Which clinical finding best distinguishes an upper trunk (Erb's palsy) injury of the brachial plexus from a lower trunk (Klumpke's palsy) injury?

Question

Accepted Answer

A. Loss of shoulder abduction and external rotation with preserved hand intrinsic muscle function. ## Distinguishing Upper vs Lower Trunk Injuries

**Key Point:** Upper trunk (C5–C6) and lower trunk (C8–T1) injuries produce opposite patterns of motor loss, and the **proximal vs distal** gradient is the primary discriminator.

### Upper Trunk (Erb's Palsy) — C5–C6
- **Affected nerves:** Suprascapular, axillary, musculocutaneous
- **Motor loss:** Shoulder abduction (supraspinatus), shoulder external rotation (infraspinatus), elbow flexion (biceps, brachialis)
- **Preserved:** Wrist/finger movements, hand intrinsics (lower trunk spared)
- **Classic posture:** "Waiter's tip" — arm hangs at side, internally rotated, elbow extended

### Lower Trunk (Klumpke's Palsy) — C8–T1
- **Affected nerves:** Ulnar, medial cutaneous, medial head of median
- **Motor loss:** Wrist flexion, finger flexion, hand intrinsics (lumbricals, interossei)
- **Preserved:** Shoulder and elbow movements (upper trunk spared)
- **Classic sign:** Claw hand — hyperextended MCP, flexed IP joints (unopposed extensor action)

### Comparison Table

| Feature | Upper Trunk (Erb's) | Lower Trunk (Klumpke's) |
|---------|---------------------|------------------------|
| **Shoulder abduction** | ✗ Lost | ✓ Preserved |
| **Shoulder external rotation** | ✗ Lost | ✓ Preserved |
| **Elbow flexion** | ✗ Lost | ✓ Preserved |
| **Wrist flexion** | ✓ Preserved | ✗ Lost |
| **Finger flexion** | ✓ Preserved | ✗ Lost |
| **Hand intrinsics** | ✓ Preserved | ✗ Lost |
| **Claw hand** | Absent | Present |
| **Waiter's tip posture** | Present | Absent |

**High-Yield:** The **proximal-distal rule** is absolute: upper trunk = proximal loss (shoulder/elbow), lower trunk = distal loss (wrist/hand). Option B captures this perfectly — loss of shoulder abduction/external rotation (proximal) with preserved hand intrinsics (distal) = upper trunk.

**Clinical Pearl:** In birth injuries, Erb's palsy (upper trunk) is far more common (~95% of brachial plexus injuries) than Klumpke's because upper trunk is more vulnerable to traction during delivery.

![Brachial Plexus diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3406.webp)

Answer

B. Claw hand deformity with preserved shoulder movements

Answer

C. Loss of wrist flexion with preserved finger extension

Answer

D. Loss of elbow flexion with intact wrist and finger movements

Which clinical finding best distinguishes an upper trunk (Erb's palsy) injury of the brachial plexus from a lower trunk (Klumpke's palsy) injury?

Explanation

Distinguishing Upper vs Lower Trunk Injuries

Upper Trunk (Erb's Palsy) — C5–C6

Lower Trunk (Klumpke's Palsy) — C8–T1

Comparison Table

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Feature	Upper Trunk (Erb's)	Lower Trunk (Klumpke's)
Shoulder abduction	✗ Lost	✓ Preserved
Shoulder external rotation	✗ Lost	✓ Preserved
Elbow flexion	✗ Lost	✓ Preserved
Wrist flexion	✓ Preserved	✗ Lost
Finger flexion	✓ Preserved	✗ Lost
Hand intrinsics	✓ Preserved	✗ Lost
Claw hand	Absent	Present
Waiter's tip posture	Present	Absent