Spinal Accessory Nerve Mononeuropathy Following Trapezius Lipoma Resection: A Case Report

The spinal accessory nerve (SAN) is crucial for the motor function of the trapezius and sternocleidomastoid muscles, playing a significant role in scapular stability and upper limb mobility. While SAN injuries are commonly associated with iatrogenic causes such as lymph node biopsies, neck dissections, and posterior cervical trauma, injury following trapezius lipoma resection has not been well-documented. Given the SAN's superficial location and proximity to surgical fields in the posterior cervical triangle, it is vulnerable to inadvertent injury during tumor excision. To the best of our knowledge, this case report presents the first documented instance of isolated SAN mononeuropathy following trapezius lipoma resection and underscores the importance of early recognition, diagnosis, and management. A 54-year-old male developed SAN mononeuropathy following an elective surgical removal of a deep right-sided trapezius lipoma. One week following resection, the patient exhibited new-onset paresthesias in the right upper extremity, scapular dyskinesis, shoulder weakness, and lateral scapular winging. Prior electrodiagnostic (EDX) studies for different complaints showed no signs of peripheral nerve injury. Given the new presentation, repeat EDX were performed six months postoperatively, revealing severe focal SAN neuropathy with ongoing active denervation. The findings suggest direct or traction-related SAN injury rather than an isolated muscular pathology. Given the course of symptoms, a referral to a peripheral nerve surgeon was warranted for recommendations regarding further management. SAN mononeuropathy following lipoma resection is an important and uncommon complication. Most SAN injuries result from iatrogenic causes, such as neck dissections and lymph node biopsies or lateral neck trauma. The SAN's close proximity to the surgical site makes it susceptible to iatrogenic injury in posterior cervical triangle procedures. This case highlights the importance of identifying lipoma location and recognizing subtle nerve injuries often overlooked without thorough postoperative assessment. Clinical signs include lateral scapular winging during active external rotation, vague shoulder pain, limitations in range of motion, and potential muscle atrophy. In this case, resection of a deeply situated trapezius lipoma led to SAN dysfunction within one week of surgery. Clinical suspicion, combined with examination and confirmation through EDX, highlights the SAN's vulnerability in this region. SAN injury outcomes vary widely, with treatment options ranging from conservative management to surgical interventions such as nerve grafting, nerve repair, or Eden-Lange muscle transfer. Successful recovery is more likely when repairs are performed early, ideally within seven months, with poorer results after 20 months. This case emphasizes the potential for SAN injury during deep trapezius lipoma resection. Early recognition through physical exam and EDX is essential for distinguishing SAN mononeuropathy from other shoulder dysfunctions, as prompt diagnosis improves functional outcomes. Additionally, preoperative nerve mapping is crucial when operating near critical neural structures, stressing the importance of intraoperative nerve monitoring to reduce the risk of injury. Further research is needed to better understand SAN injury incidence in posterior cervical and scapular surgeries and to develop standardized management guidelines.