A 28-year-old male labourer presents to the emergency department following a motorcycle accident with a severe traction injury to his left arm. On examination, he has loss of sensation over the lateral aspect of the forearm and dorsal aspect of the thumb and index finger. He also demonstrates weakness of shoulder abduction and external rotation. There is a characteristic 'waiter's tip' position of the left arm. Which nerve root(s) of the brachial plexus is/are most likely injured?

Explanation

## Clinical Presentation Analysis The patient presents with a classic **Erb's palsy** (upper trunk brachial plexus injury), characterized by: ### Motor Deficits - **Loss of shoulder abduction** → Supraspinatus and infraspinatus paralysis (axillary nerve, C5–C6) - **Loss of external rotation** → Infraspinatus paralysis (suprascapular nerve, C5–C6) - **'Waiter's tip' position** → Arm held in adduction and internal rotation due to unopposed action of pectoralis major and latissimus dorsi ### Sensory Deficits - **Lateral forearm** → Lateral antebrachial cutaneous nerve (continuation of musculocutaneous nerve, C5–C6) - **Dorsal thumb and index finger** → Radial nerve (C5–C6 contribution) ### Nerve Root Involvement | Nerve Root | Motor Supply | Sensory Supply | |---|---|---| | **C5** | Deltoid, supraspinatus, infraspinatus | Lateral shoulder | | **C6** | Biceps, brachialis, musculocutaneous | Lateral forearm, dorsal thumb/index | | C7 | Triceps, wrist extensors | Dorsal middle finger | | C8–T1 | Intrinsic hand muscles | Medial forearm/hand | **Key Point:** Erb's palsy results from injury to the **upper trunk** of the brachial plexus, formed by **C5 and C6 nerve roots** only. The traction mechanism (shoulder depression + neck flexion away) preferentially damages these proximal roots. **High-Yield:** The classic triad of Erb's palsy is: 1. Loss of shoulder abduction (deltoid) 2. Loss of elbow flexion (biceps) 3. Loss of external rotation (infraspinatus) **Clinical Pearl:** Waiter's tip position is pathognomonic—the arm hangs in adduction and internal rotation because the muscles that abduct and externally rotate are paralyzed, while the internal rotators (pectoralis major, latissimus dorsi) remain intact and unopposed.