Acute Flaccid Myelitis in 2018: The Warning Signs in 238 Confirmed Cases

Acute flaccid myelitis (AFM) is a serious neurologic syndrome that mostly strikes previously healthy children. Like the paralysis caused by polio, it brings on sudden, floppy (flaccid) weakness in the arms or legs, with lesions mainly in the gray matter of the spinal cord. It can progress over hours or days to permanent paralysis and life-threatening respiratory failure.

A pattern every two years

National surveillance began in 2014, after California and Colorado reported clusters of AFM or sudden limb weakness in previously healthy children — none with evidence of poliovirus. Since then, U.S. cases have peaked in late summer and early fall every two years. Looking back, investigators found scattered cases before 2014 and jumps in 2014, 2016 and 2018, which suggests something changed around 2014 — likely a new or emerging cause.

Confirmed AFM cases reported to CDC by month of onset, August 1, 2014–June 30, 2020 (633 cases, as of July 24, 2020). CDC

What causes it

Several viruses — West Nile virus, adenovirus and non-polio enteroviruses — cause AFM in a small share of the people they infect. Pathogens are rarely found in the cerebrospinal fluid, but enteroviruses are the commonest finds in respiratory and stool samples, and enterovirus D68 (EV-D68) is the type seen most. Poliovirus has never been detected in a case. Animal studies and antibody studies point to non-polio enteroviruses, especially EV-D68, as a likely main cause of U.S. AFM since 2014 — though other viruses may add to the peaks: a cluster of 11 Colorado cases in 2018 was linked to enterovirus A71 (EV-A71).

The 2018 cases

CDC reviewed the medical records of all 238 confirmed cases with onset in 2018. A case was confirmed when sudden flaccid limb weakness came with an MRI showing a spinal cord lesion largely confined to gray matter.

Before the weakness

97% had an illness first — most often a respiratory illness (80%) or fever (77%), typically starting a median of 6 days before the weakness. Many also had neck or back pain (46%) or headache (37%) beforehand. 11% were already in hospital a day or more before the weakness began, which suggests the early illness can be severe.

When they were seen for weakness

Weakness was more often in the arms (64%) than the legs (36%) at the first neurologic exam. Only 13% had sensory problems; 21% had cranial nerve problems and 5% altered mental status. On MRI, lesions were most common in the cervical (neck) cord (94% of those scanned) and thoracic cord (86%); 44% of brain scans showed brainstem lesions.

How sick they got

Steroids and intravenous immunoglobulin (IVIG) were the usual treatments.

Where they went first

Of 211 patients whose weakness began outside hospital, 76% sought care within a day, and 64% went first to an emergency department (23% to a primary care provider). Most of those hospitalized after weakness began were admitted within a day, but 10% weren't admitted until four or more days later — possibly a sign of delayed recognition.

EV-D68 vs EV-A71

Of the 238, 107 of the 219 tested (49%) had an enterovirus or rhinovirus in at least one specimen, most often from the respiratory tract.

EV-A71 patients more often had abnormal brain MRIs, but EV-D68 patients more often needed intensive care and a ventilator. The authors caution that with nearly all EV-A71 cases from one state, and virus testing likelier in some kinds of patient, the comparison is uncertain — though different viruses are probably tied to different forms of AFM.

What clinicians should do

• Suspect AFM in any child with sudden flaccid limb weakness — especially from August to November in a peak year, after a recent fever and respiratory illness, and with neck or back pain or any other neurologic symptom.
• In young children, weakness may show only as using a limb less, which is easily mistaken for pain or an injury.
• Hospitalize suspected cases immediately and watch for breathing problems: AFM can progress fast. Hospital care also makes specialist consultation and brain and spine MRI easier.
• Emergency departments are where most patients arrive, so awareness there matters most.
• Early recognition also helps collect the right specimens to find the cause.

2020 and COVID-19

Going by the two-year pattern, the authors expected another peak in late summer and fall 2020. How the COVID-19 pandemic and social distancing would affect enteroviruses and AFM was unknown. With non-COVID emergency visits down in 2020, and more care by phone or telemedicine, they warned that recognition could be delayed — making it critical for parents and clinicians to know the signs.

Limits

Cases reported to CDC likely undercount AFM, and the review relied on medical records, often from early in the hospital stay. Long-term outcomes weren't available; those data were being collected for future reports.

Sources

Based on Kidd S, Lopez A, Nix WA, Anyalechi G, Itoh M, Yee E, Oberste MS, Routh J, "Vital Signs: Clinical Characteristics of Patients with Confirmed Acute Flaccid Myelitis, United States, 2018," MMWR volume 69, number 31, Centers for Disease Control and Prevention; a work of the United States government in the public domain.