A 35-year-old male laborer presents with a birth-related brachial plexus injury sustained during a difficult delivery 35 years ago. He has persistent paralysis of the intrinsic hand muscles with sensory loss over the medial aspect of the forearm and hand. Which nerve of the brachial plexus is most commonly injured in such obstetric injuries?

Explanation

## Most Common Brachial Plexus Injury in Obstetric Trauma **Key Point:** The lower trunk (C8–T1) is the most commonly injured component in obstetric brachial plexus injuries, accounting for approximately 20–30% of birth-related injuries. However, when considering the **entire lower trunk distribution**, Klumpke's palsy (lower trunk injury) is the second most common presentation after Erb's palsy in birth injuries. ### Mechanism of Lower Trunk Injury in Birth Trauma Lower trunk injury occurs when: 1. The arm is forcibly abducted and externally rotated during delivery 2. Traction is applied upward on the arm (hyperabduction) 3. The lower trunk is stretched across the first rib and anterior scalene muscle 4. This is less common than upper trunk injury but still significant ### Clinical Presentation of Lower Trunk Injury (Klumpke's Palsy) | Finding | Nerve Affected | |---------|----------------| | **Paralysis of intrinsic hand muscles** | Ulnar nerve (C8–T1) and deep branch of median nerve | | **Sensory loss over medial forearm and hand** | Medial antebrachial cutaneous nerve (C8–T1) | | **"Claw hand" deformity** | Hyperextension at MCP joints, flexion at IP joints | | **Loss of finger abduction/adduction** | Ulnar intrinsics (interossei and lumbricals) | | **Horner's syndrome (if T1 root avulsed)** | Sympathetic fibers in T1 root | **High-Yield:** In obstetric injuries, the lower trunk is injured in hyperabduction scenarios, whereas the upper trunk is injured in lateral neck flexion with shoulder depression (Erb's palsy—the most common obstetric injury overall at ~50%). ### Comparative Frequency of Obstetric Brachial Plexus Injuries | Injury Type | Frequency | Mechanism | |-------------|-----------|----------| | **Upper trunk (Erb's)** | ~50–60% | Shoulder depression + lateral neck flexion | | **Lower trunk (Klumpke's)** | ~20–30% | Arm hyperabduction | | **Complete plexus** | ~10–20% | Severe traction | | **Middle trunk only** | ~5% | Rare | **Clinical Pearl:** Klumpke's palsy with Horner's syndrome (miosis, ptosis, anhidrosis) indicates T1 root avulsion—a severe injury with poor prognosis for spontaneous recovery. **Mnemonic for Obstetric Injuries:** **ERK** = **E**rb's (upper trunk) is most common; **R**oot avulsion is worst; **K**lumpke's (lower trunk) is second most common. [cite:Standring Anatomy 41e Ch 57]