A 32-year-old man presents with wrist drop and inability to extend the thumb at the interphalangeal joint after a fracture of the mid-shaft humerus. Which clinical finding best distinguishes a proximal median nerve lesion (above the elbow) from a distal lesion (at the wrist)?

## Distinguishing Proximal vs Distal Median Nerve Lesions ### Anatomical Basis of Motor Innervation **Key Point:** The pronator teres and pronator quadratus muscles receive motor innervation from the median nerve proximal to the wrist. A proximal lesion (above the elbow) damages these pronators, while a distal lesion (at the wrist) spares them. ### Motor Branches of the Median Nerve | Structure | Innervation Level | Effect of Proximal Lesion | Effect of Distal Lesion | |-----------|-------------------|--------------------------|-------------------------| | Pronator teres | Above elbow | Lost | Preserved | | Pronator quadratus | Distal forearm | Lost | Preserved | | Flexor carpi radialis | Above elbow | Lost | Preserved | | Flexor digitorum superficialis | Above elbow | Lost | Preserved | | Thenar muscles (Abductor pollicis brevis, Flexor pollicis brevis, Opponens pollicis) | Distal (recurrent branch) | Lost | Lost | | Lumbricals (1st & 2nd) | Distal | Lost | Lost | ### Clinical Differentiation **High-Yield:** Loss of **pronation of the forearm** is the single best discriminator between proximal and distal median nerve lesions. - **Proximal lesion (above elbow):** Pronation is lost because both pronator teres and pronator quadratus are denervated. The forearm assumes a supinated position and cannot be actively pronated. - **Distal lesion (at wrist):** Pronation is preserved because pronator teres (innervated above the elbow) remains functional. **Clinical Pearl:** To test pronation, ask the patient to flex the elbow to 90° and attempt to pronate the forearm against resistance. Loss of this movement indicates a proximal lesion. ### Why Other Features Are Not Discriminators - **Loss of opposition of the thumb:** Occurs in BOTH proximal and distal lesions (recurrent branch arises distal to the wrist). - **Loss of flexion of the interphalangeal joint of the thumb:** Flexor pollicis longus is innervated above the elbow, so this is lost in both proximal and distal lesions. - **Loss of sensation over the lateral palm:** Occurs in both proximal and distal lesions; sensory loss is not a discriminator.