Acute Flaccid Myelitis

Acute flaccid myelitis is an acute, poliomyelitis like illness of the spinal cord grey matter, causing rapid asymmetric flaccid limb weakness, mostly in children, after a febrile respiratory illness. Enterovirus D68 and enterovirus A71 are the main causes.

It is rare, with an incidence of about 0.1 in every 100,000 children each year in outbreak years. It follows a pattern of biennial outbreaks in late summer and autumn, with a median age of about 5 years.

Causes. Enterovirus D68, enterovirus A71, coxsackieviruses, West Nile virus, Japanese encephalitis virus and others. In many cases no pathogen is found.

Course. Peak weakness within days; recovery is incomplete in most, with long term motor deficits in more than 80% of survivors and persisting respiratory support in some.

Pathophysiology.

Direct viral infection of anterior horn motor neurons causes neuronal death, with secondary inflammation of the grey matter.

Fewer motor neurons survive in the segments affected, so recovery depends on collateral reinnervation.

• Prodrome of fever, cough, rhinorrhoea or gastrointestinal illness 3 to 10 days before weakness.
• Rapid asymmetric flaccid weakness of one or more limbs over hours to days, with reduced tone and reflexes.
• Neck or back pain, cranial nerve involvement (facial, bulbar, extraocular) in some.
• Respiratory failure in a minority; bladder and bowel dysfunction in some.
• Sensation is mostly preserved.

Red flags.

• Respiratory muscle weakness, bulbar failure or inability to handle secretions.
• Altered consciousness or seizures (encephalitis).
• Sensory level or sphincter loss (spinal cord compression).

• Clinical case definition: acute flaccid limb weakness with MRI showing longitudinal T2 hyperintensity restricted to grey matter, spanning one or more spinal segments.
• MRI of the spine and brain with contrast; nerve root enhancement can be seen.
• Cerebrospinal fluid shows mild pleocytosis and normal or raised protein; polymerase chain reaction for enteroviruses.
• Respiratory secretion, stool and rectal swabs for enterovirus polymerase chain reaction; serum and cerebrospinal fluid for other viruses.
• Nerve conduction studies and electromyography show motor axonal loss with preserved sensory responses; test for poliovirus in suspected cases.

Differential diagnosis. Guillain-Barré syndrome, transverse myelitis, poliomyelitis, myasthenia gravis, botulism, spinal cord compression or infarction, tick paralysis and West Nile virus poliomyelitis.

• No proven specific treatment; management is supportive and by a multidisciplinary team.
• Admit to a unit where respiratory and bulbar function can be monitored; intubate for respiratory failure or loss of airway protection.
• Treat pain with paracetamol 15 mg/kg orally every 4 to 6 hours as needed. Maximum 4 doses in 24 hours (60 mg/kg daily, up to 4 g). Monitor total daily dose and liver function. Reduce dose in hepatic impairment; extend the interval in renal impairment (eGFR under 30).
• Immunotherapy (corticosteroids, immunoglobulin, plasma exchange) has no proven benefit, and its use is a specialist decision based on overlapping features of other conditions.
• Antivirals have no proven benefit.
• Early physiotherapy and occupational therapy: positioning, splints, range-of-movement exercises and strengthening.
• Nutrition and swallowing support, bladder and bowel care, pressure care and psychological and school support.
• Nerve transfer surgery in selected children with persistent limb paralysis after 6 to 12 months, at specialist centres.
• Infection control: hand hygiene and respiratory precautions; notify public health.

Monitoring. Respiratory and swallowing observations at least every 4 hours in the acute phase; review at 1, 3, 6 and 12 months; track motor recovery.

Referral and escalation. Paediatric neurology, intensive care and infectious diseases for all; rehabilitation medicine and orthopaedics for long term care; public health notification.

Special groups. Children: most affected; family support and school integration. Immunocompromised patients: prolonged viral shedding.