During a radical mastectomy with axillary lymph node dissection, a 55-year-old woman develops wrist drop and loss of thumb extension, with preserved elbow flexion and shoulder abduction. Which nerve was iatrogenically injured, and what is the anatomical basis for its vulnerability during this procedure?

Explanation

## Radial Nerve Injury During Axillary Dissection **Nerve Anatomy and Course:** - **Origin:** Terminal branch of the posterior cord (C5–C6–C7–C8) - **Course in axilla:** Runs posterior to the axillary artery, then travels posteriorly to enter the spiral groove of the humerus - **Motor innervation:** - Triceps (elbow extension) - Extensor carpi radialis longus and brevis (wrist extension) - Extensor carpi ulnaris (wrist extension) - Extensor digitorum (finger extension) - Extensor pollicis longus (thumb extension) - Abductor pollicis longus **Clinical Presentation Analysis:** - **Wrist drop** → Loss of wrist extensors (extensor carpi radialis/ulnaris) - **Loss of thumb extension** → Extensor pollicis longus paralyzed - **Preserved elbow flexion** → Musculocutaneous nerve intact - **Preserved shoulder abduction** → Axillary nerve intact **Anatomical Vulnerability:** The radial nerve is vulnerable during axillary dissection because: 1. It runs **posteriorly in the axilla** along the axillary artery 2. It is **stretched during traction** of the axillary contents during lymph node dissection 3. The nerve can be **compressed or lacerated** by retractors or direct manipulation 4. Its posterior location makes it less obvious during the dissection compared to anterior structures **High-Yield Fact:** Radial nerve injury during axillary dissection is one of the most common nerve injuries in breast surgery, occurring in 0.5–2% of cases. **Clinical Pearl:** Wrist drop + thumb extension loss = radial nerve injury until proven otherwise.