Acute Flaccid Myelitis (AFM)

2021 Case Definition

CSTE Position Statement Number: 21-ID-02

Clinical Criteria

An illness with onset of acute flaccid* limb 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

** Cases with a clear alternative diagnosis attributable to a nationally notifiable condition (NNC) should be reported only once using the event code for the NNC to avoid duplicate reporting.

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 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 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.

✝ Terms in the spinal cord MRI report such as “affecting 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, 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. **

** Cases with a clear alternative diagnosis attributable to a nationally notifiable condition (NNC) should be reported only once using the event code for the NNC to avoid duplicate reporting.

Case Classifications

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.

Comment

To provide consistency in case classification, review of case information and assignment of final case classification for all suspected AFM cases will be done by experts in national AFM surveillance. This is similar to the review required for final classification of paralytic polio cases.

Case Classification Table

Criterion	Suspected	Probable	Confirmed
Clinical Evidence:
Acute flaccid* weakness of one or more limbs	N	N	N
Absence of a clear alternative diagnosis attributable to a nationally notifiable condition**	N	N	N
Laboratory/Imaging Evidence:
MRI showing spinal cord lesion with predominant gray matter involvement† and spanning one or more vertebral segments			N
MRI showing spinal cord lesion where gray matter involvement† is present, but predominance cannot be determined		N
MRI showing spinal cord lesion in at least some of the gray matter† and spanning one or more vertebral segments	N
Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities	N	N	N
Other Evidence:
Autopsy findings that include histopathologic evidence of inflammation largely involving the anterior horn of the spinal cord spanning one or more vertebral segments			N
Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities			N
Absence of a clear alternative diagnosis attributable to a nationally notifiable condition**			N
Insufficient information to classify case as probable or confirmed	N
Criteria to distinguish a new case:
N/A	N/A	N/A	N/A

Notes:

S = This criterion alone is SUFFICIENT to classify a case.

N = All “N” criteria in the same column are NECESSARY to classify a case. A number following an “N” indicates that this criterion is only required for a specific disease/condition subtype (see below). If the absence of a criterion (i.e., criterion NOT present) is required for the case to meet the classification criteria, list the absence of criterion as a necessary component.

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

** Cases with a clear alternative diagnosis attributable to a nationally notifiable condition (NNC) should be reported only once using the event code for the NNC to avoid duplicate reporting.

† 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.

Date Posted: August 31, 2021