Vital Signs

Surveillance for Acute Flaccid Myelitis — United States, 2018

Adriana Lopez, MHS; Adria Lee, MPH; Angela Guo, MPH; Jennifer L. Konopka-Anstadt, PhD; Amie Nisler, MPH; Shannon L. Rogers, MS; Brian Emery; W. Allan Nix; Steven Oberste, PhD; Janell Routh, MD; Manisha Patel, MD


Morbidity and Mortality Weekly Report. 2019;68(27):608-614. 

In This Article


National AFM surveillance using a standardized case definition was established following the first recognized outbreak in 2014. Subsequently, two nationwide outbreaks occurred, one in 2016 with 149 confirmed cases (,* and the largest in 2018, which accounted for 42% of the 559 cases reported from August 2014 through December 2018. As in previous years, most AFM cases occurred in children, during the late summer and early fall. Findings such as presence of fever or respiratory symptoms before the onset of limb weakness, predominance of upper limb involvement, and detection of viruses in respiratory specimens in approximately 50% of patients with specimens submitted were also consistent with those in previous outbreak years. The accumulation of national surveillance data since 2014 has been pivotal to refining the AFM case definition, allowing for better differentiation of epidemiologic, clinical, and laboratory features and risk factors of confirmed cases from those of probable and non-AFM cases. In addition, the shorter interval between limb weakness and diagnostic evaluations in 2018 compared with that in 2016, suggests that support to health departments for strengthened surveillance and increased provider outreach activities has improved awareness of AFM among providers, particularly during outbreak years. Prompt recognition, early specimen collection, and reporting of all suspected cases to public health are important goals for AFM national surveillance.

Early recognition and specimen collection from suspected AFM patients are essential to optimizing pathogen detection and determining whether single or multiple etiologies are responsible for the recent outbreaks. Data from the 2014 and 2016 outbreaks suggested that early specimen collection resulted in higher pathogen yield[1] (CDC unpublished data, 2019), and the median interval from onset of limb weakness to CSF collection was short in 2016 (3 days) and again in 2018 (2 days). However, only 3% of CSF specimens yielded a pathogen in 2018, similar to data from previous outbreak years. These data suggest that routine EV/RV PCR testing of CSF is unlikely to confirm the cause of these outbreaks.[1,8,14] This could represent the absence of viral shedding into the CSF or might reflect limitations in the timing of specimen collection from viremia, because CSF is collected only after onset of neurologic symptoms and not during the febrile or respiratory illness.

Although 44% of confirmed AFM cases in 2018 had an enterovirus or rhinovirus identified in respiratory specimens, approximately half were negative. Timing of respiratory specimen collection improved in 2018 compared with that in 2016, but still occurred a median of approximately 3 days after the onset of limb weakness and 5 days after the onset of any respiratory illness. Shedding of viruses in the respiratory tract can be transient, so delays in specimen collection could contribute to negative findings. Because conventional testing has not been successful in identifying pathogens in the majority of AFM cases, testing strategies have been expanded to include approaches to identifying immune responses directed toward viruses implicated in AFM, such as measuring pathogen-specific antibody responses to infection. Timely specimen collection can optimize both conventional and novel testing approaches.

Surveillance data from 2018 indicate that most patients received recommended medical care,[13] with evidence of prompt hospitalization, and lumbar puncture and/or MRI shortly after the onset of limb weakness, indicating heightened clinical awareness and successful public health outreach since surveillance for AFM was implemented in 2014. Improved understanding of the different characteristics of confirmed and non-AFM cases, including a history of febrile or respiratory symptoms preceding onset of limb weakness and a predominance of upper limb involvement has helped to differentiate AFM from other forms of acute limb weakness.[15]

AFM can progress rapidly and might require respiratory support;[14–17] patients evaluated with signs and symptoms consistent with AFM should be hospitalized for close monitoring. In the absence of a confirmatory diagnostic test for AFM, management decisions for individual patients in the acute setting should be informed by careful review of the patient's signs and symptoms, laboratory testing, MRI results, and other test results, including electromyography, and in close consultation with infectious disease specialists and neurologists. To help with clinical management, the Acute Flaccid Myelitis Workgroup and the Transverse Myelitis Association ( offer a 24-hour clinical consultation service with physicians at the University of Texas Southwestern's Transverse Myelitis Center or Johns Hopkins University Transverse Myelitis Center, established in 2019, for providers with questions about patients suspected to have AFM. Although studies on treatment have not been systematically evaluated for effectiveness, CDC, in collaboration with experts in multiple disciplines, developed interim considerations for the management of AFM patients (, which do not indicate a preference for or against any of the commonly employed treatments for AFM, including intravenous immunoglobulin, steroids, and plasmapheresis. Physical rehabilitation might also improve long-term outcomes if implemented during the acute phase of illness;[14] thus, early recognition of AFM is important so that clinicians might implement targeted clinical management with the potential to improve patient outcomes.

Because AFM is uncommon, ensuring that all suspected cases are reported to public health is vital to collecting clinical information and specimens from each patient. There continues to be a notable delay in reporting suspected cases of AFM by clinicians to public health authorities. Delays might impede important provider outreach activities to increase awareness and the early collection of specimens for pathogen detection, particularly when there is an increase in cases.

To provide additional specificity for reporting of patients with suspected AFM to health departments, the Council of State and Territorial Epidemiologists modified the clinical criteria for reporting of patients suspected of AFM in June 2019 to include MRI evidence of spinal lesions with at least some gray matter involvement, in addition to acute flaccid limb weakness These changes to the case definition more clearly reflect the cumulative clinical and epidemiologic surveillance data collected for AFM over the past 5 years. It is important to note that the clinical diagnosis of AFM by a physician might differ from case definitions used for public health surveillance. Whereas an AFM diagnosis is based on a physician's comprehensive assessment of the affected patient, public health surveillance requires standardized criteria to evaluate overall morbidity, mortality, and seasonal trends and provide consistency in measurements from year to year.

The findings in this report are subject to at least three limitations. First, national AFM surveillance relies on passive reporting and clinician awareness, which can result in underreporting. Second, misclassification might occur since a confirmatory test for AFM is not available. Lack of submission of all requested specimen types from each patient suspected to have AFM limits the ability to fully characterize the laboratory profile of all AFM cases. Finally, although national data on long-term outcomes are not yet available, CDC and state and local health departments are investigating long-term outcomes of AFM patients and other risk factors that might affect the development of AFM.

Improving the understanding of AFM is a public health priority. The overall rarity of this condition and absence of a confirmatory test highlight the need for increased vigilance among providers seeing pediatric patients with acute onset of flaccid limb weakness in the late summer and fall. Ongoing national AFM surveillance will provide an important bridge between research and public health response and will be critical for the development of optimal treatment and prevention recommendations.

* Numbers for 2018 include patients under investigation that have not yet been classified.