CDC Advisory: Acute Flaccid Myelitis

Although considered rare, early recognition and reporting are critical for treating this still mysterious, potentially serious disease.


Late summer is the season to be especially alert for possible cases of acute flaccid myelitis (AFM), the CDC says.

Since 2014, when the CDC began tracking AFM, 570 cases, mostly in children, have been reported. Outbreaks have followed a pattern: every 2 years, spiking between August and October. Nearly all states and DC have reported cases. The largest outbreak, 233 cases, was in 2018. Theoretically, 2019 would be an off year, but too little is known about AFM to say outbreaks are unlikely.

AFM starts with symptoms similar to those of a viral infection but can progress rapidly to limb weakness, then respiratory failure. Most patients are previously healthy children, average age 5 years old, who had respiratory symptoms or fever consistent with a viral infection less than a week before they experienced sudden weakness in their arms or legs. On average, the CDC receives reports of suspected AFM cases 18 days after the patient develops limb weakness.

The CDC believes viruses play a role, but which ones is still unclear. Symptoms have been found to develop after poliovirus, West Nile virus, and adenovirus infections. In an analysis of confirmed cases from 2018, CDC researchers detected enteroviruses and rhinoviruses in nearly half of stool and respiratory specimens. However, of 74 cases with a cerebral spinal fluid specimen, only 2 were positive for enteroviruses. All specimens tested negative for poliovirus.

But even when it is associated with a viral infection, it is not known how the infection triggered the AFM, or why it triggers AFM in some people and not others. AFM is rare—affecting ≤ 2 children per million in the US every year. Viral infections from enteroviruses are common, especially in children—and especially in the late summer/early autumn months. It is not known why a small number of people develop AFM while most others recover.

AFM can be difficult to diagnose because the symptoms are similar to those of neurologic diseases, such as Guillain-Barré syndrome. As of yet, no laboratory test is available; diagnosis is done through physical examination and magnetic resonance imaging (MRI) scans of the spinal cord.

There also are no proven ways to treat or prevent AFM. That is why timing is so key. The CDC says as soon as AFM is suspected, collect cerebral spinal fluid, serum, stool, and nasopharyngeal swabs. If an MRI shows a spinal lesion with some gray matter involvement, alert the health department and send specimens and medical records. Refer to specialists, monitor the patient for worsening symptoms, hospitalize if indicated, and begin treatment and rehabilitation.

In short: no specific etiology, no specific way to diagnose, and no specific treatment exist for AFM. Treatments, including immunoglobulin, corticosteroids, and antivirals have been tried, but no clear evidence exists that any have affected recovery. Other treatment is supportive, with physical and occupational therapy.

The length of recovery time varies. Some people make a full recovery, most have continued muscle weakness even after a year.

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