Acute flaccid myelitis is a rare, but serious neurological condition most often affecting children. It is characterized by acute onset limb weakness following a febrile viral illness. Yet depending on age, some children or their caregivers may not view the problem as limb weakness. They might instead report suddenly favoring one arm over the other, being unable to raise an arm (to throw a ball, for example), limping or dragging a leg, or having difficulty standing up from a seated position without help.

The weakness in one or more limbs is often asymmetric and proximal, with a loss of muscle tone and reflexes. Some patients complain about pain in the affected extremity, as well as in the neck, shoulder, or back prior to or concurrent with limb weakness. Cranial nerve abnormalities may also be present, including facial or eyelid droop, difficulty swallowing, slurred speech, or having a hoarse or weak cry (in infants). For suspected cases, an age-appropriate neurological exam should assess muscle tone, strength, reflexes, and cranial nerve deficiencies. The sensory exam is often normal.

Lumbar puncture should be done as soon as possible to test cerebral spinal fluid for cell counts and differential, protein, oligoclonal bands, and a meningitis/encephalitis panel. Additionally, serum testing should include inflammatory and metabolic markers and pathogen-specific tests; stool tests may be used to detect enterovirus; and a naso- or oropharyngeal swab should be collected for a respiratory pathogen panel that includes enterovirus. Specimens from suspected cases of acute flaccid myelitis should be sent to the Centers for Disease Control and Prevention (CDC) for public health surveillance.

Neuroimaging is essential to look for lesions in the brain and spinal cord and is required for a definitive diagnosis of acute flaccid myelitis. Grey matter lesions detected in the spinal cord spanning ≥ 1 vertebral segment on MRI confirms diagnosis in patients with acute onset limb weakness whereas compatible clinical criteria and pleocytosis of cerebral spinal fluid establish a probable diagnosis.  

As acute flaccid myelitis can progress rapidly, patients should be hospitalized both for monitoring and consultation with neurology and infectious disease specialists. Autonomic instability, often with changes in body temperature and blood pressure, may be life threatening. Supportive management should focus on monitoring and ensuring that the patient’s airway is secure. Therapies may include intravenous immunoglobulin, corticosteroids, and plasmapheresis, though there is insufficient evidence for any therapy over another. Muscle strength and function may improve over time, but long-term neurologic impairment is common.

The preceding febrile illness of acute flaccid myelitis and its temporal association with viral outbreaks suggest a viral etiology, but no single viral pathogen has been identified. Suspected viral causes include non-polio enteroviruses (EV-D68 and EV-A71), flaviviruses (West Nile virus and Japanese Encephalitis virus), herpesviruses, and adenoviruses. Notably, poliovirus has not been detected in patients with acute flaccid myelitis. Muscle weakness is primarily caused by damage to spinal motor neurons, which may be destroyed directly by the infecting virus or indirectly due to inflammation.

Outbreaks of acute flaccid myelitis occur most often in late summer and early fall. Since the initiation of national surveillance in the United States in 2014, nationwide outbreaks were reported in 2014, 2016, and 2018, with 120, 153, and 238 cases, respectively. Upsurges of EV-D68 respiratory illness also followed this biennial cycle, leading the CDC to forecast another outbreak of acute flaccid myelitis in 2020. Adding another layer of concern to the year’s public health challenges, physicians should be on the lookout this fall for acute flaccid myelitis in children with acute onset limb weakness following a febrile illness.

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