Case Definitions

Clinicians should continue to be vigilant and send information to their state or local health department for all patients with sudden onset of neurologic illness associated with limb weakness that meet the case definition below for acute flaccid myelitis. Clinicians should send information for patients who meet the case definition regardless of any laboratory results.

In June 2015, the Council of State and Territorial Epidemiologists (CSTE) adopted a standardized case definition for acute flaccid myelitis. As of August 1, 2015, a patient must meet the clinical criteria below to be considered either a confirmed or probable case of acute flaccid myelitis:

Confirmed Case

To be considered a confirmed case, a patient must meet the following criteria:

1. Acute onset of focal limb weakness, AND
2. An MRI showing a spinal cord lesion largely restricted to gray matter and spanning one or more spinal segments.

Probable Case

To be considered a probable case, a patient must meet the following criteria:

1. Acute onset of focal limb weakness, AND
2. Cerebrospinal fluid (CSF) with pleocytosis (white blood cell count >5 cells/mm³, adjusting for presence of red blood cells by subtracting 1 white blood cell for every 500 red blood cells present).

The changes in the case definition (removal of an age limit and addition of CSF findings to identify a “probable” case), were made for several reasons: the removal of the age limit was done to more accurately determine the overall occurrence of AFM, recognizing that certain etiologies of AFM (e.g., West Nile virus, herpesviruses) more often affect older people than children (though children still represent the majority of AFM cases overall). The addition of CSF findings was included to add additional sensitivity to the case definition, recognizing that some patients may not undergo MRI, or MRI findings may be normal despite the presence of AFM, when the MRI is performed early in the illness.

The case definition has been expanded to include all ages to provide a more complete picture of the full spectrum of illness; however, the focus of increased vigilance continues to be children because AFM occurs more often in children.

For more information, see COCA Clinical Reminder (August 27, 2015) –  Notice to Clinicians: Continued Vigilance Urged for Cases of Acute Flaccid Myelitis.

A brief summary of the status of the investigation through November 13, 2014 is available in CDC’s MMWR Acute Flaccid Myelitis Among Persons Aged ≤21 Years — United States, August 1–November 13, 2014.

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