Acute flaccid myelitis: A mystery disease

Acute flaccid myelitis, or AFM, is not a new syndrome or disease. The classical cause of the syndrome is infection with the poliomyelitis virus. Of course, there has been no poliomyelitis originating in the United States since 1979. Sporadic cases of nonpolio AFM have been recognized in the U.S. for many years after the eradication of polio, but it was not until 2014, when there was a surge of cases in the U.S., that it reached the national consciousness and was given the name “acute flaccid myelitis” rather than “polio-like syndrome,” “acute flaccid paralysis” or other names that had been previously attached to the syndrome.

The CDC’s criteria for a confirmed diagnosis of AFM include acute onset of flaccid limb weakness in one or more limbs with confirmation by MRI of a spinal cord lesion largely restricted to gray matter and spanning one or more vertebral segments. If there is no MRI confirmation available, the case is considered probable, provided there is cerebrospinal fluid (CSF) showing pleocytosis (white blood cell count [WBC] > 5 cells/mm3). There was no official tracking of cases of AFM in the U.S. before 2014; however, from August through December of that year, there was a national outbreak of the disease that resulted in 120 cases. As of Feb. 15 — including those 120 cases from 2014 — there has been a total of 552 AFM cases confirmed after review by the CDC. The agency is attempting to collect retrospective MRI data to determine the annual number of yearly cases before 2014.

Epidemiology in the US

AFM causes disease primarily in children. More than 90% of cases have been aged younger than 18 years, and most of these have been aged 2 to 8 years. The majority of cases occur during August through October. The male-to-female ratio has been about 60:40. A very interesting observation is that larger outbreaks of AFM have occurred in alternate years from 2014 to 2018. For example, there were 120 cases in 2014, 22 cases in 2015, 149 cases in 2016, 35 cases in 2017 and 215 cases in 2018. This last number will probably increase because records are still under review.

The 2014 outbreak occurred against a backdrop of a large national outbreak of human enterovirus D68 (EV-D68) infection in children. EV-D68 (one of more than 100 nonpolio enteroviruses) was first described in 1962, but case clusters were not recognized until about 2008, when reports of respiratory infection, some severe, caused by EV-D68 came in from around the world. In 2014, EV-D68 was demonstrated in respiratory specimens of 20 of the 120 cases of AFM, and non-D68 enteroviruses were found in 21 of the cases. Cases of AFM associated with EV-D68 were also reported during the 2016 U.S. outbreak and from multiple other countries as well.

Source: Infectious Disease News, March 2019

Donald Kaye, MD, MACP

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