median nerve

**Origin and course:** The median nerve is formed by the lateral cord (C5–C7) and medial cord (C8–T1) of the brachial plexus. The two roots wrap around the axillary artery. The nerve descends in the medial arm next to the brachial artery. It passes through the cubital fossa, medial to the brachial artery and deep to the bicipital aponeurosis. It then passes between the two heads of pronator teres and runs between flexor digitorum superficialis and profundus. It enters the hand through the carpal tunnel, deep to the flexor retinaculum. It gives no branches in the arm.

**Branches and motor function:** - Forearm (direct branches): pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis. - Anterior interosseous nerve: flexor pollicis longus, lateral half of flexor digitorum profundus (digits 2–3), pronator quadratus. - Recurrent (thenar) branch in the hand, the "LOAF" muscles: Lateral two lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis (superficial head).

**Sensory function:** - Palmar cutaneous branch: thenar eminence and central palm. It arises proximal to the carpal tunnel and passes superficial to the flexor retinaculum, so it is spared in carpal tunnel syndrome. - Digital branches: palmar surface of the lateral 3½ digits (thumb, index, middle, and radial half of ring), plus the dorsal fingertips and nail beds of those digits.

**Clinical correlates (high yield):** 1. **Carpal tunnel syndrome:** The nerve is compressed under the flexor retinaculum. - Causes: repetitive use, pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, amyloidosis, obesity. - Findings: nocturnal pain and paresthesias in the lateral 3½ digits, with thenar sparing of sensation (palmar cutaneous branch). Late findings are thenar atrophy and weak thumb abduction and opposition ("ape hand"). - Tests: positive Tinel sign (percussion over the wrist) and Phalen sign (wrist flexion). Electromyography/nerve conduction studies confirm the diagnosis. - Treatment: wrist splinting in neutral position, corticosteroid injection, and surgical release for refractory or severe cases. 2. **Pronator syndrome:** Compression between the heads of pronator teres. It causes forearm pain and sensory changes that include the thenar eminence. 3. **Anterior interosseous syndrome:** A purely motor deficit with a weak "OK sign" (can't flex the DIP of the index finger and IP of the thumb) due to loss of FPL and lateral FDP. Pronator quadratus weakness also occurs. There is no sensory loss. 4. **Proximal lesion (e.g., supracondylar humerus fracture, elbow dislocation):** - "Hand of benediction": on attempted fist formation, digits 2–3 stay extended because lateral FDP is lost, while digits 4–5 flex (ulnar nerve intact). - Weak wrist flexion with ulnar deviation (FCR lost, FCU intact), weak forearm pronation, and loss of thumb opposition. 5. **Laceration at the wrist:** Thenar muscle paralysis (ape hand) and sensory loss in the lateral 3½ digits, but forearm muscles are spared.

**Mnemonics and pearls:** - "LOAF" for the median-innervated thenar and lumbrical muscles. - Median nerve = "median" in the cubital fossa, medial to the brachial artery. - Median vs ulnar: Median nerve injury at the elbow gives "hand of benediction" on attempted fist. Ulnar nerve injury gives "claw hand" at rest (digits 4–5), because the medial lumbricals are lost. - Pronator teres is the key landmark: the nerve can be entrapped between its two heads. - Palmar cutaneous branch sparing differentiates carpal tunnel syndrome from a more proximal median nerve lesion.