Defining a danger zone for iatrogenic long thoracic nerve injury in gender-affirming mastectomy

The long thoracic nerve's (LTN) superficial location on the chest wall renders it vulnerable to iatrogenic injury. Plastic surgeons' gender-affirming mastectomy volumes are rapidly increasing. This operation involves lateral chest contouring placing the distal LTN at risk of injury along the chest wall. Although proximal LTN injury can cause debilitating shoulder dysfunction, more distal injury can cause chronic postoperative shoulder pain and dysfunction without frank scapular winging, making diagnosis and treatment difficult. Therefore, injury is best avoided. In this study, the course of the LTN relative to the lateral border of the pectoralis major muscle was mapped to delineate and define a danger zone for LTN injury in gender-affirming mastectomy. The course of the LTN along the lateral chest wall relative to the pectoralis major lateral border was prospectively mapped using intraoperative nerve stimulation. Twelve individuals were enrolled. The LTN was mapped bilaterally and was most reliably located directly lateral to the intersection of the 4th rib and the lateral border of the pectoralis major muscle. The LTN was found a median of 4.3 cm (interquartile range [IQR] = 0.9 cm) lateral to the pectoralis major at the 3rd rib level, 5.0 cm (IQR = 0.5 cm) lateral to the pectoralis border at the 4th rib level, and 7.0 cm (IQR = 0.5 cm) lateral to the pectoralis border at the 5th rib level. In conclusion, this study defines a danger zone for LTN injury in gender-affirming mastectomy. With recent increases in the number of plastic surgeons performing gender-affirming mastectomies, awareness of this LTN danger zone is important to avoid iatrogenic injuries.