INNERVATION OF THE TRAPEZIUS MUSCLE BY THE INTRAOPERATIVE MEASUREMENTOF MOTOR ACTION-POTENTIALS

Although the surgical anatomy of the spinal accessory nerve and the ce rvical plexus has been extensively described, the exact motor innervat ion of the trapezius has been controversial. Attempts to resolve this question have involved anatomic or electrophysiologic studies in human embryos and animals. Extrapolation of the results to adult humans may not be correct. Accurate identification of muscle innervation is obta inable by intra-operative measurement of motor action potentials produ ced by direct stimulation of the accessory nerve and the cervical plex us. The study involved 14 patients undergoing supraomohyoid or modifie d neck dissections. Under direct vision, stimulating electrodes were p laced on the identified nerves and motor action potentials, and latenc ies were recorded by surface electrodes placed over the three portions of the trapezius. In 13 patients, when the accessory nerve was stimul ated, motor action potentials were obtained in 13 of 13 in the first p ortion, 11 of 13 in the second portion, and 10 of 13 in the third port ion of the trapezius. In the last patient, the accessory nerve ended i n the sternocleidomastoid muscle, and innervation of the trapezius was via C3 as demonstrated by motor action potentials. Responses when the roots of the cervical plexus were stimulated varied. Three patterns w ere seen: In the first group (seven patients), motor action potentials were distinct from those recorded when the accessory nerve was stimul ated. Additionally, latencies were different from those of the accesso ry nerve. The second group (four patients) had motor action potentials that were similar to those obtained from stimulation of the accessory nerve, although their corresponding latencies were different. In two patients, no motor action potentials were recorded when the cervical p lexus was stimulated. The results suggest that motor innervation of th e trapezius is variable. The accessory nerve, when present, provides t he most important input to the trapezius. Motor innervation from the c ervical plexus is unpredictable, although it appears to be present in the majority of patient studies.