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 possible AFM patients to help us better understand the spectrum of AFM illness, all possible causes, risk factors, and outcomes for this condition.

Case definitions

Case definitions are used by the team of expert neurologists and CDC to classify patients under investigation (PUIs) as “confirmed,” “probable,” “suspect,” or “not a case” for surveillance purposes. Case definitions should not be used to decide whether to report a PUI or to diagnose a patient with AFM, and they have no impact on treatment or patient care.

Case definitions are approved by the Council of State and Territorial Epidemiologists (CSTE) to standardize collection of data for specific diseases and are updated as appropriate.

  • In June 2015, CSTE adopted a standardized case definition for AFM used to classify PUIs as confirmed or probable cases.
  • In June 2017, the case definition was updated to better describe the clinical presentation of cases and provide guidance on results of magnetic resonance images (MRIs) conducted early (e.g., within 72 hours).
  • In June 2019, the case definition was updated to include laboratory/imaging criteria for case ascertainment and a “suspect” category for classification.
  • In October 2020, the case definition was updated to clarify the intent of the changes from 2019, to allow for reporting of possible AFM cases identified post-mortem, and to expand the definition of confirmed cases to include persons who died and did not have an MRI performed but had evidence of myelitis on autopsy (see CSTE’s Revision to the Standardized Case Definition, Case Classification, and Public Health Reporting for Acute Flaccid Myelitispdf iconexternal icon).

The updates to the 2020 case definitions are described below.


Case Ascertainment

Illness that meets any of the following criteria should be considered a possible AFM case and reported to the health department:

  • A person with clinical AND laboratory/imaging criteria for reporting, OR
  • A person whose death certificate lists AFM as the cause of death or a contributing cause of death, OR
  • A person with autopsy findings that include histopathologic evidence of inflammation largely involving the anterior horn of the spinal cord

Clinical Criteria

An illness with onset of acute flaccid* limb weakness.

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

Laboratory/Imaging Criteria

An 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.

Vital Records Criteria

Any person whose death certificate lists acute flaccid myelitis as a cause of death or a condition contributing to death.

Other Criteria

Autopsy findings that include histopathologic evidence of inflammation largely involving the anterior horn of the spinal cord spanning one or more vertebral segments.


Case Classification

Cases meeting case ascertainment criteria that are reported by the health department to CDC are classified as:

case classification
Confirmed
  • Meets clinical criteria with confirmatory laboratory/imaging evidence,
    OR
  • Meets other classification criteria.
Probable
  • Meets clinical criteria with presumptive laboratory/imaging evidence.
Suspect
  • Meets clinical criteria with supportive laboratory/imaging evidence,
    AND
  • Available information is insufficient to classify case as probable or confirmed.

Clinical criteria

  • An illness with onset of acute flaccid* weakness of one or more limbs, AND
  • Absence of a clear alternative diagnosis attributable to a nationally notifiable condition.

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

Laboratory/Imaging criteria

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.

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.

Supportive laboratory/imaging evidence:

  • 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.

* 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.

Other classification criteria

  • Autopsy findings that include histopathologic evidence of inflammation largely involving the anterior horn of the spinal cord spanning one or more vertebral segments.

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.


2020 case definition update

The updates to the case definition, released October 2020, include the following:

  1. Addition of language for suspect cases to include supportive laboratory/imaging criteria
  2. Addition of criteria under case ascertainment to allow for reporting of possible AFM cases identified post-mortem
  3. Addition of language for confirmed cases to allow for the inclusion of persons who died and did not have an MRI performed but have evidence of myelitis on autopsy

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.


Page last reviewed: November 20, 2020