Acute Flaccid Myelitis (AFM): Complete Guide to Symptoms, Causes & Treatment
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
Quick Answer
Acute flaccid myelitis (AFM) is a rare but serious condition that affects the spinal cord, causing sudden weakness in the arms or legs, loss of muscle tone, and loss of reflexes. It mainly affects young children, and most have had a mild respiratory illness or fever from a viral infection one to four weeks before symptoms appear. The condition might be caused by an enterovirus infection, the same family of viruses behind common childhood colds, and symptoms can resemble polio, though no U.S. cases have been caused by poliovirus. There is currently no specific treatment; care focuses on managing symptoms, and hospitalization is needed, sometimes with a ventilator. If you or your child develops any signs of AFM, seek medical care immediately, because symptoms can progress rapidly.
TL;DR
What it is: A rare but serious spinal cord condition causing sudden limb weakness, lost muscle tone, and lost reflexes, mainly in young children.
The telltale pattern: A mild respiratory illness or fever about one to four weeks before the weakness strikes.
Likely cause: An enterovirus infection, though it is not clear why some infected children develop AFM.
Not polio: Symptoms look similar to polio, but no U.S. AFM cases have been caused by poliovirus.
Emergency situation: Symptoms can progress rapidly. Hospitalization is needed, and a ventilator may be required for breathing support.
Treatment: No specific treatment exists; care manages symptoms through physical/occupational therapy and, in some cases, immunoglobulin, corticosteroids, antiviral drugs, plasma exchange, or surgery, with unclear benefit for the medicine options.
Prevention: Hygiene basics: handwashing, avoiding sick contact, cleaning surfaces, covering coughs, keeping sick children home.
Acute Flaccid Myelitis at a Glance
What it is: A rare but serious condition affecting the spinal cord.
Core symptoms: Sudden arm or leg weakness, sudden loss of muscle tone, sudden loss of reflexes.
Who it mainly affects: Young children.
Typical pattern: Mild respiratory illness or fever from a viral infection 1–4 weeks before symptoms.
Suspected cause: Enterovirus infection (the exact reason why some develop AFM is unclear).
Is it polio? No. Symptoms look similar to polio, but no U.S. cases are caused by poliovirus.
Outbreak timing: Initial clusters tracked from 2014; U.S. outbreaks in 2016 and 2018; tends to occur August–November.
Diagnosis: Medical history, physical exam, nervous system exam, MRI, lab tests (spinal fluid, respiratory fluid, blood, stool), nerve check (EMG).
Treatment: None specific; symptom management with therapy, possible immunoglobulin/steroids/antivirals/plasma exchange, sometimes surgery.
Main complication: Muscle weakness continuing for months to years.
Note: This article is for general information only and is not a substitute for professional medical advice. Always consult a qualified healthcare professional for diagnosis and treatment.
What Is Acute Flaccid Myelitis?
Acute flaccid myelitis (AFM) is a rare but serious condition that affects the spinal cord. The word "flaccid" describes what happens to the muscles: they become loose, weak, and unable to move. The condition can cause sudden weakness in the arms or legs, loss of muscle tone, and loss of reflexes, and it mainly affects young children.
Understanding the timing is essential for recognizing AFM. Most children have a mild respiratory illness or fever caused by a viral infection about one to four weeks before developing AFM symptoms. By the time the weakness appears, the original "cold" may have been forgotten, so always mention any recent illness when speaking with a doctor.
The condition is new to modern surveillance: experts began tracking AFM following initial clusters in 2014, with outbreaks in the United States occurring in 2016 and 2018. Outbreaks tend to occur between August and November.
The AFM pattern: a viral illness weeks earlier, sudden weakness when inflammation strikes the spinal cord, and the need for fast hospital care.
Symptoms: Recognizing the Warning Signs
The three most common signs appear suddenly, which is the defining feature of AFM:
Sudden arm or leg weakness: one limb, or more, suddenly cannot move normally.
Sudden loss of muscle tone: muscles become loose and flaccid instead of firm.
Sudden loss of reflexes: normal reflex responses disappear.
Beyond these hallmark signs, other symptoms may develop:
Eye and face: difficulty moving the eyes or drooping eyelids; facial droop or weakness.
Swallowing and speech: difficulty with swallowing or slurred speech.
Pain: pain in the arms, legs, neck, or back.
Uncommon signs: numbness or tingling; inability to pass urine.
Severe Symptoms
In the most serious cases, AFM causes respiratory failure, because the muscles involved in breathing become weak. It is also possible to experience life-threatening body temperature changes and blood pressure instability. These are true medical emergencies.
When to See a Doctor
Act immediately. If you or your child has any of the signs or symptoms listed above, seek medical care as soon as possible. AFM is a time-sensitive condition: symptoms can progress rapidly, hospitalization is needed, and sometimes a ventilator is required for breathing support. Waiting to see if symptoms improve on their own is not an option with this condition.
What Causes Acute Flaccid Myelitis?
AFM might be caused by an infection with a type of virus known as an enterovirus. Here is the important nuance: respiratory illnesses and fevers from enteroviruses are common, especially in children, and most people recover. It is not clear why some people with an enterovirus infection develop acute flaccid myelitis while others simply shake off the cold.
Seasonality matters too. In the United States, many viruses, including enteroviruses, circulate between August and November, which is precisely when AFM outbreaks tend to occur.
The Polio Comparison
The symptoms of AFM can look similar to those of the viral disease polio. However, none of the acute flaccid myelitis cases in the United States have been caused by poliovirus. AFM is a distinct condition with its own cause.
Risk Factors
The single main risk factor is simple: AFM mainly affects young children. Adults can be affected, but young children bear the overwhelming share of cases.
Complications
The most significant long-term complication is persistence: muscle weakness caused by AFM can continue for months to years. The weakness may be permanent in affected limbs, which is why early therapy matters so much.
Prevention: Basic Hygiene Is Your Best Defense
There is no specific way to prevent acute flaccid myelitis. However, preventing a viral infection can help reduce the risk of developing AFM. Since enteroviruses spread like common colds, standard hygiene measures apply:
Hand hygiene: wash your hands often with soap and water.
Face discipline: avoid touching your face with unwashed hands.
Distance from illness: avoid close contact with people who are sick.
Surface care: clean and disinfect frequently touched surfaces.
Cough etiquette: cover coughs and sneezes with a tissue or upper shirt sleeve.
Keep sick kids home: do not send sick children to school or group care.
A parent's quick reference: the common symptoms, the other warning signs, when to seek emergency care, the August–November outbreak window, and the prevention habits.
How Acute Flaccid Myelitis Is Diagnosed
Diagnosis begins with a thorough medical history and physical exam, followed by targeted testing. AFM is hard to diagnose because it shares many of the same symptoms as other neurological diseases, such as Guillain-Barre syndrome, so these tests exist partly to distinguish AFM from its look-alikes:
Nervous system exam: the doctor examines the places on the body where weakness, poor muscle tone, and decreased reflexes occur.
MRI: imaging that lets the doctor look at the brain and spinal cord.
Lab tests: samples of the fluid around the brain and spinal cord (cerebrospinal fluid), respiratory fluid, blood, and stool are tested in the laboratory.
Nerve check (EMG): tests how fast an electrical impulse moves through the nerves and how muscles respond to messages from the nerves.
Treatment: Managing Symptoms
This is the honest truth about AFM today: there is no specific treatment. Treatment is aimed at managing symptoms, and decisions are made by a neurologist, a doctor who specializes in treating brain and spinal cord illnesses.
Therapy First
The neurologist might recommend physical or occupational therapy to help with arm or leg weakness. This recommendation carries real weight: if physical therapy is started during the initial phase of the illness, it might improve long-term recovery, which is a strong argument for acting fast.
Medicines and Procedures
The doctor might also recommend one or more of the following, though it is important to note that it is not clear whether these treatments have any benefits:
Immunoglobulin: contains healthy antibodies from healthy donors.
Corticosteroids: drugs that lower inflammation in the body.
Antiviral drugs: target the viral infection itself.
Plasma exchange: a treatment that removes and replaces blood plasma.
Sometimes nerve and muscle transfer surgeries are done to improve limb function in children with lasting weakness.
Preparing for Your Appointment
If you have symptoms of AFM, seek immediate medical care. This is not a condition for a routine waiting-room visit.
What You Can Do
Make a list of the following before you go: signs and symptoms (including any that seem unrelated to the reason for the appointment), all medications you or your child is taking, including vitamins, herbs, and over-the-counter medicines, with their dosages, key personal information such as recent illnesses, travel, and activities, and the questions you want to ask.
Helpful questions to bring: Will additional tests be needed? What are the treatment options, and what are the benefits and risks of each? Is there one treatment you feel is best? Should additional specialists be seen, what will that cost, and will insurance cover it? Are there brochures or recommended websites?
What to Expect From Your Doctor
The doctor is likely to ask: When did you or your child first begin experiencing symptoms? Are they continuous or occasional? How severe are they? What seems to improve or worsen the symptoms? And the critical question: Did you or your child have a viral infection in the past month?
Frequently Asked Questions
What is acute flaccid myelitis?
Acute flaccid myelitis (AFM) is a rare but serious condition that affects the spinal cord. It causes sudden weakness in the arms or legs, loss of muscle tone, and loss of reflexes, and it mainly affects young children.
What causes AFM?
AFM might be caused by an infection with an enterovirus, the same family of viruses that causes common respiratory illnesses in children. Most children with enterovirus infections recover normally; it is not clear why some develop AFM.
How is AFM different from polio?
AFM symptoms can look similar to polio, but no cases of AFM in the United States have been caused by poliovirus. It is a distinct condition.
What are the main symptoms to watch for?
The three hallmark signs are sudden arm or leg weakness, sudden loss of muscle tone, and sudden loss of reflexes. Other possible signs include drooping eyelids, facial droop, trouble swallowing, slurred speech, and pain in the limbs, neck, or back.
When should I seek emergency care?
Seek medical care as soon as any symptoms appear. In severe cases the breathing muscles can weaken, causing respiratory failure that may require a ventilator. Symptoms can progress rapidly, so do not wait.
Is there a cure or specific treatment?
Currently there is no specific treatment for AFM. Care focuses on managing symptoms, often with physical or occupational therapy started early, which may improve long-term recovery. Some doctors also use immunoglobulin, corticosteroids, antiviral drugs, or plasma exchange, though their benefits are unclear.
Conclusion
Three lessons anchor this guide: the weakness arrives suddenly (one arm, one leg, a drooping eyelid, and the reflexes simply vanish), so any sudden limb weakness in a child deserves immediate attention, not a "wait and see" approach; the viral prelude matters, because a mild cold or fever one to four weeks earlier is part of the AFM story, and telling your doctor about it helps get to the diagnosis faster; and therapy timing is everything, because while no medicine is proven to work, starting physical therapy during the initial phase of the illness may improve long-term recovery, and in the worst cases the breathing muscles themselves can fail, requiring a ventilator.
This guide walked you through what AFM is, every symptom to recognize, the enterovirus link, the polio distinction, the outbreak seasons, the four diagnostic steps, all treatment options, and the six hygiene habits that reduce risk. If your child shows sudden weakness, especially after a recent viral illness, the single most important step you can take is to get medical care without delay.
Related reading: Absence Seizures (Petit Mal): Symptoms, Causes, Treatment, which covers sudden neurological symptoms in children.
This content is provided for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified healthcare provider with any questions about a medical condition.
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