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Acute flaccid myelitis

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An acute or sudden onset of focal limb weakness that is associated mainly with gray matter abnormalities or CSF ]]pleocytosis]], but which is without an apparent cause.

Seasonal AFM
Seasonal AFM

Other names



  • Acute flaccid myelitis (AFM) is an uncommon but serious neurologic condition.
  • It affects the nervous system, specifically the area of the spinal cord called gray matter, which causes the muscles and reflexes in the body to become weak.
  • There has been an increase in AFM cases in the United States since 2014, according to the CDC.
  • Most AFM cases (more than 90%) have been in young children.

Differences with poliomyelitis

  • Poliomyelitis is the term used to describe the syndrome of acute flaccid limb weakness and lesions in the grey matter of the spinal cord. Poliomyelitis caused by poliovirus no longer occurs in the United States.
  • In 2014, the term AFM was adopted to describe poliomyelitis without a known cause and not caused by poliovirus.
  • We now have stronger evidence that points to AFM being caused by non-polio enteroviruses, such as EV-D68 and EV-A71.
  • Poliovirus can be prevented by a vaccine.
  • However, there is no vaccine yet available for EV-A71 in the United States, and no vaccine for EV-D68 or for the other enteroviruses that are believed to cause AFM.
  • All the stool specimens from AFM patients that we received tested negative for poliovirus.

AFM symptoms

  • sudden arm or leg weakness
  • Arm or leg weakness
  • Loss of muscle tone and reflexes
  • Some people will also have:
  • Difficulty moving the eyes or drooping eyelids
  • Facial droop or weakness
  • Difficulty with swallowing or slurred speech
  • Pain in arms or legs
  • Pain in neck or back
  • In uncommon cases, people may also:
  • Have numbness or tingling
  • Be unable to pass urine (pee)

The most severe symptoms of AFM are:

  • Respiratory failure: This happens when the muscles involved with breathing become weak and can require a ventilator (a machine to help them breathe).
  • Serious neurologic complications such as body temperature changes and blood pressure instability that could be life threatening.

Possible Causes of AFM

Our five years of surveillance data have contributed important evidence to support the role of viruses, including enteroviruses, in AFM.

Characteristics of AFM

  • Most patients had respiratory illness or fever
  • More than 90% of patients with AFM had a mild respiratory illness or fever consistent with a viral infection before they developed AFM.
  • Respiratory illnesses and fever from viral infections such as from enteroviruses are common, especially in children, and most people recover. Enteroviruses can also cause neurologic illness, such as meningitis, encephalitis, and acute flaccid limb weakness, but these are rare.
  • We don’t know why a small number of people with enterovirus infection develop AFM, while most others recover.
  • We are continuing to investigate possible ways that people develop AFM including:
  • Direct virus infection of the motor neurons (nerves that make the muscles move)
  • Indirect infection where a virus may lead to an inflammatory or immune response directed toward motor neurons
  • Host genetic factors in which certain children may be more susceptible than others
  • Increase in cases between August and November
  • Most patients had onset of AFM between August and November, with increases in AFM cases every other year starting in 2014.
  • Many viruses, including enteroviruses, commonly circulate between August and November, at the same time that AFM outbreaks have occurred. CDC is working with national partners to understand the annual circulation of enteroviruses, including EV-D68, and their association with AFM.
  • The 2014 AFM outbreak coincided with a national outbreak of severe respiratory illness caused by EV-D68. EV-D68 outbreaks have also been detected in 2016 and 2018.

Detecting the causes of AFM

  • In some patients, AFM can be caused by coxsackievirus A16, EV-A71, and EV-D68 in the spinal fluid of a small number of patients with AFM, which points to the cause of their AFM.
  • For many other patients, no pathogen (germ) has been detected in spinal fluid to confirm a cause.
  • Studies have shown that AFM patients had antibodies specific for enteroviruses in their spinal fluid more often than persons without AFM. Having antibodies for enterovirus means that a person was previously infected with the virus.
  • All the stool specimens from AFM patients that we received tested negative for poliovirus.
  • Often, despite extensive testing of AFM patients, no pathogens are found in the spinal fluid.
  • This may be because the body has cleared the pathogen, or it is hiding in tissues that make it difficult to detect.
  • Another possibility is that the pathogen triggers an immune response in the body that causes damage to the spinal cord.


  • Clinicians diagnose AFM by taking a thorough medical history, doing a physical exam of the patient, and performing an MRI to review pictures of the spinal cord.
  • Currently, there is no specific treatment for AFM, but clinicians may recommend different interventions based on each patient.
  • Diagnosis
  • It is important that the tests (MRI and laboratory testing of the CSF, respiratory fluid, blood and stool) are done as soon as possible after a patient develops symptoms.


  • AFM is diagnosed by reviewing a patient’s medical history along with examining their nervous system and reviewing pictures of the spinal cord. A clinician may:
  • Examine the nervous system
  • MRI
  • Spinal tap
  • Lab tests on cerebrospinal fluid (the fluid around the brain and spinal cord).
  • Check nerve conduction

Difficult to diagnose

  • AFM can be difficult to diagnose because it shares many of the same symptoms as other neurologic diseases, like transverse myelitisexternal icon and Guillain-Barre syndrome.
  • Consider consulting with specialists in neurology and infectious diseases to assist with the diagnosis of AFM.
  • With the help of testing and examinations, clinicians can distinguish between AFM and other neurologic conditions.


There is no specific treatment for AFM, but a clinician who specializes in treating brain and spinal cord illnesses (neurologist) may recommend certain interventions on a case-by-case basis.

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