Purpose
Clinicians should send information about patients who meet the clinical criteria AND laboratory/imaging criteria for AFM to their health department regardless of any laboratory results.
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 June 2021, 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.
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:
- Meets clinical criteria with confirmatory laboratory/imaging evidence,
OR - Meets other classification criteria.
- Meets clinical criteria with presumptive laboratory/imaging evidence.
- 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.
Report a patient under investigation for AFM
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.