Case Definitions for AFM

Clinicians should continue to be vigilant and send information about patients who meet the clinical criteria (sudden onset of flaccid limb weakness) and laboratory/imaging criteria (MRI showing a spinal cord lesion in at least some gray matter and excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities) for AFM  to their health department regardless of any laboratory results.

Since AFM is a newly recognized 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.

Case definitions

The case definitions should not be used to decide whether to report a patient under investigation (PUI); it is used by the team of expert neurologists who classify PUIs as “confirmed,” “probable,” “suspect,” or “not a case.”


Case Ascertainment

Clinical Criteria

An illness with onset of acute flaccid* limb weakness.

* Low muscle tone, limp, hanging loosely, not spastic or contracted.

Laboratory/Imaging Criteria

A magnetic resonance image (MRI) showing a spinal cord lesion in at least some gray matter† and spanning one or more vertebral segments, AND

Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities.

† 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.


Case Classification

Confirmed Case
  • Clinically compatible case WITH
  • Confirmatory laboratory/imaging evidence: MRI showing spinal cord lesion with predominant gray matter involvement* and spanning one or more vertebral segments, AND
  • Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities, AND
  • Absence of a clear alternative diagnosis attributable to a nationally notifiable condition.
Probable Case
  • Clinically compatible case WITH
  • Presumptive laboratory/imaging evidence: MRI showing spinal cord lesion where gray matter involvement* is present but predominance cannot be determined, AND
  • Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities, AND
  • Absence of a clear alternative diagnosis attributable to a nationally notifiable condition.
Suspect Case
  • Clinically compatible case WITH
  • Available information is insufficient to classify case as probable or confirmed.
    The intent of the case definition is that the criteria for suspect cases should also include laboratory/imaging evidence as follows:
    • MRI showing a spinal cord lesion in at least some gray matter* and spanning one or more vertebral segments, AND
    • Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities, AND
    • Absence of a clear alternative diagnosis attributable to a nationally notifiable condition.

* 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.


2019 case definition update

The updates to the case definition, released June 2019, include the following:

  1. addition of MRI criteria for case ascertainment and reporting to increase specificity,
  2. addition of language for confirmed and probable cases to include the absence of an alternative diagnosis due to a nationally notifiable condition to avoid double counting of cases in different surveillance systems,
  3. removal of criteria for pleocytosis and the addition of MRI criteria for probable cases to ensure spinal cord involvement, and
  4. addition of a suspect category to allow for classification of cases where sufficient information is not available to classify as probable or confirmed.

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.