A 28-year-old male motorcyclist sustains a traction injury to the right brachial plexus during a high-speed collision. On examination, he has weakness of wrist and finger flexion, loss of hand grip strength, and a claw hand deformity. Sensation is lost over the medial aspect of the forearm and hand. Which nerve root injury best explains this clinical picture, and what is the most reliable distinguishing feature from a complete upper trunk injury?

Explanation

## Lower Trunk (C8–T1) Injury: Clinical Recognition **Key Point:** The combination of **distal motor loss** (wrist/finger flexion, hand intrinsics) with **preserved proximal function** (shoulder abduction, elbow flexion) is pathognomonic for lower trunk injury and definitively excludes upper trunk involvement. ### Clinical Findings in This Case 1. **Weakness of wrist and finger flexion** → Ulnar nerve (C8–T1) and medial median nerve (C8–T1) 2. **Loss of hand grip strength** → Intrinsic hand muscles (lumbricals, interossei) — ulnar and medial median 3. **Claw hand deformity** → Unopposed extensor digitorum (radial nerve, C5–C8) acting on denervated lumbricals 4. **Sensory loss over medial forearm and hand** → Medial antebrachial cutaneous (C8–T1) and ulnar nerve (C8–T1) ### Why C8–T1 (Lower Trunk), Not Upper Trunk? | Finding | Upper Trunk (C5–C6) | Lower Trunk (C8–T1) | This Patient | |---------|---------------------|---------------------|---------------| | **Shoulder abduction** | ✗ Lost | ✓ Preserved | ✓ Preserved | | **Elbow flexion** | ✗ Lost | ✓ Preserved | ✓ Preserved | | **Wrist flexion** | ✓ Preserved | ✗ Lost | ✗ Lost | | **Finger flexion** | ✓ Preserved | ✗ Lost | ✗ Lost | | **Hand intrinsics** | ✓ Preserved | ✗ Lost | ✗ Lost | | **Claw hand** | Absent | Present | Present | **High-Yield:** The **preserved shoulder abduction and elbow flexion** are the single most reliable discriminators. These proximal movements depend on C5–C6 (suprascapular and musculocutaneous nerves), which are **spared** in lower trunk injury but **lost** in upper trunk injury. This patient has both intact, proving lower trunk pathology. **Clinical Pearl:** Klumpke's palsy from traction injury is often associated with **Horner syndrome** (miosis, ptosis, anhidrosis) if the T1 root is avulsed, because sympathetic fibers to the eye travel with T1. Always examine for this sign in suspected lower trunk injuries. **Mnemonic:** **CLAW = C8–T1 Loss Affects Wrist** — Remember claw hand = lower trunk.