A 28-year-old male presents with weakness of finger abduction and adduction, loss of wrist flexion, and sensory loss over the medial forearm. Which finding best distinguishes this presentation from a posterior cord injury?

Explanation

## Distinguishing Lower Trunk from Posterior Cord Injury ### Clinical Presentation Analysis The patient presents with: - Weakness of finger abduction/adduction (ulnar nerve) - Loss of wrist flexion (ulnar/median nerves) - Sensory loss over medial forearm (ulnar nerve) **Key Point:** This constellation of findings indicates a **lower trunk (C8–T1) injury**, not a posterior cord injury. ### Lower Trunk (C8–T1) Injury - **Motor loss:** Intrinsic hand muscles (finger abduction/adduction via ulnar), wrist flexion, finger flexion - **Preserved:** Elbow extension (radial nerve from posterior cord), wrist extension (radial nerve) - **Sensory loss:** Medial forearm and hand (ulnar distribution) - **Hand appearance:** Claw hand deformity ### Posterior Cord Injury - **Motor loss:** Elbow extension (triceps), wrist extension (ECRB, ECRL), finger extension, shoulder external rotation (infraspinatus) - **Preserved:** Wrist flexion, finger flexion, intrinsic hand muscles - **Sensory loss:** Dorsal forearm and hand (radial distribution) - **Hand appearance:** Wrist drop, extended fingers cannot be extended ### Comparison Table | Feature | Lower Trunk | Posterior Cord | |---------|-------------|----------------| | **Nerve roots** | C8–T1 | C5–C8 (all levels) | | **Elbow extension** | Preserved | Lost | | **Wrist extension** | Preserved | Lost | | **Wrist flexion** | Lost | Preserved | | **Finger abduction** | Lost | Preserved | | **Sensory loss** | Medial forearm | Dorsal forearm/hand | | **Claw hand** | Present | Absent | | **Wrist drop** | Absent | Present | **High-Yield:** The **preservation of elbow extension and wrist extension** is the best discriminator. In lower trunk injury, the radial nerve (from posterior cord) remains intact, so these functions are preserved. In posterior cord injury, both are lost. **Mnemonic:** **RAD** = Radial nerve (Posterior cord) → **R**adial nerve loss = **R**adial-innervated muscles lost (wrist **E**xtension, **E**lbow extension). If radial function is preserved, posterior cord is spared. **Clinical Pearl:** Posterior cord injury is often seen in axillary nerve compression (crutch palsy) or in complete brachial plexus avulsion; lower trunk injury is more common in birth trauma and traction injuries.