Lateral Scapular Winging and Shoulder Pain Following Posterior Fossa Surgery: A Case Report

The spinal accessory nerve (SAN) provides motor innervation to the sternocleidomastoid and trapezius muscles. Injury to the distal segment of this nerve can result in trapezius denervation, leading to lateral scapular winging (LSW), shoulder pain, and impaired arm elevation. While iatrogenic SAN injury most commonly occurs during cervical lymph node biopsies and neck dissections, it may also arise following posterior fossa neurosurgical procedures due to prolonged traction, compression, or mechanical stress during surgical positioning or, less commonly, accidental direct ligation. Recognizing this complication is essential, as symptoms may mimic intrinsic shoulder pathology and delay appropriate diagnosis. A 36-year-old man presented with right shoulder pain, perceived instability, and scapular protrusion beginning shortly after posterior fossa brain tumor resection performed in the prone position. Physical examination revealed right-sided scapular winging with inferior and lateral scapular displacement and shoulder asymmetry. MRI of the right shoulder demonstrated trapezius edema and fatty streaking consistent with subacute denervation, along with mild rotator cuff tendinosis and a labral tear. Electromyography (EMG) and nerve conduction studies (NCS) confirmed severe right spinal accessory mononeuropathy with active denervation, though the nerve remained in continuity. Ultrasound (US) corroborated trapezius atrophy and loss of muscle architecture on the affected side. This case illustrates SAN injury as an underrecognized complication of posterior fossa surgery, likely resulting from positioning-related traction or compression. The clinical and electrodiagnostic findings localized the injury distal to the sternocleidomastoid branch, consistent with classical LSW. A distinction between lateral and medial scapular winging is reviewed: LSW results from trapezius dysfunction and is accentuated by arm abduction, whereas medial scapular winging results from serratus anterior weakness and is accentuated by forward flexion. Management begins with physical therapy targeting compensatory scapular stabilizers, with microsurgical repair or tendon transfer procedures reserved for refractory cases. Clinicians should maintain a high index of suspicion for SAN injury in patients presenting with new shoulder pain or scapular winging following posterior fossa surgery, as early recognition facilitates timely rehabilitation and improved functional outcomes.