Acute flaccid myelitis (AFM) is a rare but serious neurological condition that attacks the gray matter of the spinal cord, causing sudden weakness in one or more limbs. It primarily affects children and resembles polio in how it damages the nerve cells responsible for muscle movement. Since 2014, when health authorities began formally tracking it, the United States has confirmed several hundred cases, with peaks occurring every two years in 2014, 2016, and 2018.
How AFM Affects the Spinal Cord
The spinal cord contains two types of tissue: gray matter and white matter. Gray matter houses the motor neurons, the nerve cells that send signals from the brain to your muscles telling them to contract. AFM specifically targets this gray matter, damaging or destroying motor neurons across one or more segments of the spine. When those neurons stop working, the muscles they control go limp. That’s what “flaccid” means in the name: the affected limbs lose tone and become floppy rather than stiff.
This is the same basic mechanism behind polio, which is why AFM is sometimes called a “polio-like” illness. The difference is the virus involved. Polio has been virtually eliminated through vaccination, but other viruses can attack the same vulnerable cells.
What Causes It
Most AFM cases are linked to viral infections, particularly a respiratory virus called enterovirus D68 (EV-D68). The major U.S. outbreaks in 2014, 2016, and 2018 all coincided with widespread circulation of EV-D68, and it was the most common virus detected in respiratory specimens from AFM patients. Researchers have also found antibodies against enteroviruses in the spinal fluid of AFM patients more often than in people without the condition, strengthening the connection.
Other viruses can trigger AFM as well, including enterovirus A71, West Nile virus, Japanese encephalitis virus, certain herpesviruses, and adenoviruses. In a small number of patients, scientists have recovered virus directly from spinal fluid, but in many cases no specific virus is identified. The condition remains difficult to pin down because the virus may clear from the body before testing happens, or it may trigger an immune response that does the actual damage to the spinal cord.
Symptoms and How They Develop
AFM typically begins with what looks like an ordinary cold or respiratory illness. Days later, the hallmark symptom appears: sudden weakness in an arm or leg, along with loss of muscle tone and loss of reflexes. The weakness can affect one limb or multiple limbs, and it develops quickly, sometimes over hours.
Beyond limb weakness, some people experience:
- Facial involvement: drooping on one side of the face, difficulty moving the eyes, or drooping eyelids
- Trouble speaking or swallowing: slurred speech or difficulty getting food and liquid down
- Pain: in the arms, legs, neck, or back
- Numbness or tingling: in the arms or legs, though this is uncommon
The most dangerous scenarios involve the muscles that control breathing. When those weaken, a patient may need mechanical ventilation. Some patients also develop instability in body temperature and blood pressure, which can become life-threatening. These severe complications are why AFM requires immediate medical attention, even though the condition is rare.
Who Gets AFM
AFM overwhelmingly affects children, with the average age of patients in U.S. outbreaks falling around 5 years old. Adults can develop it, but they represent a small fraction of cases. Most cases cluster between August and November, overlapping with the peak season for enterovirus circulation.
The CDC confirmed 25 cases in 2024 and 15 in 2025. These numbers are far lower than the peak years, when over 200 cases were confirmed in 2018 alone. The earlier biennial pattern (spikes every two years) appears to have shifted, possibly influenced by changes in viral circulation patterns after the COVID-19 pandemic disrupted the usual respiratory virus seasons.
How AFM Is Diagnosed
Diagnosis relies heavily on MRI imaging of the spinal cord. A confirmed case requires an MRI showing lesions concentrated in the gray matter, spanning one or more spinal segments. This pattern distinguishes AFM from other causes of sudden weakness, like Guillain-Barré syndrome, which typically affects peripheral nerves rather than the spinal cord itself.
Doctors also collect spinal fluid for analysis. Elevated white blood cell counts in the spinal fluid support a probable diagnosis, even when the MRI findings are less clear-cut. Blood and respiratory samples are tested for viruses, though a specific virus isn’t identified in every case. The clinical picture, a child who had a recent respiratory illness and then developed sudden limb weakness, is what typically triggers the diagnostic workup.
Treatment Options Are Limited
There are currently no FDA-approved drugs or therapies specifically for AFM. This is one of the most frustrating aspects of the condition for families. Several treatments have been tried, but none has enough evidence to be considered a standard of care.
Immunoglobulin therapy (antibodies given through an IV) has shown some promise in animal studies when given early, but there’s no clear evidence it works in human AFM patients. It hasn’t been studied systematically enough to measure its effectiveness. Steroids, which are commonly used for other types of spinal cord inflammation, carry a potential concern with AFM: animal studies suggest they could actually worsen outcomes if a viral infection is still active, since steroids suppress the immune system. Plasma exchange, a procedure that filters the blood, has also been tried without clear evidence of benefit.
What does help is supportive care and rehabilitation. Physical and occupational therapy are central to recovery, helping patients rebuild strength and adapt to any lasting weakness. Some children recover fully, while others are left with permanent weakness in the affected limbs. Recovery varies widely and can continue over months or even years.
Nerve Transfer Surgery
For children with persistent limb weakness that doesn’t improve with rehabilitation, nerve transfer surgery has emerged as an option. This procedure reroutes working nerves from other parts of the body to take over for the damaged motor neurons. It doesn’t work for every patient, and outcomes depend on factors like which limbs are affected and how soon after onset the surgery is performed. Specialized neurosurgical centers have reported meaningful improvements in some children’s ability to move their arms or legs after the procedure.
Experimental Treatments in Development
The National Institutes of Health is sponsoring a clinical trial for an experimental antibody therapy targeting EV-D68. Scientists at Vanderbilt University Medical Center isolated a neutralizing antibody from patients who had recovered from EV-D68 infection. An engineered version of that antibody, called EV68-228-N, entered a Phase 1 safety trial in healthy adult volunteers. The study is testing three different doses in 36 participants. If the antibody proves safe, it would move into trials in patients with active EV-D68 infection or AFM, but that is likely years away from becoming an available treatment.
Warning Signs That Need Immediate Attention
If your child develops sudden weakness in an arm or leg, especially following a respiratory illness, seek emergency medical care. Other symptoms that warrant an immediate visit include facial drooping, difficulty moving the eyes, drooping eyelids, trouble swallowing, and slurred speech. AFM progresses quickly, and early evaluation gives doctors the best chance to manage complications like breathing difficulties before they become critical.