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Acute flaccid myelitis (AFM) cases remained low in 2022, despite increased circulation of the enterovirus D68 (EV-D68) in the U.S., according to an analysis of CDC data.

AFM cases ranged from 28 to 47 per year from 2019 to 2022 -- far lower than the 238 confirmed cases in 2018, Sarah Kidd, MD, from the CDC's National Center for Immunization and Respiratory Diseases in Atlanta, and colleagues reported in .

"Current trends do not indicate when the next increase of AFM might be expected," the authors wrote. "Clinicians should continue to suspect AFM in any child with acute flaccid limb weakness, especially those with a recent respiratory or febrile illness."

AFM is a rare, paralytic condition that most often affects children. It is a known complication of viral infections, most notably enteroviruses -- particularly EV-D68 -- and can result in acute respiratory failure and permanent paralysis.

As part of national surveillance of AFM, which began in 2014, U.S. health departments have submitted case reports of acute flaccid limb weakness with any spinal cord gray matter lesion on MRI to the CDC. AFM cases have tended to peak in even-numbered years, due to biennial outbreaks of EV-D68. Although cases were predicted to peak in 2020 and 2022, they remained low at 33 and 47, respectively. In 2020, COVID-19 prevention measures such as social distancing and masking may have reduced EV-D68 and other respiratory infections. However, in 2022, EV-D68 and other respiratory viruses were circulating at the highest levels since 2018, the authors pointed out.

"It is unclear why EV-D68 circulation in 2022 was not associated with an increase in AFM cases or when the next increase in AFM cases will occur," wrote Kidd and co-authors.

"Possibly, EV-D68 viruses circulating in 2022 were less neurotropic or less likely to cause neurologic disease," they speculated. "Another possibility is that infection with respiratory viruses, including other [rhinoviruses/enteroviruses], SARS-CoV-2, or respiratory syncytial virus that were frequently circulating in 2022 affected immune responses to EV-D68 and provided protection against neurologic disease." However, data to support these hypotheses are lacking, the authors said.

The clinical and laboratory characteristics of AFM cases that occurred during 2019-2021 were different from those diagnosed in 2018, with less upper limb involvement, prodromal respiratory or febrile illness, or cerebrospinal fluid (CSF) pleocytosis, but more lower limb involvement. During 2022, when EV-D68 was circulating, cases did have a higher proportion of upper limb involvement, respiratory illness or fever, and CSF pleocytosis compared to those in 2019-2021, but a lower proportion than those seen in 2018.

The median age of affected children was lowest in 2018, at 5.1 years, as compared with 6.3 years in 2019, 8 years in both 2020 and 2021, and 7.1 years in 2022.

These differences did not appear to affect the overall severity of AFM. During all years, 98% to 100% of all patients with confirmed AFM were hospitalized, and 51% to 75% were admitted to an intensive care unit. Between 18% to 34% required ventilatory support and 15% to 28% required mechanical ventilation.

Notably, in 2018, EV-D68 was the most common enterovirus found in samples from patients with AFM. In 2019, EV-D68 was detected in only one patient, and since then none of the patients with AFM have tested positive for EV-D68. However, researchers noted that laboratory detection of enteroviruses/rhinovirus is often difficult due to sample availability and time of sample collection.

In this analysis, CDC researchers evaluated patient summary forms, medical records, and laboratory data from 2018 through 2022. The CDC also analyzed CSF, serum, respiratory, and stool samples submitted by health departments and clinicians. Confirmed AFM was defined as acute flaccid limb weakness plus spinal cord gray matter lesions spanning one or more vertebral segments on MRI.

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