A 28-year-old woman develops carpal tunnel syndrome with progressive loss of thumb opposition and wasting of the thenar eminence. Which single clinical finding best distinguishes carpal tunnel syndrome from a proximal median nerve lesion at the elbow?

Explanation

## Carpal Tunnel Syndrome vs Proximal Median Nerve Lesion at Elbow ### Anatomical Course of the Median Nerve **Key Point:** The median nerve gives off motor branches at different levels. Proximal branches (above the carpal tunnel) innervate forearm muscles, while distal branches (within and beyond the carpal tunnel) innervate thenar muscles. ### Comparative Motor Deficits | Motor Function | Carpal Tunnel Syndrome | Proximal Median Lesion (Elbow) | |---|---|---| | **Forearm pronation** | Preserved | Lost | | **Wrist flexion** | Preserved | Lost | | **Finger flexion (DIP/PIP)** | Preserved | Lost | | **Thumb opposition** | Lost | Lost | | **Thumb abduction (APB)** | Lost | Lost | | **Thumb flexion (IPJ)** | Preserved | Lost | ### Clinical Discriminator: Pronation **High-Yield:** **Preserved pronation of the forearm** is the best single discriminator between carpal tunnel syndrome and proximal median nerve lesion at the elbow. **Mechanism:** - **Carpal tunnel syndrome:** The median nerve is compressed within the carpal tunnel at the wrist. Pronator teres and pronator quadratus (innervated proximal to the carpal tunnel) remain fully functional → pronation is preserved. - **Proximal median lesion at elbow:** The nerve is damaged above the origin of pronator teres → both pronators are denervated → pronation is lost or severely weakened. **Clinical Pearl:** A patient with carpal tunnel syndrome can actively pronate the forearm against resistance, whereas a patient with a proximal median nerve lesion cannot. This single test reliably distinguishes the two conditions. ### Why Sensory and Other Motor Findings Are Not Discriminators - **Sensation in dorsal hand:** The dorsal cutaneous branch of the median nerve arises proximal to the carpal tunnel, so dorsal sensation is preserved in both conditions (the median nerve primarily supplies palmar sensation via the recurrent branch). - **Wrist flexion:** Flexor carpi radialis is innervated above the carpal tunnel, so wrist flexion is preserved in carpal tunnel syndrome but lost in proximal lesions — this is also a discriminator, but pronation testing is more specific and commonly used. - **Finger abduction:** Abduction is mediated by ulnar and radial nerves; median nerve does not abduct fingers.