Median Nerve Neuropathy

The median nerve (C5 to T1, lateral and medial cords) supplies the pronators, wrist and finger flexors (except flexor carpi ulnaris and the ulnar half of flexor digitorum profundus), the thenar muscles, and the first two lumbricals, with sensation to the palmar radial three and a half digits.

Sites of injury and compression proximal to the wrist

• Ligament of Struthers: a fibrous band running from a supracondylar process (present in 1 to 3% of people) to the medial epicondyle. The nerve and brachial artery pass beneath it.
• Bicipital aponeurosis (lacertus fibrosus).
• Pronator teres: between its two heads.
• Flexor digitorum superficialis arch: a fibrous arch between humeral and radial heads.
• Anterior interosseous nerve: arises about 5 to 8 cm distal to the elbow, purely motor, supplying flexor pollicis longus, the radial half of flexor digitorum profundus (index and often middle), and pronator quadratus.

Syndromes

• Pronator syndrome: compression at the elbow and proximal forearm. It is rare, and accounts for under 1% of median nerve entrapments.
• Anterior interosseous syndrome (Kiloh-Nevin): rare. Many apparent cases are in fact neuralgic amyotrophy with fascicular constriction.
• High median nerve injury: supracondylar humerus fracture, elbow dislocation, laceration, forearm fracture, compartment syndrome, brachial artery puncture with haematoma, injections and repeat venepuncture in the antecubital fossa, and arteriovenous fistula for dialysis.

Other causes include diabetes, rheumatoid arthritis (Mannerfelt syndrome with tendon rupture in the differential), amyloidosis, gout, hypertrophied muscles, anomalous muscles (Gantzer muscle), and hereditary neuropathy with liability to pressure palsies.

Distal compression at the wrist is discussed under carpal tunnel disease and not here.

Prognosis varies. Many anterior interosseous palsies recover spontaneously over 6 to 18 months, while traumatic transections require surgery and recover slowly.

Pronator syndrome

• Aching pain in the proximal volar forearm, worsened by repetitive pronation, gripping and lifting.
• Paraesthesia in the median distribution, including the thenar eminence and palm (the palmar cutaneous branch leaves before the wrist, so palm numbness distinguishes this from distal compression).
• Mild weakness of grip and thenar muscles, with nocturnal symptoms uncommon.
• Provocative tests: resisted pronation with elbow extension (pronator teres), resisted elbow flexion with forearm supination (lacertus fibrosus), and resisted middle finger flexion at the proximal interphalangeal joint (flexor digitorum superficialis arch). Tinel sign over the proximal forearm.

Anterior interosseous nerve syndrome

• Often begins with acute forearm or arm pain lasting hours to days, followed by weakness.
• Inability to flex the thumb interphalangeal joint and the index distal interphalangeal joint, producing a failed OK sign ("pinch sign"), with flattened pinch between thumb and index pulp (tear drop sign).
• Weak pronation in elbow flexion, because pronator quadratus is weak.
• No sensory loss.

High median nerve palsy

• Loss of forearm pronation, wrist flexion (weak, with ulnar deviation), and flexion of the index and middle fingers (benediction sign on attempted fist).
• Loss of thenar abduction and opposition (ape hand), and sensory loss over the radial three and a half digits and thenar eminence.
• Loss of the median-supplied lumbricals contributes to impaired index and middle finger extension at the interphalangeal joints.

Red flags: acute painful onset with preceding illness (neuralgic amyotrophy), vascular compromise, a mass or pulsatile swelling, and rapidly progressive deficit after a supracondylar fracture.

Differential diagnosis: C6 to C7 radiculopathy, thoracic outlet syndrome, brachial plexopathy, tendon rupture (lost tenodesis effect), flexor pollicis longus rupture in rheumatoid arthritis, stroke, and cervical myelopathy.

Diagnosis is clinical, with electrodiagnostic and imaging support.

Electrodiagnostic testing

• Pronator syndrome: nerve conduction studies are often normal. They may show slowed forearm conduction or reduced SNAP and CMAP amplitudes in advanced disease. A normal study does not exclude the diagnosis.
• Anterior interosseous nerve syndrome: needle EMG of flexor pollicis longus, pronator quadratus and flexor digitorum profundus to the index shows denervation, with normal median sensory studies and normal abductor pollicis brevis CMAP.
• Include paraspinal muscles and other nerves to exclude radiculopathy or neuralgic amyotrophy.

Imaging

• Ultrasound shows nerve swelling, hourglass constriction, tendon rupture and mass lesions, and can examine the nerve dynamically.
• MRI shows denervation oedema in pronator quadratus, flexor pollicis longus and flexor digitorum profundus, and identifies masses, muscle anomalies and cervical pathology.
• Radiographs of the distal humerus for a supracondylar process, and of the elbow for fractures.
• Doppler or CT angiography for vascular injury or pseudoaneurysm.

Laboratory tests: HbA1c, vitamin B12, TSH, ESR, rheumatoid factor and anti-CCP, serum protein electrophoresis, and Lyme and viral serology where relevant.

Pronator syndrome

• Conservative treatment for at least 3 to 6 months: rest, activity modification to avoid repetitive pronation and gripping, a forearm splint in neutral rotation, and NSAIDs (naproxen 500 mg twice daily for 2 weeks).
• Local steroid injection near the pronator teres (methylprednisolone 40 mg with 3 to 5 mL lidocaine 1%) can be tried.
• Neuropathic agents: gabapentin 300 mg at night titrated to 1800 mg/day, or amitriptyline 10 to 50 mg at night.
• Surgical decompression for failure of conservative treatment: release of the lacertus fibrosus, pronator teres, and the flexor digitorum superficialis arch, with exploration of the nerve. Success is reported in about 70 to 90%.

Anterior interosseous nerve syndrome

• Observe for 8 to 12 weeks with splinting and hand therapy, since about 70% recover spontaneously, especially neuralgic amyotrophy related cases.
• Consider prednisolone 60 mg daily for 5 to 7 days then taper if within the first weeks and a neuralgic amyotrophy picture is present.
• No recovery at 3 months (clinically or on EMG): surgical exploration and decompression, with neurolysis of fascicular constrictions or grafting.
• Persistent deficit beyond 6 to 12 months: nerve transfer (branch of the brachialis to the anterior interosseous nerve) or tendon transfers: brachioradialis to flexor pollicis longus, and extensor carpi radialis longus to flexor digitorum profundus of the index and middle fingers.

Ligament of Struthers

• Surgical resection of the ligament and supracondylar spur with neurolysis of the nerve and artery.

Traumatic median nerve injury

• Lacerations: exploration and primary tension-free repair within days, with grafting for gaps over 2 to 3 cm.
• Closed injuries after supracondylar fractures: observation for 3 to 4 months, then exploration if no recovery. Vascular injury requires urgent exploration and repair.
• Compartment syndrome and haematoma: urgent fasciotomy or evacuation.

Reconstruction for high median palsy

• Opponensplasty (Camitz, Bunnell, or Huber transfers) to restore thumb opposition.
• Brachioradialis to flexor pollicis longus and extensor carpi radialis longus to flexor digitorum profundus for finger and thumb flexion.
• Sensory re-education and protection of insensate skin.

Supportive care

• Hand therapy for range of motion and strength, splinting to prevent contractures, and occupational therapy.
• Control of diabetes, thyroid disease and inflammatory arthritis.
• Pain control as for neuropathic pain, and early referral for persistent or progressive deficits.