Background Secondary peripheral nerve injuries remain a significant perioperative problem due to patient positioning and contribute to reduced patient quality of life and exacerbated professional liability. Comorbidities and concomitant lesions can further elicit these injuries in patients undergoing spinal surgeries. Case Presentation We report a case of a 70-year-old male polytrauma patient presenting with a left first-rib fracture and an adjacent hematoma around the brachial plexus without preoperative deficits. Subsequent to a lumbar spinal fusion in the prone position, he developed a postoperative left upper extremity monoplegia. The postoperative magnetic resonance imaging revealed an enhanced asymmetric signal in the trunks and cords of the left brachial plexus. He progressively improved with rehabilitation, a year after the initial presentation, with a residual wrist drop. Conclusions Pan brachial plexus monoplegia, following spine surgery, is rare and under-reported pathology. To minimize the occurrence of this rare morbidity, appropriate considerations in preoperative evaluation and counseling, patient positioning, intraoperative anesthetic, and electrophysiological monitoring should be performed. We emphasize an unreported risk factor in polytrauma patients, predisposing this rare injury that is associated with prone spinal surgery positioning, SEPs being an extremely sensitive test intraoperatively and highlight the importance of counseling patients and families to the possibility of this rare occurrence.