A 38-year-old carpenter presents with weakness of finger abduction and adduction, loss of sensation over the medial 1.5 fingers, and wasting of the hypothenar eminence. Examination reveals a palpable mass in the medial aspect of the palm near the wrist crease. Regarding ulnar nerve lesions at the wrist, all of the following are true EXCEPT:

Question

Accepted Answer

D. Claw hand deformity affects all four fingers (2nd to 5th) in ulnar nerve palsy at the wrist. ## Ulnar Nerve Lesions at the Wrist: Clinical Correlations

### Anatomical Basis for Wrist-Level Lesions
**Key Point:** At the wrist, the ulnar nerve divides into superficial and deep branches distal to the pisiform bone. Lesions at different levels produce distinct clinical patterns:

| Feature | Superficial Branch Lesion | Deep Branch Lesion | Complete Ulnar Nerve Lesion |
|---------|---------------------------|-------------------|-----------------------------|
| **Motor deficit** | Hypothenar muscles only | Interossei + lumbricals (medial 2) + adductor pollicis | All intrinsic hand muscles |
| **Sensory deficit** | Medial 1.5 fingers | None (purely motor) | Medial 1.5 fingers |
| **Claw hand** | Absent | 4th–5th fingers only | 2nd–5th fingers |
| **Froment's sign** | Negative | Positive | Positive |

### Claw Hand Deformity: The Critical Distinction
**High-Yield:** Claw hand in ulnar nerve palsy affects **only the 4th and 5th fingers** (medial two), NOT all four fingers:
- **Mechanism:** Loss of lumbricals (medial 2) and interossei → unopposed action of extensor digitorum → hyperextension at MCP joints + flexion at IP joints
- **2nd and 3rd fingers are spared** because their lumbricals are supplied by the **median nerve**

**Clinical Pearl:** A patient with "claw hand affecting all four fingers" suggests a **combined median + ulnar nerve lesion** (e.g., anterior interosseous syndrome + ulnar nerve palsy), not isolated ulnar nerve injury.

### Froment's Sign
**Mnemonic:** **FIP** = **F**roment's sign tests **I**ntrinsic hand muscles (adductor pollicis) via **P**inch test.
- **Positive:** Thumb IP joint flexes when pinching paper (patient compensates for weak adductor pollicis by recruiting flexor pollicis longus)
- **Present in:** Complete ulnar nerve palsy, deep branch lesion
- **Absent in:** Superficial branch lesion (sensory only)

### Guyon's Canal Syndrome
**Key Point:** A ganglion cyst or other mass in Guyon's canal can compress the deep branch of the ulnar nerve **after** it has separated from the superficial branch. This causes:
- **Motor:** Weakness of interossei, medial lumbricals, adductor pollicis
- **Sensory:** Preserved (superficial branch intact)
- **Clinical presentation:** Selective deep branch palsy with normal sensation

## Why Option 3 Is Wrong
Claw hand deformity in ulnar nerve palsy affects **only the 4th and 5th fingers**, not all four fingers. The 2nd and 3rd fingers are spared because their lumbricals are median-innervated. Option 3 incorrectly states that all four fingers are affected, making it the exception.

Answer

A. A deep branch ulnar nerve lesion at the wrist would spare sensation over the medial 1.5 digits

Answer

B. Froment's sign (flexion of the thumb IP joint when pinching paper) is positive in complete ulnar nerve palsy

Answer

C. Guyon's canal syndrome from a ganglion cyst can cause selective deep branch palsy while preserving sensation

Explanation

Ulnar Nerve Lesions at the Wrist: Clinical Correlations

Anatomical Basis for Wrist-Level Lesions

Claw Hand Deformity: The Critical Distinction

Froment's Sign

Guyon's Canal Syndrome

Why Option 3 Is Wrong

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Feature	Superficial Branch Lesion	Deep Branch Lesion	Complete Ulnar Nerve Lesion
Motor deficit	Hypothenar muscles only	Interossei + lumbricals (medial 2) + adductor pollicis	All intrinsic hand muscles
Sensory deficit	Medial 1.5 fingers	None (purely motor)	Medial 1.5 fingers
Claw hand	Absent	4th–5th fingers only	2nd–5th fingers
Froment's sign	Negative	Positive	Positive