A 42-year-old woman with a history of repetitive gripping work presents with weakness of thumb adduction and sensory loss in the medial 1.5 fingers. Wrist flexion is normal. Which clinical sign best distinguishes a deep branch (motor) lesion in Guyon's canal from a superficial branch (sensory) lesion at the same site?

Explanation

## Discriminating Feature: Froment's Sign with Preserved Sensation ### Anatomical Basis of Guyon's Canal Branching Within Guyon's canal, the ulnar nerve divides into: 1. **Superficial (sensory) branch** → supplies sensation to medial 1.5 fingers and palm 2. **Deep (motor) branch** → supplies hypothenar muscles, medial lumbricals, interossei, and adductor pollicis **Key Point:** The two branches can be selectively damaged depending on the lesion location within Guyon's canal. A deep branch lesion causes motor deficit (weak thumb adduction = weak adductor pollicis) with preserved sensation. A superficial branch lesion causes sensory loss with preserved motor function. ### Froment's Sign Explained **Mnemonic: FROMENT = Flexion Reflex Of Thumb MEdiaN (actually ulnar, but the sign is classic)** — When the patient attempts to adduct the thumb (hold a piece of paper between thumb and index finger), weakness of adductor pollicis forces compensatory flexion of the thumb IP joint via flexor pollicis longus (innervated by anterior interosseous nerve, a branch of median nerve). A positive Froment's sign = thumb IP flexion during attempted adduction = adductor pollicis weakness = deep branch lesion. ### Comparison Table | Feature | Deep Branch Lesion (Motor) | Superficial Branch Lesion (Sensory) | | --- | --- | --- | | **Froment's sign** | Positive (thumb IP flexes) | Negative (normal adduction) | | **Sensation medial 1.5 fingers** | Intact | Lost | | **Sensation palm** | Intact | Lost | | **Adductor pollicis strength** | Weak | Normal | | **Hypothenar weakness** | Yes | No | | **Interossei weakness** | Yes | No | ### Clinical Pearl **High-Yield:** Froment's sign is pathognomonic for adductor pollicis weakness (ulnar nerve motor lesion). A positive Froment's sign with normal sensation = pure deep branch (motor) lesion in Guyon's canal. ### Why This Discriminates - **Deep branch lesion** → Positive Froment's sign + normal sensation = motor-only deficit - **Superficial branch lesion** → Negative Froment's sign + sensory loss = sensory-only deficit The combination of a positive motor sign (Froment's) with preserved sensation is pathognomonic for deep branch involvement. [cite:Clinically Oriented Anatomy 8e Ch 6]