muscles creates a definite deficit of the shoulder function. Therefore, the modified radical neck dissection(MRND) preserving one or more structures of the spinal accessory nerve, internal jugular vein or sternocleidomastoid muscle was introduced to minimize the postoperative morbidities. We studied the shoulder function by clinical examinations and electrodiagnosis for the various types of neck dissection in 39 cases of 24 patients and compared the results of each test according to the types of neck dissection. Correlation between the clinical parameter and electrodiagnostic results showed a statistical significancy. The functional results of trapezius muscle in the group of modified radical neck dissection were better than those of the radical neck dissection. The fact that 80% of the cases in the RND group presented incomplete denervation of the trapezius muscle, suggests the innervation of other nerves to this muscle. Forty five percents of the cases in the MRND group which presented partial denervation of the trapezius muscle, might be due to the damages during operations. To preserve the spinal accessory nerve, a careful manipulation of the nerve is required. Further studies including an anatomic dissection and intraoperative electrophysiologic evaluation of the trapezius muscle should be performed for the better rehabilitation outcomes.