A 38-year-old female tailor from Mumbai presents with a 6-month history of progressive weakness affecting the 4th and 5th fingers of her right hand. She reports difficulty with fine motor tasks such as threading needles. On examination, there is atrophy of the first dorsal interosseous muscle, weakness of finger abduction and adduction, and a positive Froment's sign. Notably, sensation is completely intact over the dorsal and volar surfaces of the hand, including the medial one-and-a-half fingers. Which anatomical variant or site best explains this presentation?

Explanation

## Differential Diagnosis of Isolated Motor Ulnar Nerve Lesion **Key Point:** A pure motor ulnar nerve deficit with completely preserved sensation indicates a lesion distal to the point where the sensory branch has separated from the motor trunk. ### Clinical Findings Breakdown | Finding | Significance | |---------|-------------| | Atrophy of first dorsal interosseous | Intrinsic hand muscle denervation (motor) | | Weakness of finger abduction/adduction | Interossei paralysis (motor) | | Positive Froment's sign | Thumb IP joint flexion during pinch (FPL compensation for weak adductor pollicis) | | **Completely intact sensation** | Sensory branch NOT affected | **High-Yield:** The **superficial sensory branch of the ulnar nerve separates at the wrist level, approximately 5–8 cm proximal to the pisiform bone**. A lesion distal to this point affects motor only. ### Anatomical Course at the Wrist ```mermaid flowchart TD A[Ulnar Nerve at Wrist]:::outcome --> B{Superficial Sensory<br/>Branch Separated?}:::decision B -->|No - Proximal to separation| C[Motor + Sensory Loss]:::action B -->|Yes - Distal to separation| D{Which Motor Branch?}:::decision D -->|Superficial Motor Only| E[Hypothenar muscles only<br/>Sensation intact]:::action D -->|Deep Motor Branch Only| F[Intrinsic hand muscles<br/>Sensation intact]:::action F --> G[Guyon's Canal Lesion]:::outcome G --> H[Claw hand + Froment's sign<br/>NO sensory loss]:::outcome ``` ### Why Guyon's Canal? **Guyon's canal** is a fibro-osseous tunnel at the wrist bounded by: - Medially: pisiform bone and pisohamate ligament - Laterally: hook of hamate - Superficially: palmaris brevis and palmar fascia - Deeply: flexor carpi ulnaris tendon Within Guyon's canal, the ulnar nerve divides into: 1. **Superficial sensory branch** — exits the canal early (proximal to Guyon's canal proper) 2. **Deep motor branch** — continues through the canal to innervate: - Hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi) - Lumbricals 3 and 4 - Interossei (all) - Adductor pollicis **Clinical Pearl:** A lesion within Guyon's canal affects the **deep motor branch only**, producing: - Claw hand deformity (4th and 5th digits) - Weakness of interossei and medial lumbricals - **Completely preserved sensation** (sensory branch already separated) - Positive Froment's sign (adductor pollicis weakness) This is the **pure motor ulnar nerve syndrome** at the wrist. ### Why Not Other Sites? **Cubital tunnel:** Would cause sensory loss (dorsal and volar) because the dorsal cutaneous branch has not yet separated. **Arcade of Struthers:** A rare anatomical variant proximal to the cubital tunnel; would cause both motor and sensory loss. **Superficial sensory branch lesion:** Would cause sensory loss only, not motor deficit.