What Is Acute Flaccid Myelitis? Symptoms & Causes

Acute flaccid myelitis is a rare but serious neurological condition that causes sudden weakness or paralysis in one or more limbs, driven by damage to the gray matter of the spinal cord. It overwhelmingly strikes children and has been linked to outbreaks of a common respiratory virus, enterovirus D68, though proving that connection took years of detective work. The condition draws frequent comparisons to polio for good reason, and its emergence since 2012 has caught public health authorities off guard.

How AFM Announces Itself

The pattern is unsettlingly consistent. A child develops what looks like an ordinary cold or fever, then within days experiences sudden weakness in an arm or leg. That prodromal illness, usually fever or respiratory symptoms, precedes the limb weakness in most cases.¹ The weakness comes on fast and is “flaccid,” meaning the affected limbs feel floppy rather than stiff. This distinguishes AFM from conditions where spasticity or tightness dominates.

While a single arm or leg is the most recognizable presentation, the weakness can extend further. Muscles that control breathing, the trunk, face, and even eye movement can be involved.¹ When respiratory muscles are affected, children may need ventilator support, making AFM a potential medical emergency. Some patients also develop difficulty swallowing or speaking if the bulbar muscles, the group that controls the throat and mouth, are hit.

The speed of progression catches many families and even clinicians off guard. The interval between the first sign of weakness and its worst point is short, often just a few days. In one comparative study, the median time from weakness onset to its lowest point was about three days in children with AFM, which is notably faster than the roughly eight-day timeline seen in Guillain-Barré syndrome, a condition AFM can initially resemble.²

The Enterovirus D68 Connection

For years, the cause of AFM was one of the more frustrating puzzles in infectious disease. The leading suspect has been enterovirus D68, a respiratory virus that circulates widely but only rarely causes neurological problems. Surges of EV-D68 in 2014 and 2016 lined up both in time and geography with spikes in AFM cases, and respiratory specimens from AFM patients frequently tested positive for the virus.³ During one investigation during the 2014 outbreak, EV-D68 was detected in the respiratory secretions of roughly half of patients with AFM.⁴

But proving that the virus actually causes the paralysis was harder than it sounds. The classic way to nail down a cause in infectious disease is to find the pathogen at the site of damage. With AFM, researchers struggled on this front. EV-D68 was almost never found in spinal fluid: among hundreds of confirmed AFM patients, only a tiny fraction, around 3%, had positive cerebrospinal fluid tests.⁵ And direct virus isolation from affected spinal cord tissue was essentially absent in living patients. This gap left the causal relationship formally unproven for years, even as circumstantial evidence piled up.³

Animal studies eventually strengthened the case. When researchers injected neonatal mice with a clinical EV-D68 isolate from the 2014 epidemic, the mice developed many of the same features seen in human AFM: the virus infected the spinal cord, killed motor neurons, and produced progressive paralysis.⁶ Blood-based antibody testing has also helped. A technique called pan-viral serology, which screens for immune responses against thousands of viruses at once, implicated enteroviruses in AFM patients even when standard tests came up empty.⁷

Other Viruses That Can Cause It

EV-D68 is not the only virus that can trigger AFM. Enterovirus A71, another member of the same viral family, has also been linked to cases of acute flaccid paralysis.⁸ With the global phaseout of oral polio vaccine reducing vaccine-associated poliomyelitis, EV-D68 and EV-A71 have emerged as the main enteroviral causes of acute flaccid paralysis.⁹ Historically, West Nile virus, other enteroviruses, and adenoviruses have occasionally been associated with similar presentations, but the recent AFM outbreaks have been dominated by EV-D68.

One reason the virus family keeps producing these cases is that enteroviruses are extremely common. Most children who catch EV-D68 get nothing worse than a cold. AFM appears to be a rare but severe outcome in susceptible individuals, which is part of why it has been so hard to predict who will develop paralysis.⁴ What makes a particular child vulnerable remains an open question. Researchers have called for investigations into genetic, immunological, and virus-host interaction factors, but no clear answer has emerged yet.¹⁰

Why Children Are the Primary Victims

AFM overwhelmingly affects young children. Multiple sources describe it as a condition that mainly strikes pediatric patients, and the U.S. outbreak data consistently show a median age in the single digits.¹¹ Adults can develop it, but they represent a small minority of confirmed cases. The age skew resembles what was seen with polio before widespread vaccination: young children encountered the virus with immune systems that hadn’t developed protection from prior exposures.

The parallel to polio is not superficial. Reviews comparing historical poliomyelitis data to modern AFM cases have found striking similarities in clinical features, epidemiology, and lab findings.¹² Both conditions target motor neurons in the spinal cord’s anterior horn, both follow a respiratory or gastrointestinal illness, and both cause asymmetric flaccid paralysis that can be permanent. The clinical syndrome itself is not new; what has changed is the virus doing the damage.¹³

Outbreak Patterns in the United States

Since 2014, confirmed AFM cases in the United States have followed a biennial cycle, with peaks occurring every other year.¹⁴ The pattern went: a spike in 2014, lower numbers in 2015, another spike in 2016, a lull in 2017, and a large outbreak in 2018. This two-year rhythm mirrors the circulation pattern of EV-D68, which itself tends to peak biennially as new cohorts of susceptible children are born. After 2018, an expected 2020 peak did not materialize in the expected form, likely because COVID-19 mitigation measures such as masking and school closures suppressed respiratory virus transmission broadly.

Mathematical modeling has suggested that this biennial cycle may not be stable over the long run.¹⁵ In other words, after a period of disrupted transmission, a larger pool of susceptible children could build up, potentially leading to a bigger or differently timed outbreak when EV-D68 returns to circulation. Public health officials have remained watchful for this rebound scenario.

How AFM Is Diagnosed

Diagnosis relies heavily on MRI imaging rather than on catching the virus itself. For surveillance purposes, confirmed AFM requires acute flaccid limb weakness plus an MRI showing a spinal cord lesion largely restricted to gray matter, spanning one or more vertebral segments.¹⁶ The emphasis on gray matter is important because it distinguishes AFM from other causes of spinal cord inflammation, like transverse myelitis, which often involves white matter more extensively.

The MRI appearance evolves over time. Early on, within the first few days of symptoms, the lesions appear as diffuse, somewhat blurry areas of signal abnormality spread through the central gray matter of the spinal cord. Over the following weeks, they become more defined and settle into a distinctive pattern confined to the anterior horns, the specific region where motor neurons live.¹⁷ This evolution has been documented both in U.S. and Japanese cohorts. In the 2014 U.S. outbreak, patients imaged earlier showed the diffuse pattern, while those imaged later showed the more defined anterior horn pattern, with no overlap in timing between the two groups.¹⁸

The cervical cord, the portion of the spinal cord in the neck, is almost always affected. From there, involvement extends downward in decreasing frequency. In some patients, lesions stretch the entire length of the spinal cord from the base of the skull to the lowest segments.¹⁸ A Japanese study found that children whose MRI showed involvement of both gray and white matter tended to have more severe weakness than those with gray-matter-only lesions.¹⁹

Why Lab Tests Often Come Up Empty

One of the most frustrating aspects of diagnosing AFM is that standard viral testing frequently fails to identify the culprit. The virus does its damage in the spinal cord, but by the time a child develops weakness and gets a lumbar puncture, EV-D68 is almost never detectable in the spinal fluid.⁷ The best yield comes from respiratory specimens, where about 44% test positive, with EV-D68 as the most common finding.⁵ But even respiratory testing misses a large share of cases, partly because the initial respiratory illness may have cleared before anyone suspects a neurological problem.

This diagnostic gap has real consequences. It makes it harder to confirm the causal link to any particular virus, and it means clinicians cannot rely on a positive viral test to make the AFM diagnosis. The diagnosis is clinical and radiological: sudden limb weakness plus the right MRI pattern, regardless of whether the virus is caught.

Telling AFM Apart from Guillain-Barré Syndrome

The condition most commonly confused with AFM is Guillain-Barré syndrome, since both cause rapid-onset limb weakness in children. But there are reliable differences. AFM weakness is typically asymmetric, affecting one side more than the other. In one direct comparison, asymmetric weakness appeared in more than half of AFM patients but in none of the Guillain-Barré patients.² Sensory deficits, numbness and tingling, are common in Guillain-Barré but absent in AFM. And the spinal fluid tells different stories: AFM tends to produce higher white cell counts but lower protein levels, whereas Guillain-Barré classically shows the reverse.

Crucially, spinal cord lesions on MRI appeared only in AFM patients in that study, not in any Guillain-Barré cases. This makes MRI the single most useful tool for distinguishing the two. No child with confirmed Guillain-Barré met the CDC criteria for definite AFM.² The distinction matters because treatments differ: Guillain-Barré responds to immune therapies like intravenous immunoglobulin and plasma exchange, while these interventions have not shown clear benefit in AFM.

Long-Term Recovery and What Families Can Expect

The prognosis for AFM is sobering, and parents searching for reassurance will find the data honest but not cheerful. Most children retain some impairment at one year. In a study tracking 49 confirmed AFM patients from the 2018 outbreak, significant or severe impairment at six months ranged from 2% to 59% depending on which outcome was measured. By twelve months, the proportions had decreased but most patients still had some degree of impairment.²⁰

Longer-term data are somewhat more encouraging, though far from universally so. In a three-year follow-up study, about a quarter of patients showed continued improvement even in the later recovery period. The likelihood of full recovery depended heavily on how many limbs were affected at the start. Among children who initially had weakness in just one limb, about two in thirteen recovered completely, while the remainder improved to varying degrees. Children initially paralyzed in all four limbs had the worst trajectories, with only two of seven recovering fully. Daily functioning scores improved even in patients with persistent weakness, suggesting that children adapt and compensate over time even when full muscle strength does not return.²¹

Nerve Transfer Surgery

One of the more promising developments in AFM management has been nerve transfer surgery, a technique borrowed from brachial plexus injury repair. The idea is to reroute a working nerve to supply a paralyzed muscle when the original motor neurons have been destroyed. For children with severe, persistent weakness that has not recovered on its own, this approach has shown meaningful results.

In a study comparing muscles that received a nerve transfer to muscles that did not, the transferred muscles improved significantly while the untreated muscles showed no improvement at all.²² Another study of eight children who underwent upper limb nerve transfers found that all but one showed functional improvement, with procedures targeting elbow function performing better than those targeting the shoulder.²³ Surgery was typically performed more than a year after disease onset, suggesting there is a window during which surgeons wait to see how much spontaneous recovery occurs before intervening.

Nerve transfer is not a cure. It does not restore normal function. But for a child who cannot bend an arm at the elbow, getting even partial movement back is transformative for daily life. Timing matters: the longer a muscle goes without nerve input, the harder it is to revive. Families of children with AFM are increasingly being referred to peripheral nerve specialists early in the course of the disease so that surgical options remain on the table.

Rehabilitation and Recovery Strategies

Physical rehabilitation plays a central role in the long-term management of AFM, even when it begins well after the initial paralysis. In one pediatric cohort, children who underwent structured, comprehensive rehabilitation made significant neurological and functional gains even when therapy started long after the acute illness.²⁴ The goals shift over time: early rehabilitation focuses on maintaining range of motion and preventing complications like joint contractures, while later phases emphasize strengthening, functional independence, and adapting to residual deficits.

Activity-based restorative therapy, which emphasizes high-repetition, task-specific exercises designed to promote nervous system recovery, has shown particular promise. In one inpatient program, children with AFM who participated in this type of intensive therapy increased muscle strength and showed functional improvement across all measured outcomes.²⁵ The approach draws on the principle that the nervous system retains some capacity to rewire even after motor neuron loss, especially in young children whose brains are still developing. For many families, rehabilitation is not a short chapter but an ongoing effort that continues for years, often combining physical therapy, occupational therapy, and assistive devices as the child grows.

Why There Is No Vaccine Yet

Given the strong link to EV-D68, the obvious question is why there is no vaccine. Developing one faces several hurdles. EV-D68 belongs to a large family of enteroviruses with many circulating strains, and the virus evolves quickly enough that a vaccine targeting one strain may not protect against the next outbreak’s variant. Additionally, AFM is rare enough that designing a clinical trial to prove vaccine efficacy is logistically difficult. You would need enormous numbers of participants, vaccinate them, and wait for a naturally occurring outbreak to see if cases are prevented. Unlike polio, which paralyzed tens of thousands per year at its peak, AFM confirms only a few hundred cases per cycle in the United States, making the math for traditional vaccine trials challenging.

Research is underway. Several groups are working on candidate vaccines and monoclonal antibodies that could neutralize EV-D68. The animal models that helped establish causality are now being used to test these interventions. But given the regulatory pathway and the unpredictable timing of outbreaks, a licensed vaccine likely remains years away. In the meantime, standard respiratory hygiene, handwashing and keeping sick children home, remains the only practical prevention advice, though it offers no guarantee against a virus that spreads easily through respiratory droplets and often causes nothing more than a cold.

1
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