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:

Explanation

## 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.