Acute Motor and Sensory Axonal Neuropathy

Acute motor and sensory axonal neuropathy is a severe axonal variant of Guillain-Barré syndrome in which both motor and sensory axons degenerate. It causes rapid, profound weakness with sensory loss, a high rate of respiratory failure, and slow, often incomplete recovery.

It makes up about 5 to 10% of Guillain-Barré syndrome, and is more frequent in Asia and Central America. It occurs in all ages and affects both sexes.

Triggers. Campylobacter jejuni infection is the commonest antecedent, and anti-GM1 and anti-GD1a antibodies are found in many.

Prognosis. Recovery is slow, over months to 2 years, with residual weakness in about half; mortality is about 5 to 10% and is higher than for other types. Early severe disease, older age, preceding diarrhoea and low compound muscle action potentials predict a poor outcome.

Pathophysiology.

Antibodies against gangliosides on axolemma of motor and sensory fibres activate complement and macrophages, causing Wallerian like degeneration of axons rather than demyelination.

Loss of axons explains the slow, incomplete recovery.

• Rapidly ascending flaccid weakness and areflexia, reaching nadir within days to 4 weeks.
• Distal sensory loss, paraesthesia and neuropathic pain.
• Respiratory failure, bulbar and facial weakness, and autonomic instability in many.
• Muscle wasting develops later, with prolonged disability.

Red flags.

• Falling vital capacity, weak cough, bulbar weakness or aspiration.
• Autonomic instability: arrhythmia, labile blood pressure, ileus.
• Sensory level, sphincter loss or upper motor neuron signs (spinal cord lesion).

• Nerve conduction studies show markedly reduced motor and sensory amplitudes with relatively preserved conduction velocities, and electromyography shows denervation after 2 to 3 weeks.
• Cerebrospinal fluid shows raised protein with a normal cell count, which may be normal in week 1.
• Anti-ganglioside antibodies (GM1, GD1a) and stool test for Campylobacter.
• MRI of the spine with contrast to exclude spinal cord lesions and show nerve root enhancement.
• Measure forced vital capacity and negative inspiratory force regularly; electrolytes, glucose, creatine kinase, HIV, porphyrins and toxic screen.

Differential diagnosis. Acute inflammatory demyelinating polyneuropathy, acute motor axonal neuropathy, acute flaccid myelitis, transverse myelitis, critical illness polyneuropathy, toxic neuropathy, porphyria, diphtheria and vasculitic neuropathy.

• Admit to a high dependency or intensive care unit for monitoring of respiratory and autonomic function; intubate early for vital capacity under 15 to 20 mL/kg, rapid decline, or bulbar failure.
• Intravenous immunoglobulin: 0.4 g/kg daily for 5 days (total 2 g/kg), started within 2 weeks of onset. Maximum 2 g/kg in a course. Infusion rate is set by the product label, starting slowly. Monitor renal function, blood pressure, hypersensitivity, thrombosis, haemolysis and aseptic meningitis; check IgA before the first dose. Avoid in IgA deficiency with anti-IgA antibodies; use caution in renal impairment, heart failure and older adults; acceptable in pregnancy.
• Plasma exchange as an alternative: 5 exchanges of 200 to 250 mL/kg plasma over 1 to 2 weeks, started within 4 weeks. Monitor calcium, blood pressure, coagulation and line infection. Avoid in unstable cardiovascular disease and sepsis. Do not combine with immunoglobulin.
• Corticosteroids do not help.
• Thromboprophylaxis: enoxaparin 40 mg subcutaneously once daily. Maximum 40 mg daily (20 mg daily when creatinine clearance is under 30 mL/min). Monitor platelets, renal function and bleeding. Avoid in active bleeding and heparin induced thrombocytopenia; acceptable in pregnancy.
• Neuropathic pain: gabapentin 300 mg orally at night on day 1, 300 mg twice daily on day 2, then 300 mg three times daily on day 3, increased every 3 to 7 days to 900 to 2400 mg daily. Maximum 3600 mg daily. Monitor sedation, dizziness and oedema. Adjust for renal function: creatinine clearance 50 to 79 mL/min, 600 to 1800 mg daily; 30 to 49 mL/min, 300 to 900 mg daily; 15 to 29 mL/min, 150 to 600 mg daily; under 15 mL/min, 150 to 300 mg daily.
• Autonomic care: cardiac monitoring, avoid drugs that cause hypotension or bradycardia, and treat arrhythmia as needed.
• Early and prolonged rehabilitation, nutrition, tracheostomy planning, contracture prevention and psychological support.

Monitoring. Respiratory, cardiovascular and swallowing observations at least every 4 hours initially; review at 4 to 6 weeks, 3 months, 6 months and 12 months.

Referral and escalation. Neurology and intensive care for all; rehabilitation medicine for prolonged recovery.

Special groups. Children and pregnancy: the same treatment, with weight based dosing and obstetric input.