Radial Neuropathies

The radial nerve (C5 to T1, posterior cord) supplies the triceps, anconeus, brachioradialis, wrist and finger extensors, and the supinator, with sensation to the posterior arm, forearm and dorsoradial hand.

Anatomical course determines the syndromes

• It leaves the axilla and passes into the spiral groove on the posterior humerus, where it is closely applied to bone.
• It pierces the lateral intermuscular septum and runs between brachialis and brachioradialis.
• At the elbow it divides into the superficial radial (sensory) nerve and the posterior interosseous nerve, which passes through the arcade of Frohse into the supinator.

Sites and causes of injury

• Axilla: crutch compression, prolonged arm hanging over a chair, and shoulder dislocation.
• Spiral groove: compression during deep sleep or intoxication ("Saturday night palsy"), tourniquets, humeral shaft fractures (radial palsy in 10 to 18%, higher in distal third oblique fractures, Holstein-Lewis), callus entrapment, and injection injury.
• Proximal forearm: posterior interosseous nerve syndrome from the arcade of Frohse, lipoma, ganglion, synovitis of the elbow in rheumatoid arthritis, and radial head fracture or dislocation.
• Radial tunnel syndrome: a contested pain syndrome from compression of the same nerve with no weakness.
• Wrist: superficial radial sensory neuropathy (Wartenberg syndrome, cheiralgia paraesthetica) from handcuffs, watch straps and tight casts.

Systemic and diffuse causes include lead poisoning, diabetes, alcohol, vasculitis, hereditary neuropathy with liability to pressure palsies, multifocal motor neuropathy, neuralgic amyotrophy (isolated posterior interosseous palsy), and ischaemia from vascular disease.

Prognosis is good in most compressive injuries. Most spiral groove neurapraxias recover within 2 to 4 months, while axonotmetic and neurotmetic lesions need regeneration at about 1 mm/day.

Features by lesion level

• Axilla: triceps weakness with loss of the triceps reflex, plus wrist drop and finger drop, with sensory loss on the posterior arm and forearm.
• Spiral groove: wrist drop, finger drop and thumb drop, with sparing of triceps. Brachioradialis is weak, and sensory loss is limited to the dorsal first web space and the dorsoradial hand.
• Posterior interosseous nerve: finger and thumb extension weakness without sensory loss. Because extensor carpi radialis longus is spared (it is supplied above the supinator), the wrist extends with radial deviation.
• Superficial radial nerve: numbness, burning pain or tingling over the dorsoradial hand and thumb, worsened by wrist flexion and ulnar deviation.
• Radial tunnel syndrome: deep aching pain in the lateral proximal forearm, with tenderness about 4 to 5 cm distal to the lateral epicondyle, pain on resisted supination and resisted middle finger extension, and no weakness.

Characteristic signs

• Wrist drop with grip weakness caused by loss of the wrist extensor stabilising synergy.
• In finger drop, extension at the metacarpophalangeal joints is lost but interphalangeal extension is preserved through the intrinsics.
• Thumb extension and abduction are lost.

Differential diagnosis

• C7 radiculopathy (weak triceps and flexor carpi radialis, neck pain).
• Posterior cord or upper trunk plexopathy.
• Extensor tendon rupture in rheumatoid arthritis (preserved passive tenodesis effect is absent).
• Lateral epicondylitis.
• Cortical hand knob stroke, which mimics a radial palsy.
• Lead toxicity, which causes bilateral wrist drop with abdominal pain and anaemia.

Diagnosis is clinical, with localisation by examination of muscles above and below the suspected site. Document triceps, brachioradialis, extensor carpi radialis, extensor digitorum, extensor pollicis longus, and sensory loss.

Electrodiagnostic testing at 2 to 3 weeks or later

• Reduced radial CMAP amplitudes recorded from the extensor indicis, and reduced or absent superficial radial SNAP, with comparison to the opposite side.
• Conduction block across the spiral groove indicates neurapraxia and predicts good recovery.
• Needle EMG localises the lesion, with denervation in extensor digitorum and extensor indicis, and sparing of triceps in groove lesions.
• Fibrillation potentials indicate axon loss and slower recovery.

Imaging

• Radiographs of the humerus, elbow and wrist.
• High resolution ultrasound shows nerve continuity, swelling, entrapment, neuroma, and tumours such as lipoma or ganglion, and can be used dynamically.
• MRI shows denervation oedema in supinator and extensors, and masses compressing the nerve.

Laboratory tests: HbA1c, vitamin B12, ESR, and blood lead levels in unexplained wrist drop.

Conservative care for neurapraxia and axonotmesis

• Neutral to slight extension wrist cock-up splint, with a dynamic outrigger finger extension splint to prevent contracture and allow function.
• Passive range of motion exercises, followed by strengthening as function returns.
• Pain control with gabapentin or pregabalin, and amitriptyline if needed.
• Remove the compressive cause, adjust crutches, and avoid pressure.
• Observe for 3 months with serial clinical and EMG assessment.

Humeral shaft fractures with radial palsy

• A palsy present at the time of a closed fracture is observed, since most are neurapraxia. Immobilise or fix the fracture per orthopaedic indication.
• Early exploration is indicated for open fractures, vascular injury, palsy that develops after manipulation or surgery, and suspected nerve transection or entrapment in the fracture.
• Absence of Tinel sign advancement, EMG reinnervation or clinical recovery at 3 to 4 months: explore with neurolysis, repair or nerve grafting (sural nerve graft for gaps beyond 2 to 3 cm).
• Nerve transfers: median nerve branches (to flexor digitorum superficialis or pronator teres) to the posterior interosseous nerve or extensor carpi radialis brevis branch.

Tendon transfers (late or irreparable palsy)

• Pronator teres to extensor carpi radialis brevis (wrist extension).
• Flexor carpi radialis or flexor carpi ulnaris to extensor digitorum communis (finger extension).
• Palmaris longus to extensor pollicis longus (thumb extension).

Posterior interosseous nerve compression

• If a mass or synovitis is present, surgical excision with decompression at the arcade of Frohse.
• Idiopathic palsy without mass: observe for 3 months, then decompress if there is no improvement.
• Neuralgic amyotrophy causes are observed with steroids as appropriate.

Radial tunnel syndrome

• Rest, activity modification, NSAIDs (naproxen 500 mg twice daily), and a forearm splint for 4 to 6 weeks.
• Injection of methylprednisolone 20 to 40 mg with 3 to 5 mL lidocaine 1% to the tunnel.
• Surgical release of the arcade of Frohse and supinator after 3 to 6 months of failed therapy, with variable success.

Superficial radial neuropathy

• Remove constricting items, short term wrist splint, and NSAIDs.
• Triamcinolone 10 to 20 mg local injection for persistent symptoms.
• Neurolysis, or neurectomy with burial in muscle, for refractory pain and neuroma.

Other causes

• Lead poisoning: remove the source, chelation with succimer 10 mg/kg three times daily for 5 days then twice daily for 14 days, or calcium disodium EDTA for severe cases.
• Diabetes: optimise glucose control.
• Compartment syndrome: urgent fasciotomy.

Prevention includes correct arm positioning during anaesthesia and sleep, avoiding tight tourniquets, and fixing fractures with careful radial nerve protection.