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Acute Flaccid Myelitis (AFM)
2020 Case Definition

NOTE: A surveillance case definition is a set of uniform criteria used to define a disease for public health surveillance. Surveillance case definitions enable public health officials to classify and count cases consistently across reporting jurisdictions. Surveillance case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs.

CSTE Position Statement(s)

  • 19-ID-05

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 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.

Clinical Criteria

An illness with onset of acute flaccid* weakness of one or more limbs

Laboratory Criteria

Confirmatory laboratory/imaging evidence:

  • MRI showing spinal cord lesion with predominant gray matter involvement and spanning one or more vertebral segments
  • 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
  • Excluding persons with gray matter lesions in the spinal cord resulting from physician diagnosed malignancy, vascular disease, or anatomic abnormalities

Case Classification

Suspect

  • Clinically compatible case, AND
  • Available information is insufficient to classify case as probable or confirmed

Probable

  • Clinically compatible case with presumptive laboratory/imaging evidence, AND
  • Absence of a clear alternative diagnosis attributable to a nationally notifiable condition

Confirmed

  • Clinically compatible case with confirmatory laboratory/imaging evidence, AND
  • Absence of a clear alternative diagnosis attributable to a nationally notifiable condition

Comments

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 (1).

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

✝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.

Reference(s)

  1. CSTE. National Surveillance for Paralytic Poliomyelitis and Nonparalytic Poliovirus Infection (09-ID-53).


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