A lesion affecting the upper trunk of the brachial plexus (C5–C6 nerve roots) results in which characteristic clinical presentation?

Explanation

## Upper Trunk Brachial Plexus Lesion (Erb's Palsy) **Key Point:** Upper trunk injury (C5–C6) results in **Erb's palsy**, characterized by the "waiter's tip" position due to paralysis of shoulder abductors and external rotators. ### Anatomical Basis The upper trunk (C5–C6) gives rise to: - **Suprascapular nerve** → Supraspinatus and infraspinatus (shoulder abduction and external rotation) - **Nerve to subclavius** - Contributes to the **upper and posterior cords** ### Clinical Features of Upper Trunk Lesion | Feature | Mechanism | Result | |---------|-----------|--------| | Loss of shoulder abduction | Supraspinatus paralysis | Arm hangs at side | | Loss of external rotation | Infraspinatus paralysis | Shoulder internally rotated | | Characteristic posture | Combined effect | **Waiter's tip** (arm adducted, internally rotated, elbow extended) | | Sensory loss | C5–C6 dermatomes | Lateral shoulder and lateral forearm | **Mnemonic:** **RUDIN** for upper trunk branches: - **R**otator cuff (supraspinatus, infraspinatus) - **U**pper subscapular - **D**orsal scapular - **I**nfratrochlear - **N**erve to subclavius **High-Yield:** Erb's palsy is the most common type of brachial plexus injury, accounting for ~50% of cases, often from shoulder dystocia in neonates or motorcycle accidents in adults. **Clinical Pearl:** The "waiter's tip" position is pathognomonic — the arm is held in adduction, internal rotation, and elbow extension, resembling a waiter carrying a tray with the arm at the side.