Acute flaccid myelitis (AFM) is a rare neurologic disease presenting as rapid-onset flaccid weakness due to selective inflammation of the central gray matter of the spinal cord.

AFM predominantly affects children (90% of cases), with a median age of 8 years. Neurologic symptoms may be preceded by a febrile respiratory illness and sometimes pain in the extremities. This is followed by sudden-onset flaccid paralysis in a single limb or in multiple limbs with areflexia. Progression is rapid, with nadir of weakness within hours to days. Cranial nerve involvement may result in facial weakness, ophthalmoplegia, dysphagia, dysarthria, and difficulty breathing. Respiratory failure can result in the need for mechanical ventilation. Typically, there are no sensory or cognitive changes. Bowel and bladder involvement are rare. Severity of the disease can range from mild weakness in one limb to complete paralysis and inability to breathe. There is currently no established treatment. Prognosis is variable, but the disease can result in permanent paralysis.

This clinical syndrome has been associated with several pathogens, including enteroviruses (EV; eg, EV-D68, EV-A71), flaviviruses (eg, West Nile virus), and herpes virus, among others, but causation has not been established.