A 38-year-old right-handed carpenter from Delhi presents with a 3-week history of progressive weakness and wasting of the thenar eminence. He reports pain and paresthesias in the lateral three and a half fingers of the right hand, worse at night. Examination reveals loss of thumb opposition and abduction, and sensory loss over the lateral palm and dorsal aspects of the index and middle fingers. Tinel's sign is positive at the wrist. Which nerve is compressed, and at what anatomical location?

Explanation

## Clinical Diagnosis: Carpal Tunnel Syndrome ### Key Anatomical Features **Key Point:** The clinical triad of carpal tunnel syndrome is (1) thenar muscle atrophy, (2) sensory loss in the lateral 3½ fingers, and (3) positive Tinel's sign at the wrist. The patient's presentation is classic for median nerve compression at the carpal tunnel: - **Motor deficit:** Thenar eminence wasting indicates loss of abductor pollicis brevis (APB) and opponens pollicis, both innervated by the recurrent (motor) branch of the median nerve distal to the carpal tunnel. - **Sensory deficit:** Loss of sensation in the lateral three and a half fingers (thumb, index, middle, and lateral half of ring finger) matches the median nerve's cutaneous distribution. - **Tinel's sign at the wrist:** Percussion over the median nerve at the carpal tunnel reproduces paresthesias, localizing the lesion to this site. - **Nocturnal symptoms:** Classic for carpal tunnel syndrome due to wrist flexion during sleep increasing intracarpal pressure. ### Median Nerve Course and Compression Sites | Anatomical Site | Nerve Branch Affected | Clinical Features | Exam Finding | |---|---|---|---| | **Carpal tunnel** | Recurrent motor branch + sensory branches | Thenar atrophy, sensory loss in lateral 3½ fingers | Tinel's at wrist, positive Phalen's | | **Pronator teres** | Anterior interosseous nerve (AIN) | Weakness of FPL and FDP (index), no sensory loss | Loss of thumb IP flexion, index DIP flexion | | **Ligament of Struthers** | Proximal median nerve | Proximal forearm pain, variable motor/sensory | Weakness proximal to pronator | **High-Yield:** The recurrent motor branch of the median nerve arises distal to the carpal tunnel and is the ONLY motor branch that can be affected by carpal tunnel compression. Lesions proximal to the carpal tunnel (e.g., at pronator teres) spare the thenar muscles because the recurrent branch arises distal to that level. ### Clinical Pearl **Clinical Pearl:** Dorsal sensory loss in the index and middle fingers (as described) is a key distinguishing feature. The median nerve's dorsal sensory branch (which supplies the dorsal aspects of the lateral 3½ fingers) arises proximal to the carpal tunnel, so it is preserved in pure carpal tunnel syndrome affecting only the recurrent motor and palmar sensory branches. However, in this case, the patient has dorsal sensory loss, which suggests either a more proximal lesion OR advanced carpal tunnel syndrome with retrograde involvement. Re-reading the stem: "sensory loss over the lateral palm and dorsal aspects of the index and middle fingers" — this is consistent with carpal tunnel syndrome because the dorsal branch can be involved in severe compression. ### Mnemonic for Median Nerve Motor Branches **Mnemonic:** **LOAF** = Lateral lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis (all innervated by the recurrent motor branch distal to carpal tunnel).