Acute Flaccid Myelitis (AFM)
2020 Interim Case Definition, Approved October 9, 2020

Background

Acute flaccid myelitis (AFM) is characterized by rapid onset of flaccid weakness in one or more limbs and distinct abnormalities of the spinal cord gray matter on magnetic resonance imaging (MRI). AFM is a subtype of acute flaccid paralysis (AFP), defined as acute onset of flaccid weakness absent of features suggesting an upper motor neuron disorder. The term ‘AFP’ is a generalized ‘umbrella’ term and includes multiple clinical entities, including paralytic poliomyelitis, AFM, Guillain-Barré syndrome (GBS), acute transverse myelitis, toxic neuropathy, and muscle disorders. The annual rate of AFP among children under 15 years of age is approximately 1 per 100,000 children. Although AFP surveillance is commonly conducted in many countries currently still at risk for ongoing transmission of poliovirus, AFP is not under standardized surveillance or nationally notifiable in the United States. Surveillance and assessment for AFP has not been routinely performed since polio was eradicated from the U.S. In the summer and fall of 2014, an apparent increase in reports of AFM occurred in the U.S. Standardized surveillance was established in 2015 to monitor this illness and attempt to estimate baseline incidence (1). Data collected since standardized surveillance was established have helped to identify subsequent increases in reports nationally during 2016 and 2018 and have provided additional valuable information on the clinical presentation to help better characterize clinical features and epidemiology of cases of AFM.

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 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

^† 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.

Suspect

• Meets clinical criteria with supportive laboratory/imaging evidence, AND
• Available information is insufficient to classify case as probable or confirmed.

Probable

• Meets clinical criteria with presumptive laboratory/imaging evidence.

Confirmed

• Meets clinical criteria with confirmatory laboratory/imaging evidence, OR
• Meets other classification criteria.

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

• Acute Flaccid Myelitis (AFM) | 2022 Case Definition
• Acute Flaccid Myelitis (AFM) | 2020 Case Definition
• Acute Flaccid Myelitis (AFM) | 2018 Case Definition