Case Definitions

Clinicians should continue to be vigilant and send all information about patients that meet the clinical criteria for acute flaccid myelitis (AFM) (sudden onset of flaccid limb weakness) to their state or local health department. Since AFM is a relatively new condition, we need information on all patients to help us better understand the spectrum of AFM illness, all possible causes, risk factors, and outcomes for this condition. Clinicians should send information on patients who meet the clinical criteria regardless of any laboratory results or MRI findings.

In June 2015, the Council of State and Territorial Epidemiologists (CSTE) adopted a standardized case definition for AFM that is used by CDC to classify patients under investigation (PUIs) for AFM as confirmed or probable cases. The case definition should not be used to decide whether to report a PUI; it is used by the team of expert neurologists who classify PUIs as “confirmed,” “probable,” or “not a case.” The case definition was updated in June 2017 (see Standardized Case Definition for Acute Flaccid MyelitisCdc-pdfExternal) and is described below:

Clinical Criteria

An illness with onset of acute flaccid limb weakness

Laboratory Criteria
  • Confirmatory Laboratory Evidence: a magnetic resonance image (MRI) showing spinal cord lesion largely restricted to gray matter*† and spanning one or more vertebral segments
  • Supportive Laboratory Evidence: cerebrospinal fluid (CSF) with pleocytosis (white blood cell count >5 cells/mm3)
Case Classification


  • Clinically compatible case AND
  • Confirmatory laboratory evidence: MRI showing spinal cord lesion largely restricted to gray matter*† and spanning one or more spinal segments


  • Clinically compatible case AND
  • Supportive laboratory evidence: CSF showing pleocytosis (white blood cell count >5 cells / mm3)

* Spinal cord lesions may not be present on initial MRI; a negative or normal MRI performed within the first 72 hours after onset of limb weakness does not rule out AFM.

† Terms in the spinal cord MRI report such as “affecting mostly gray matter,” “affecting the anterior horn or anterior horn cells,” “affecting the central cord,” “anterior myelitis,” or “poliomyelitis” would all be consistent with this terminology.

Final case classification

To provide consistency in case classification, review of case information and assignment of final case classification for all patients under investigation (PUIs) for AFM is done by experts in national AFM surveillance. This is similar to the review required for final classification of paralytic polio cases.

2017 case definition update

The updates to the case definition, released June 2017, include the following:

  1. the word “flaccid” was added to better describe the clinical presentation of cases,
  2. further guidance is provided about MRIs performed within 72 hours of onset of limb weakness that appear normal; and
  3. clarification about the process for case classification.

Note there is no age restriction for reporting PUIs. The case definition includes people of all ages to collect information on the full spectrum of the condition in both children and adults. A probable category with the addition of CSF pleocytosis is also included to add additional sensitivity as some patients may not undergo an MRI or MRI findings may be normal, especially if the MRI is performed early in the condition. If an early MRI is normal and MRI studies conducted after 72 hours are available, those should also be reviewed.

For more information, see