radial nerve palsy

**Anatomy:** The radial nerve arises from the posterior cord of the brachial plexus (roots C5–T1). It innervates the triceps, anconeus, brachioradialis, and extensor carpi radialis longus. It then divides into the deep branch (posterior interosseous nerve, which is purely motor) and the superficial branch (sensory). It gives off motor branches before and after the spiral groove, and the sensory supply is to the posterior arm, posterior forearm, and the dorsolateral hand (first dorsal web space, lateral 3.5 digits dorsally, proximal to the DIP).

**Mnemonic:** BEST = Brachioradialis, Extensors, Supinator, Triceps.

**Common causes and the level of injury:** - **Axilla (high lesion):** crutch use, "Saturday night palsy" (sleeping with the arm over a chair or bench, often after alcohol intoxication), or a dislocated shoulder. Result: triceps weakness (lost elbow extension and triceps reflex), wrist drop, and sensory loss. - **Spiral (radial) groove of the humerus (most common on exams):** midshaft humeral fracture. The triceps is spared because its branches leave the nerve proximal to the groove. Result: wrist drop, finger drop, and sensory loss over the dorsal hand and forearm, with preserved elbow extension. - **Elbow/forearm (posterior interosseous nerve syndrome):** from a supinator (arcade of Frohse) entrapment, radial head dislocation, or Monteggia fracture. Result: finger drop with *no sensory loss*. The wrist extends with radial deviation because the ECRL is spared. - **Superficial radial nerve (Wartenberg syndrome, "handcuff palsy"):** purely sensory, with numbness over the dorsolateral hand from tight watches or handcuffs. - Other causes: lead poisoning (classically wrist drop), diabetic mononeuropathy, and injection injury.

**Clinical features:** - **Wrist drop** (loss of wrist extension) and **finger drop** (loss of MCP extension). Grip strength is weak because wrist extension is needed to stabilize the wrist for flexor action. - Sensory loss over the dorsal first web space. - Weak thumb extension and abduction ("hitchhiker's thumb" cannot be done). - With a high lesion, loss of triceps strength and the triceps reflex. Brachioradialis weakness with a supinator deficit may also occur.

**Diagnosis:** Mainly clinical, based on localizing the lesion by which muscles and sensory areas are involved. EMG/NCS confirms the lesion and its severity at 2–3 weeks. Imaging (X-ray for a humeral fracture, ultrasound or MRI for entrapment) is used when a structural cause is suspected.

**Management:** - Treat the underlying cause. Most compression neurapraxias (Saturday night palsy) and many fracture-associated palsies recover spontaneously in weeks to months, because the nerve is usually in continuity. - Wrist cock-up splint to prevent contracture and to help function. - Physical and occupational therapy. - Surgical exploration if there is no recovery in about 3–6 months, if the nerve is transected (open fracture), or for a persistent entrapment. Tendon transfers are an option for permanent deficits.

**Differential for wrist drop:** cervical radiculopathy (C7), stroke/upper motor neuron lesion (the wrist is flexed and spastic, with other upper motor neuron signs), lead toxicity, and a posterior cord lesion (would also involve the deltoid and teres minor via the axillary nerve).

**High-yield pearls:** - Midshaft humerus fracture → radial nerve injury → wrist drop. - The triceps is spared in a spiral groove injury, but affected in an axillary injury. - Posterior interosseous syndrome has no sensory deficit. - Lead poisoning and alcohol-related compression are classic board associations.