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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)

  • 17-ID-01


Acute flaccid myelitis (AFM) is a syndrome 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). Beginning in the summer and fall of 2014, an apparent increase in reports of AFM occurred in the United States, and standardized surveillance was established in 2015 to monitor this illness and attempt to estimate the baseline incidence. Data collected since the establishment of standardized surveillance helped with the identification of another increase in reports nationally during 2016 and has provided additional valuable information on the clinical presentation to help better characterize the clinical features, epidemiology, and short-term outcomes of cases of AFM.

Clinical Criteria

An illness with onset of acute flaccid limb weakness

Laboratory Criteria For Diagnosis

  • Confirmatory Laboratory Evidence: a magnetic resonance image (MRI) showing spinal cord lesion largely restricted to gray matter* and spanning one or more vertebral segments
  • Supportive Laboratory Evidence: cerebrospinal fluid (CSF) with pleocytosis (white blood cell count >5 cells/mm3)

Case Classification


  • Clinically compatible case AND
  • Supportive laboratory evidence: CSF showing pleocytosis (white blood cell count >5 cells / mm3)


  • Clinically compatible case AND
  • Confirmatory laboratory evidence: MRI showing spinal cord lesion largely restricted to gray matter*✝︎ and spanning one or more spinal segments


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).

* 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.


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


Related Case Definition(s)