A 28-year-old male motorcyclist is brought to the emergency department following a high-speed collision. On examination, he has a flaccid right arm with loss of sensation over the lateral aspect of the forearm and dorsal first web space. He cannot abduct his shoulder or flex his elbow. There is also loss of sensation over the lateral shoulder. Imaging confirms avulsion of nerve roots from the spinal cord. Which nerve root(s) are most likely avulsed?

## Clinical Presentation Analysis The patient presents with a classic **upper trunk (Erb's palsy) injury** pattern resulting from brachial plexus root avulsion. ### Motor Deficits - **Loss of shoulder abduction** → Supraspinatus and infraspinatus paralysis → **Axillary nerve** (C5, C6) - **Loss of elbow flexion** → Biceps and brachialis paralysis → **Musculocutaneous nerve** (C5, C6) ### Sensory Deficits - **Lateral forearm** (lateral antebrachial cutaneous nerve) → **C6 dermatome** - **Dorsal first web space** (radial nerve) → **C6 dermatome** - **Lateral shoulder** (axillary nerve) → **C5 dermatome** ### Root-Level Anatomy | Nerve Root | Motor Supply | Sensory Supply | Key Reflex | |---|---|---|---| | **C5** | Deltoid, rotator cuff, biceps | Lateral shoulder | Biceps reflex | | **C6** | Biceps, wrist extensors | Lateral forearm, thumb, index | Brachioradialis reflex | | C7 | Triceps, wrist flexors | Middle finger | Triceps reflex | | C8 | Intrinsic hand muscles | Medial forearm, ring/little finger | — | | T1 | Intrinsic hand muscles | Medial arm | — | **Key Point:** The **upper trunk** of the brachial plexus is formed by C5 and C6 roots. Avulsion of these roots produces the classic Erb's palsy pattern: "waiter's tip" position (shoulder adducted, internally rotated; elbow extended; forearm pronated). **High-Yield:** Erb's palsy (upper trunk injury) is the most common brachial plexus injury in birth trauma and traumatic avulsions. Klumpke's palsy (lower trunk, C8–T1) presents with claw hand and medial forearm sensory loss. **Clinical Pearl:** The presence of **dorsal first web space sensory loss** is pathognomonic for C6 involvement—this is the most reliable single sensory marker for upper trunk injury.