Motor Innervation of the Trapezius by Cervical Sensitive Nerves: An Intraoperative Neuromonitoring Study and Postoperative Functional Outcomes

Background/Objectives: We investigated whether cervical sensitive nerves (CSN) provide motor input to the trapezius muscle and how this relates to short-term functional outcomes after neck dissection. Methods: A total of 22 neck dissections were performed in 17 patients; the SAN was preserved. CSN roots (C2–C4) were stimulated intraoperatively using IONM. Shoulder and neck function were evaluated preoperatively and at 3 months in 15 operated necks using goniometry, an IMU-based motion analysis system (iSen), trapezius isometric strength, the Modified Constant–Murley Score (MCMS), and the Shoulder Pain and Disability Index (SPADI). Results: CSN-evoked trapezius responses were detected in 10/22 (45.5%) dissections (C2: 6/22, 27.2%; C3: 4/22, 18.2%; C4: 0/22). Postoperatively, neck extension/rotation and shoulder abduction/external rotation decreased significantly within groups; upper and middle trapezius strength and MCMS also declined. Shoulder flexion loss was smaller when CSN motor participation was present. Median shoulder flexion (goniometry) changed from 162°→140° in CSN(+) vs. 170°→131° in CSN(−) (between-group p = 0.024). With iSen, shoulder flexion changed 120°→116° in CSN(+) vs. 122°→97° in CSN(−) (p = 0.033). Conclusions: Approximately half of the neck dissections exhibited CSN-related motor responses. Short-term shoulder flexion was better preserved when CSN motor participation was present, suggesting that documenting CSN motor input intraoperatively may inform early rehabilitation planning.