Upper Limb Neurology · Anatomy · High Yield

Radial Nerve

The radial nerve (C5–C8, T1) is the largest branch of the posterior cord and the sole nerve supplying the extensor compartments of the arm and forearm. Its anatomical course — spiralling through the radial groove — makes it vulnerable at several sites, producing distinct clinical syndromes.

✦ The Mnemonic

"Brave Soldiers Extend Everything Daily, All Pathways Explored Properly Efficiently"

Key muscles supplied by the radial nerve in proximal-to-distal order

B Brachioradialis Elbow flexion in mid-pronation; supplied by radial nerve BEFORE the groove

S Supinator Forearm supination; deep branch (PIN) passes through Arcade of Frohse

E ECRL/ECRB Extensor Carpi Radialis Longus and Brevis — wrist extension

E EDC Extensor Digitorum Communis — extends fingers at MCP joints

D EDM Extensor Digiti Minimi — extends little finger independently

A APL/EPB/EPL Abductor Pollicis Longus, Extensor Pollicis Brevis, Extensor Pollicis Longus

P EI Extensor Indicis — independent index finger extension

📚 Clinical Breakdown

The radial nerve arises from the posterior cord, passes posterior to the humerus in the radial groove (spiral groove) between the heads of the lateral and medial heads of triceps, and emerges at the lateral intermuscular septum. At the lateral epicondyle, it divides into the superficial radial nerve (purely sensory) and the deep radial nerve / posterior interosseous nerve (PIN) (motor, no skin territory).

Wrist drop is the hallmark of radial nerve injury in the forearm or radial groove — the patient cannot extend the wrist or fingers, and grip strength is severely weakened (because grip is lost when the wrist is flexed). Brachioradialis and ECRL are spared in posterior interosseous nerve (PIN) palsy because they branch off before the PIN enters the supinator.

Saturday night palsy = compression of the radial nerve in the radial groove from prolonged pressure on the axilla (e.g. sleeping with arm over a chair back). Triceps is spared (branches given off proximal to the groove). The key finding is wrist drop WITH intact triceps and preserved triceps reflex.

Origin Posterior cord (C5–T1)

Radial groove injury Wrist drop; triceps SPARED

PIN palsy Finger drop; wrist extension preserved (ECRL)

Sensory territory Superficial radial n. — dorsal hand/thumb web space

⭐ Clinical Pearl

PIN (posterior interosseous nerve) entrapment at the arcade of Frohse (fibrous arch of supinator) causes finger drop with preserved wrist extension (ECRL innervated before the PIN). This is distinct from radial nerve palsy at the groove. PIN entrapment is associated with repetitive forearm rotation, rheumatoid synovitis, and lipomas near the radial head.

⚠ Exam Trap

In Holstein-Lewis fracture (distal one-third humerus fracture), the radial nerve is tethered at the lateral intermuscular septum as it transitions anteriorly — injury rate ~12%. Always document radial nerve status before and after any proximal forearm manipulation. Nerve function should be re-assessed after fracture reduction.

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