Acute Flaccid Myelitis: Symptoms, Types, Causes and Treatment

Acute Flaccid Myelitis (AFM) has emerged as a rare but serious neurological condition, primarily affecting children and causing sudden muscle weakness or paralysis. Since 2014, clusters of cases have been reported worldwide, raising concern and prompting intense research into its features, origins, and management. This comprehensive article explores AFM’s symptoms, types, causes, and treatment, relying on recent clinical studies and expert reviews to provide a clear, human-centered guide to this mysterious and impactful condition.

Symptoms of Acute Flaccid Myelitis

Acute Flaccid Myelitis makes a dramatic entrance, often catching families and clinicians off guard. Most cases begin with mild, familiar symptoms—such as fever or cough—but within days, children suddenly develop weakness in one or more limbs. Recognizing these early signs is crucial for prompt intervention and optimal care.

Symptom	Frequency	Associated Features	Source(s)
Limb weakness	Nearly all	Sudden onset, flaccid	1 2 3 4 9
Gait difficulty	~50%	Trouble walking	1
Neck/back pain	~47%	Stiffness, discomfort	1
Fever	35–92%	Usually precedes weakness	1 2 3 4
Limb pain	34%	Aching, sensitivity	1
Cranial nerve involvement	Up to 50%	Facial weakness, eye movement problems	3 14
Respiratory failure	20–25%	May require ventilation	1 3 4 14

Table 1: Key Symptoms

Sudden Limb Weakness

The hallmark of AFM is the rapid onset of limb weakness or paralysis, which can affect one or multiple limbs. This weakness is typically “flaccid,” meaning the muscles are limp rather than stiff, and deep tendon reflexes are decreased or absent. In most cases, symptoms progress quickly, with maximum weakness developing within a few days 1 3 4 9.

Prodromal Symptoms

AFM frequently follows a mild viral illness. Children often experience fever, respiratory symptoms (such as cough or congestion), or both in the week before limb weakness appears. This prodrome is present in the majority of cases and can easily be mistaken for a routine cold or flu 1 2 3 4.

Pain and Sensory Features

Although AFM’s primary feature is motor weakness, about one-third to half of patients also report limb pain, neck or back pain, or difficulty walking. Sensory loss is generally mild or absent, helping distinguish AFM from other causes of paralysis 1 3 4.

Cranial Nerve and Bulbar Involvement

Some children develop weakness of the face, difficulty swallowing, or problems with eye movements—signs that cranial nerves are affected. These cases tend to be more severe and may require intensive care 3 14.

Respiratory Complications

Up to one-fourth of AFM patients experience respiratory muscle involvement, leading to breathing difficulties that can necessitate mechanical ventilation. This is a critical concern and requires immediate medical attention 1 3 4 14.

Types of Acute Flaccid Myelitis

AFM is often described as a single syndrome, but its presentation can vary based on the pattern and location of nervous system involvement. Understanding these types helps clinicians recognize and differentiate AFM from other neurological emergencies.

Type	Description	Distinguishing Features	Source(s)
Classic AFM	Acute limb weakness + gray matter findings	Flaccid paralysis, spinal cord MRI	3 4 9 16
Cranial/bulbar-predominant	Facial, bulbar, or extraocular weakness	Difficulty swallowing, facial droop	3 14
Respiratory muscle involvement	Weakness of breathing muscles	Respiratory distress, ICU need	1 3 4
Peripheral extension	Nerve root/plexus involvement	Brachial plexus MRI findings	6

Table 2: Clinical Types of AFM

Classic Limb-Dominant AFM

Most cases present with sudden, flaccid weakness of an arm and/or leg—often asymmetric and more severe on one side. MRI shows lesions in the spinal cord’s gray matter (anterior horn cells), the same area affected in poliomyelitis. This is considered the “classic” form of AFM 3 4 9 16.

Cranial and Bulbar Forms

In some cases, AFM targets the nerves controlling facial movement, swallowing, or eye motion. Children may show facial droop, double vision, or trouble speaking and swallowing. These features suggest cranial nerve involvement and can be accompanied by limb weakness 3 14.

Respiratory Muscle Involvement

When AFM affects the neurons that control breathing, children may develop rapid-onset breathing difficulties. These cases are the most severe and often need intensive care and mechanical ventilation 1 3 4.

Peripheral Nerve Extension

Rarely, AFM pathology can extend beyond the spinal cord to involve nerve roots or plexuses, such as the brachial plexus in the shoulder region. This is detected through specialized MRI imaging and may present as severe, localized limb weakness 6.

Causes of Acute Flaccid Myelitis

Unraveling the cause of AFM has been a major challenge. While there is growing evidence linking viral infections—especially certain enteroviruses—to AFM, the exact trigger remains elusive for many cases.

Cause	Strength of Evidence	Notes on Mechanism	Source(s)
Non-polio enteroviruses (EV-D68, EV-A71)	Strong epidemiological association	Preceding illness, antibody evidence	2 3 4 7 8 9 10 14 15 16
Other viruses	Rare/possible	West Nile, coxsackieviruses	11 14
Autoimmune factors	Unproven	No consistent evidence	2 3 15 16
Unknown	Frequent in cases	Direct cause not found	2 3 13 14

Table 3: Suspected Causes of AFM

Enterovirus Association

The strongest evidence links AFM to non-polio enteroviruses, especially enterovirus D68 (EV-D68) and enterovirus A71 (EV-A71) 2 3 4 7 8 9 10 14 15 16. These viruses commonly cause mild respiratory illnesses in children, but only a tiny fraction develop AFM. Outbreaks of AFM have coincided with spikes in EV-D68 circulation, and high levels of enterovirus-specific antibodies have been found in the spinal fluid of affected children 7 8.

• Enterovirus D68 (EV-D68): Frequently detected in respiratory samples during AFM outbreaks; animal models and antibody studies strongly implicate it as a cause 7 8 12 14.
• Enterovirus A71 (EV-A71): Known to cause neurological illness in Asia and sometimes linked to AFM clusters 3 4 10 14.

Other Viral Triggers

Other viruses, including coxsackieviruses, West Nile virus, and even influenza, have occasionally been detected in children with AFM-like presentations, but these are far less common and may not be true causal agents 11 14.

Immune and Unknown Factors

Despite intensive searching, a causative virus is not identified in many cases, possibly due to delayed sample collection or viral clearance. There is currently little evidence that autoimmunity or other non-infectious mechanisms play a primary role in most AFM cases 2 3 13 14 15 16.

Pathogenesis: How Does AFM Develop?

AFM occurs when viruses or immune responses attack the anterior horn cells in the spinal cord. This leads to the sudden loss of motor function characteristic of the disease. Imaging and pathology studies confirm that the damage is focused on these motor neurons, similar to polio 3 4 6 8 9.

Treatment of Acute Flaccid Myelitis

Treating AFM is a daunting challenge—there is currently no proven, disease-specific therapy. Management focuses on supportive care, early rehabilitation, and preventing complications. Research into targeted treatments is ongoing and brings hope for the future.

Treatment	Role/Effectiveness	Notes/Recommendations	Source(s)
Supportive care	Essential, mainstay	ICU, respiratory support	2 4 13 15 16
Rehabilitation	Critical for recovery	Physical/occupational therapy	2 4 15 16
IVIG	Unproven benefit	Sometimes used early	2 13 14 15
Corticosteroids	Unproven, may be used	No clear long-term benefit	2 13 14 15
Plasmapheresis	Unproven, anecdotal use	No established efficacy	2 13 14 15
Antivirals	No established role	Research ongoing	2 12 16
Investigational antibodies	Promising in animal models	Not yet available clinically	12

Table 4: Treatment Approaches

Supportive and Critical Care

The foundation of AFM treatment is vigilant supportive care. This means closely monitoring breathing and heart function, providing respiratory support (including mechanical ventilation if needed), and managing pain and nutrition. Most children require hospitalization; more severe cases may need intensive care 2 4 13 15 16.

Rehabilitation: The Path to Recovery

Intensive physical and occupational therapy begins as soon as possible to help children regain strength and function. Early, multidisciplinary rehabilitation is essential, as many patients experience long-term weakness and disability 2 4 15 16.

Immune and Antiviral Therapies

Numerous treatments have been tried—including intravenous immunoglobulin (IVIG), corticosteroids, and plasma exchange—but none have shown consistent benefit in clinical studies 2 13 14 15. These therapies are sometimes administered in the early phase, especially if an inflammatory process is suspected, but the evidence does not support their routine use for AFM.

Investigational Therapies and Future Directions

Exciting research is underway to develop virus-specific therapies, including monoclonal antibodies that neutralize enterovirus D68. Animal studies are promising, but these treatments are not yet available for routine clinical use 12. Ongoing research and surveillance may soon offer more targeted and effective options 12 16.

Conclusion

Acute Flaccid Myelitis is a rare yet life-altering condition, mainly affecting children and causing rapid-onset paralysis. While much remains unknown, swift recognition and supportive care are key to improving outcomes. Here’s a summary of the main points covered:

• Symptoms: Sudden limb weakness, often after a mild viral illness, sometimes with pain, cranial nerve involvement, or respiratory failure 1 2 3 4.
• Types: Diverse presentations, from classic limb paralysis to cranial or respiratory muscle weakness, sometimes with peripheral nerve extension 3 4 6 14 16.
• Causes: Strongest evidence points to non-polio enteroviruses, especially EV-D68 and EV-A71, though direct viral detection is rare 2 3 4 7 8 9 10 14 15 16.
• Treatment: Supportive care and early rehabilitation are mainstays; no specific antiviral or immune therapy is proven, but research is ongoing 2 4 12 13 14 15 16.

AFM remains a medical mystery in many respects, but growing awareness, research, and clinical vigilance offer hope for improved care and future breakthroughs. If you suspect AFM, seek medical attention immediately—early intervention can make a lasting difference.