Acute flaccid myelitis (AFM): a polio-like illness explained
Acute flaccid myelitis (AFM) is a rare but serious illness of the spinal cord that causes sudden weakness in the arms or legs. It mostly affects young children, and it often follows a cold-like illness or fever.
AFM got the nickname "polio-like" because it looks a lot like polio. Both damage the part of the spinal cord that controls muscles. But AFM is not polio, and doctors test for poliovirus to tell them apart.
Quick facts
- AFM causes sudden limb weakness, loss of muscle tone and loss of reflexes.
- More than 90 percent of cases have been in young children, according to the CDC.
- Viruses can cause it, especially enteroviruses such as EV-D68.
- The US saw rises in cases in 2014, 2016 and 2018.
- Any sudden weakness in a child needs medical care right away.
Sources: CDC: about AFM; CDC: AFM frequently asked questions.
What AFM does in the body
AFM affects the gray matter of the spinal cord. That is the area holding the nerve cells that send signals to muscles. When those cells are inflamed or damaged, the muscles they control go weak and floppy. Doctors call that kind of weakness "flaccid."
On an MRI, AFM shows up as spots in the spinal cord's gray matter. Some white matter can be involved too. This pattern helps doctors separate AFM from other spinal cord problems, such as transverse myelitis, which tends to affect sensation and bladder control along with strength.
Sources: CDC: clinical overview of AFM; CDC: about AFM; NINDS: transverse myelitis.
Warning signs to watch for
The most common sign is weakness in an arm or a leg that starts suddenly. It can get worse over hours to a few days. Parents may notice a child who stops using one arm, drags a leg, or cannot lift their head.
| How common | Signs the CDC lists |
|---|---|
| Most people | Sudden arm or leg weakness; loss of muscle tone and reflexes |
| Some people | Drooping eyelids or trouble moving the eyes; facial droop; trouble swallowing or slurred speech; pain in the arms, legs, neck or back |
| Uncommon | Numbness or tingling in the arms or legs |
| Most severe | Weak breathing muscles that need a machine to help; swings in body temperature or blood pressure |
The CDC's advice is simple: seek medical care right away if you or your child develops any of these signs. Do not wait to see if the weakness passes.
Sources: CDC: AFM symptoms.
What causes AFM
Viruses can cause AFM. The CDC names enteroviruses as a main group, and links enterovirus D68 (EV-D68) to the spikes in cases. Other viruses have also been tied to it, including West Nile virus, Japanese encephalitis virus, herpesviruses and adenoviruses.
The CDC has found coxsackievirus A16, EV-A71 and EV-D68 in the spinal fluid of only a small number of patients. That gap is one reason the illness still puzzles researchers.
Because doctors do not know exactly what triggers AFM in one child and not another, the CDC says there is no specific step that prevents it. Its general advice is to wash hands often with soap and water and to stay up to date on recommended vaccines.
Sources: CDC: causes of AFM; CDC: preventing AFM.
How doctors check for AFM
Doctors start with an exam of strength, tone and reflexes. An MRI of the brain and spinal cord looks for the gray matter pattern. A spinal tap checks the fluid around the cord for signs of inflammation. The CDC also lists lab tests on respiratory fluid, blood and stool to look for viruses.
Timing matters. The CDC says these tests are most useful when done as soon as possible after symptoms start. Stool is also tested for poliovirus. If poliovirus is found, the illness is counted as polio, not AFM.
Sources: CDC: AFM diagnosis; CDC: causes of AFM.
How AFM differs from polio and transverse myelitis
Three illnesses can make a child's limbs go weak, and doctors work hard to tell them apart. The table shows the main differences in plain terms.
| AFM | Polio | Transverse myelitis | |
|---|---|---|---|
| Cause | Viruses, often enteroviruses | Poliovirus | Many causes, including infections and immune conditions such as MS |
| Main problem | Sudden floppy weakness with lost reflexes | Weakness or paralysis in the arms, legs or both | Weakness plus numbness, tingling and bladder or bowel trouble |
| Feeling (sensation) | Numbness is uncommon | Weakness is the main feature | Often lost below a band across the trunk |
| How it is told apart | MRI of the cord's gray matter, spinal fluid, virus tests | Poliovirus found in stool | MRI, spinal fluid, and tests for other causes |
Transverse myelitis has its own guide on this site, and so does MS.
Sources: CDC: causes of AFM; CDC: AFM symptoms; CDC: about polio; NINDS: transverse myelitis.
Treatment and recovery
There is no FDA-approved medicine to treat or prevent AFM. The CDC's guidance for doctors says no targeted treatment has enough evidence to endorse or discourage it. Care focuses on support in the hospital, close watch on breathing, and quick input from neurology and infectious disease specialists.
Rehabilitation is a big part of the long road back. The CDC notes that physical and occupational therapy may help, and that starting rehab early might improve long-term results. Some children have also had nerve transfer surgery. Recovery varies a lot. Some children regain much of their strength, while others keep lasting weakness.
Sources: CDC: clinical guidance for AFM; CDC: your child's care.
For how US case numbers rose and fell from 2014 on, read acute flaccid myelitis outbreaks in the US: what the data showed. For other immune conditions of the spinal cord and nerves in children, see pediatric neuroimmunology and the NICC overview.
Who this page is for
This page is for parents and caregivers who have heard about AFM and want a clear picture of what it is, and for anyone whose child has sudden weakness and needs to know why speed matters. It is also a starting point for students and clinicians who want the CDC's current wording in one place. If your child cannot move an arm or leg normally today, stop reading and get care now.
Questions people ask
What causes acute flaccid myelitis?
Viruses can cause AFM, most often enteroviruses. The CDC links enterovirus D68 to the spikes in US cases, and other viruses such as West Nile virus have been tied to it too. Viruses turn up in the spinal fluid of only a few patients, so doctors still cannot say why one child develops AFM after an infection and another does not.
What are the symptoms of acute flaccid myelitis?
The main symptom is sudden weakness in one or more arms or legs, with loss of muscle tone and reflexes. Some children also have facial droop, drooping eyelids, trouble swallowing or slurred speech, or pain in the neck, back or limbs. Weak breathing is the most dangerous sign and needs emergency care.
Can acute flaccid myelitis be cured?
There is no cure and no approved medicine for AFM. Hospital care supports breathing and other body functions, and rehabilitation helps children rebuild strength. Some children recover a lot of function over time, while others keep some weakness. A child's own care team is the best guide to what recovery may look like for them.
How do people get AFM?
Doctors think AFM usually follows a viral infection. In the CDC's 2018 review, 96 percent of children with confirmed AFM had been sick in the four weeks before the weakness began, most often with fever or breathing symptoms. Many children catch these viruses, but AFM stays rare, and researchers are still working out why a small number develop it.
Sources: CDC: causes of AFM; CDC: AFM symptoms; CDC: clinical guidance for AFM; CDC MMWR: Vital Signs, 2018 AFM surveillance.
Sources
- CDC: about acute flaccid myelitis
- CDC: AFM symptoms
- CDC: causes of AFM
- CDC: clinical overview of AFM
- CDC: AFM diagnosis
- CDC: preventing AFM
- CDC MMWR: Vital Signs, surveillance for acute flaccid myelitis, United States, 2018
- CDC: clinical guidance for the acute medical treatment of AFM
- CDC: your child's care
- CDC: about polio
- NINDS: transverse myelitis