Case Definition for AFM

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Committee:

Infectious Disease

Title: Revision to the Standardized Surveillance and Case Definition for Acute Flaccid Myelitis
I. Statement of the Problem
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 (1). 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. To facilitate interpretation of apparent increases
in this syndrome, to improve the tracking of national trends, and to better define the etiologic agent(s), this
position statement proposes a revision to the standardized case definition and surveillance for AFM.
II. Background and Justification
AFM is a subtype of acute flaccid paralysis (AFP). AFP is the 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 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
expected to occur at 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 a
reportable condition in any U.S. state and routine surveillance and assessment for AFP is not performed.
Therefore, understanding the baseline incidence and epidemiology of AFM and its public health impact in
the United States is significantly limited. While AFM is most commonly attributable to poliovirus or West
Nile virus and other flaviviruses, there are numerous other viruses, including non-polio enteroviruses,
which may uncommonly cause AFM.
During the summer/fall of 2014, 120 confirmed cases of AFM were reported to CDC. Confirmed cases
were defined as acute onset of focal limb weakness occurring on or after August 1, 2014, and an MRI
showing spinal cord lesion largely restricted to gray matter, in patients ≤21 years of age. Most of the
patients had distinctive abnormalities of the spinal cord gray matter on MRI and reported a respiratory or
febrile illness in the days before onset of neurologic symptoms (2). Despite extensive pathogen-specific
testing, no common etiology was identified. To better understand the full spectrum of AFM and determine
baseline incidence, standardized surveillance for AFM was established in 2015 (1). The case definition
was broadened to include patients of all ages and a probable category to capture patients demonstrating a
pleocytosis (cerebrospinal fluid (CSF) white blood cells >5 mm3) who did not have an MRI performed or
had a normal MRI.
During 2015, only 21 confirmed and 3 probable cases from 17 states were reported to CDC. However, in
2016, reported cases increased to levels similar to 2014. Through December 2016, 136 confirmed and 29
probable cases have been reported from 37 states. No fatalities attributed to AFM have been reported to
date.
Testing of biological specimens, including CSF, respiratory secretions, serum, and stool, continued
through 2016, without identification of a common etiology (CDC, unpublished data). Thus, CDC has
expanded the search for potential causes of AFM by broadening laboratory approaches that test for
potential infectious and noninfectious causes, including possibly immune-mediated mechanisms.

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Because polio viruses were the classic etiology of AFM in the United States prior to elimination, vaccine
and travel history along with a stool test to rule out polio should always be obtained. Without a biological
marker to confirm cases of AFM not associated with polio virus, classification of cases is challenging. For
AFM, as with polio (3), review of case information by experts in national AFM surveillance provides
consistency to classification of AFM cases.
III. Statement of the desired action(s) to be taken
1. Utilize standard sources (e.g. reporting*) for case ascertainment for acute flaccid myelitis (AFM).
Surveillance for AFM should use the following recommended sources of data to the extent of coverage
presented in Table III.
Table III. Recommended sources of data and extent of coverage for ascertainment of cases
of Acute Flaccid Myelitis (AFM).
Coverage
Source of data for case ascertainment
Population-wide
Sentinel sites
Clinician reporting
x
Laboratory reporting
x
Reporting by other entities (e.g., hospitals,
x
veterinarians, pharmacies, poison centers)
Death certificates
x
Hospital discharge, neurology or infectious disease
x
consult notes, MRI reports and images, or outpatient
records
Extracts from electronic medical records
x
Telephone survey
School-based survey
Other _________________________
2. Utilize standardized criteria for case identification and classification (Sections VI and VII) for acute
flaccid myelitis (AFM) but do not add AFM to the Nationally Notifiable Condition List. If requested by CDC,
jurisdictions (e.g. States and Territories) conducting surveillance according to these methods may submit
case information to CDC.

IV. Goals of Surveillance
To provide a standard case definition for states electing to perform surveillance for acute flaccid myelitis
(AFM).
V. Methods for Surveillance: Surveillance for acute flaccid myelitis (AFM) should use the
recommended sources of data and the extent of coverage listed in Table III.
Surveillance for acute flaccid myelitis (AFM) should use the recommended sources of data and the extent
of coverage listed in Table III.

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VI. Criteria for case identification
Reporting refers to the process of healthcare providers or institutions (e.g., clinicians, hospitals) submitting
basic information to governmental public health agencies about cases of illness that meet certain reporting
requirements or criteria. The purpose of this section is to provide those criteria to determine whether a
specific illness should be reported.
A. Narrative: A description of suggested criteria for case ascertainment of a specific condition.
Clinical presentation criteria:
Report any illness to public health authorities that meets all of the following criteria:
• A person with onset of acute flaccid limb weakness, AND
• A magnetic resonance image showing a spinal cord lesion largely restricted to gray matter*†, and
spanning one or more vertebral segments OR
Cerebrospinal fluid (CSF) with pleocytosis (CSF white blood cell count >5 cells/mm3); CSF protein
may or may not be elevated
Other recommended reporting procedures
• 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 (3).
* 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. MRI studies performed 72 hours or more after onset should also be reviewed if available.
† 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.

B. Table of criteria to determine whether a case should be reported to public health authorities
Table VI-B. Table of criteria to determine whether a case should be reported to public health
authorities.
Criterion
Reporting
Disease or Condition Subtype
Clinical Evidence
Acute onset of flaccid limb weakness
N
Magnetic resonance image (MRI) showing spinal cord lesion largely
O
restricted to gray matter*† and spanning one or more spinal segments
Cerebrospinal fluid (CSF) with pleocytosis (CSF white blood cell count
O
>5 cells/mm3)
* 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. MRI studies performed 72 hours or more after onset should also be reviewed if available.
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.
† Terms

Notes:
S = This criterion alone is Sufficient to report a case.
N = All “N” criteria in the same column are Necessary to report a case.
O = At least one of these “O” (One or more) criteria in each category (e.g., clinical evidence and laboratory
evidence) in the same column—in conjunction with all “N” criteria in the same column—is required to
report a case.
* A requisition or order for any of the “S” laboratory tests is sufficient to meet the reporting criteria.

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C. Disease-specific data elements
Disease-specific data elements to be included in the initial report are listed below.
Basic demographics
Clinical information:
• Date of onset
• Limbs with acute onset of weakness
o Description of limb weakness: limb(s) affected; weakness symmetric or asymmetric
o Cranial nerve involvement (e.g., extraocular movement abnormalities, facial weakness)
o Reflexes and tone (flaccid or spastic) in affected limbs
• Hospitalized (include duration)
• Date of performance of MRI (if >1 MRI performed, date of each MRI study)*
• Radiographic evidence of spinal cord lesion largely restricted to gray matter** and spanning one or
more vertebral segments (if >1 MRI performed, radiographic details of each MRI)*
Laboratory data:
• Date(s) of lumbar puncture(s) (LP)
• WBC count from CSF (cells / mm3)
• Protein level in CSF (mg/dL)
*Restricted to MRIs performed in the proximate period of the suspected AFM illness; excludes neuroimaging performed for illnesses
unrelated (clinically or temporally) to AFM illness)
**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.

VII. Case Definition for Case Classification
A. Narrative: Description of criteria to determine how a case should be classified.
Clinical Criteria
An illness with onset of acute flaccid limb weakness
Laboratory Criteria
• 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
Confirmed:
• Clinically compatible case AND
• Confirmatory laboratory evidence: MRI showing spinal cord lesion largely restricted to gray
matter*† and spanning one or more spinal segments
Probable:
• Clinically compatible case AND
• Supportive laboratory evidence: CSF showing pleocytosis (white blood cell count >5 cells /
mm3).

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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 (3).
* 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.
Criteria to distinguish a new case of this disease or condition from reports or notifications which
should not be enumerated as a new case for surveillance
Not applicable.
B. Classification Tables
Table VII-B. Criteria for defining a case of Acute Flaccid Myelitis (AFM).
Criterion
Probable
Confirmed
Clinical Evidence
Acute onset of flaccid limb weakness
N
N
Laboratory Evidence
Magnetic resonance image (MRI) showing spinal cord lesion
N
largely restricted to gray matter*† and spanning one or more
spinal segments
Cerebrospinal fluid (CSF) with pleocytosis (CSF white blood cell
N
count >5 cells / mm3)
* 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.
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.
†Terms

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.
O = At least one of these “O” (One or more) criteria in each category (e.g., clinical evidence and laboratory
evidence) in the same column—in conjunction with all “N” criteria in the same column—is required to
classify a case. (These “O” criteria are alternatives, which means that a single column will have either no O
criteria or multiple O criteria; no column should have only one O.) A number following an “O” indicates that
this criterion is only required for a specific disease/condition subtype.
VIII. Period of Surveillance
Surveillance should be ongoing. Reporting should be provided as soon as available recommended
sources of data for case ascertainment have been collected and a patient summary form has been
completed.
IX. Data sharing/release and print criteria
Data may be used to measure the burden of acute flaccid myelitis (AFM).
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X. Revision History
Position
Statement
ID
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Section of Document

Revision Description

Section VII. Narrative: Description of criteria
to determine how a case should be classified.

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Table III. Recommended sources of data and
extent of coverage for ascertainment of cases
Narrative: A description of suggested criteria
for case ascertainment of a specific condition.
Narrative: A description of suggested criteria
for case ascertainment of a specific condition.

CSTE National Office clarified narrative to
match Table 7B by separating laboratory
criteria from clinical criteria (July 2017)
ADDED additional sources of data for
case ascertainment
ADDED “flaccid” to clinical presentation
criteria to help with case ascertainment
DELETED text about adjusting WBC
count in the presence of RBC as detail
not needed for case ascertainment
ADDED text to “Other recommended
reporting procedures” to explain process
for classification of cases
EDITED * footnote to clarify findings on
MRI
ADDED “flaccid” to clinical presentation
criteria to help with case classification
DELETED text about adjusting WBC
count in the presence of RBC as detail
not needed for case classification
ADDED text to “Comment” to explain
process for classification of cases
EDITED * footnote to clarify findings on
MRI

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Narrative: A description of suggested criteria
for case ascertainment of a specific condition.

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Narrative: A description of suggested criteria
for case ascertainment of a specific condition.
Narrative: Description of criteria to determine
how a case should be classified.
Narrative: Description of criteria to determine
how a case should be classified.

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Narrative: Description of criteria to determine
how a case should be classified.
Narrative: Description of criteria to determine
how a case should be classified.

XI. References
1. CSTE. Standardized Case Definition for Acute Flaccid Myelitis.
http://c.ymcdn.com/sites/www.cste.org/resource/resmgr/2015PS/2015PSFinal/15-ID-01.pdf.
2. Sejvar JJ, Lopez AS, Cortese MM, et al. Acute Flaccid Myelitis in the United States, August-December
2014: Results of Nationwide Surveillance. Clin Infect Dis. 2016;63:737-45.
3. CSTE. National Surveillance for Paralytic Poliomyelitis and Nonparalytic Poliovirus Infection.
http://c.ymcdn.com/sites/www.cste.org/resource/resmgr/PS/09-ID-53.pdf.
XII. Coordination
Agencies for Response
(1)

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Centers for Disease Control and Prevention
Brenda Fitzgerald, MD
Director
1600 Clifton Road, NE
Atlanta, GA 30333
Telephone: 404-639-7000
Email: director@cdc.gov

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Agencies for Information
N/A
XIII. Submitting Author:
(1)

Washington State Department of Health
Chas DeBolt RN, MPH
Senior Epidemiologist

1610 NE 150th Street
Shoreline, WA 98155
206-418-5431
Chas.DeBolt@DOH.WA.gov

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