Acute Motor Axonal Neuropathy

Acute motor axonal neuropathy is an axonal variant of Guillain-Barré syndrome that causes rapidly progressive, symmetrical, purely motor weakness without sensory loss. It is associated with antibodies to GM1 and GD1a gangliosides and with preceding Campylobacter jejuni infection.

It accounts for about 30 to 65% of Guillain-Barré syndrome in China, Japan and Bangladesh, and under 10% in Europe and North America. The overall incidence of Guillain-Barré syndrome is about 1 to 2 in every 100,000 people each year, and it occurs in all ages, with a peak in young adults and children in some regions.

Triggers. Campylobacter jejuni gastroenteritis in up to 60 to 70% of cases, other infections, and rarely vaccination or surgery.

Course. Nadir within 2 to 4 weeks; recovery varies, with fast recovery in some by rapid reversal of conduction block, and slow recovery in others from axonal degeneration. Mortality is about 3 to 7%.

Pathophysiology.

Antibodies against gangliosides on motor axolemma activate complement at the nodes of Ranvier, causing disruption of sodium channel clusters, conduction failure and, in severe cases, axonal degeneration.

Dorsal roots and sensory axons are spared because they lack the same antigen density.

• Ascending, symmetrical, flaccid weakness that progresses over days to 4 weeks.
• Reduced or absent reflexes (reflexes may be preserved or brisk early in some).
• No sensory loss; cranial nerve and bulbar involvement in some.
• Respiratory failure in about 20 to 30% and autonomic dysfunction in some.
• Back pain or limb pain is less common than in demyelinating disease.

Red flags.

• Falling vital capacity (under 20 mL/kg), weak cough or bulbar weakness.
• Autonomic instability: arrhythmia, labile blood pressure, ileus or urinary retention.
• Rapid progression over hours or sensory level (spinal cord lesion).

• Clinical diagnosis supported by nerve conduction studies showing reduced compound muscle action potentials with normal sensory studies (axonal pattern), sometimes with reversible conduction failure.
• Cerebrospinal fluid shows raised protein with normal cell count (albuminocytological dissociation), which may be normal in week 1.
• Anti-ganglioside antibodies (GM1, GD1a) support but are not essential for diagnosis; stool test for Campylobacter.
• MRI of the spine with contrast when a spinal cord lesion is possible; exclude hypokalaemia, porphyria, toxins and poliomyelitis like illness.
• Measure forced vital capacity and negative inspiratory force every 4 to 6 hours during progression.

Differential diagnosis. Acute inflammatory demyelinating polyneuropathy, acute flaccid myelitis, transverse myelitis, myasthenia gravis, botulism, hypokalaemic periodic paralysis, critical illness neuropathy and porphyria.

• Admit to a unit able to monitor respiratory and autonomic function; intubate for vital capacity under 15 to 20 mL/kg, falling rapidly, or for 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, with compression stockings. 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.
• Early rehabilitation, nutrition, bladder and bowel care, pressure area care and psychological support.
• Treatment related fluctuation (relapse within 8 weeks of treatment) is managed with a second course.

Monitoring. Vital capacity, heart rate, blood pressure and swallowing at least every 4 to 6 hours initially; review at 4 to 6 weeks, 3 months and 12 months.

Referral and escalation. Neurology and intensive care for all; rehabilitation services during recovery.

Special groups. Children: weight based dosing and early paediatric intensive care referral. Pregnancy: immunoglobulin preferred.