Although sporadic cases of non-polio infantile paralysis have been reported in the medical literature in the past, it was not until 2012 in California and 2014 in Colorado when unusual but significant clusters of pediatric patients with a polio-like illness started rising quickly in the USA. This prompted the CDC to begin collecting surveillance data on the disease we now call AFM. Between August 2014 and June 2020, there have been 626 confirmed cases across the USA. AFM cases have also been reported in multiple regions worldwide including Canada, Europe, Asia, Australia, and South America.
Thanks to the epidemiological surveillance efforts, it was noted that this disease usually follows a particular seasonal biennial pattern, with notable peaks of cases between August and October every 2 years (2014, 2016, 2018, and maybe 2020!).
Over 90% of cases of AFM occur in children (median age of 6 years), and no racial or ethnic predispositions have been clearly identified. A slight predisposition to males has been reported.
We are still learning about what causes this unusual illness, and how is it causing the paralysis.
There is evidence that supports a connection between the recent outbreaks of AFM with non-polio enterovirus infections as potential causative agents. The most commonly associated virus is enterovirus D68, but other enteroviruses like enterovirus A71, and some coxsackie virus strains have also been reported.
First, individuals affected by AFM usually experience a febrile respiratory illness a few days prior to the onset of weakness. Enteroviruses are known for typically causing a respiratory infection. Second, there seems to be a temporal and geographic correlation between enterovirus D68 circulation and the outbreaks of AFM. Third, enterovirus D68 is the most commonly identified pathogen in respiratory or stool specimens in patients with AFM across many regions, and it is hypothesized that new strains of the virus may have acquired the ability to cause the flaccid paralysis.
Although this reinforces the idea of a virus as a driving agent of the disease, the mechanism by which the virus might cause injury to the spinal cord is not well understood. Direct neuronal infection and indirect immunological response have been proposed as potential explanations.
Finally, in many patients with AFM, no viruses are detected in routine samples. This could be explained by shortcomings of the tests themselves, but also by the possibility of other viruses that we still have not identified.
Signs & Symptoms
Most of the individuals affected by AFM will have a fever and/or respiratory symptoms (cough, runny nose, sore throat) around 3 days to 1 week before the onset of muscle weakness. Gastrointestinal symptoms like diarrhea and vomiting have also been reported but are less common. In many cases, other members of the household are also affected by febrile illness.
Then, when most patients are starting to improve from their febrile illness, neck stiffness, headache, vomiting, back pain, and pain in the limb that will be compromised might develop.
Shortly afterward, there is a rapid onset of flaccid weakness that progresses over hours. The pattern and extent of the paralysis are widely variable between patients. It usually begins in one limb and then spreads, without a particular order, although there seems to be a predilection for upper extremities. It can affect from just one to all four limbs, as well as facial, oropharyngeal, neck, and breathing muscles. It means that it can affect the movement of the limbs as well as the ability to breathe, swallow, or move their eyes normally. Other possible symptoms include autonomic instability, meaning problems with the heart rate and blood pressure, and bladder/ bowel dysfunction.
Finally, but uncommon, sensory problems (e.g. nerve pain) in the affected limbs, neck, or back can also be present.
A good clinical examination, an MRI of the spinal cord, and the analysis of cerebrospinal fluid are essential tests that help diagnose AFM and differentiate it from other conditions that can cause a similar presentation.
A complete and thorough neurological exam is essential for diagnosis. Exam findings suggestive of AFM include weakness involving one or more limbs, decreased muscle tone, and decreased reflexes in the affected limbs.
Regarding MRI findings, lesions on the grey matter of the spinal cord, usually involving several vertebral levels, are the characteristic abnormality in AFM. The cervical cord (the upper part) is the most commonly affected section. It is important to remember that very early in the acute setting, MRI abnormalities might be very faint or ill-defined and could be misinterpreted. Also, different from other neurological illnesses, lesions in the upper part of the brain are very rare.
A lumbar puncture should also be completed in suspected cases. It usually reveals a moderate increase in white blood cells and mild protein elevation. Viral studies and cultures are generally negative but that does not exclude the diagnosis. CSF analysis can be helpful during the acute phase in differentiating AFM from other causes of flaccid paralysis less likely to produce an inflammatory profile, especially spinal cord infarction or Guillain Barre Syndrome.
Studies of the nerves and muscle conductions (called nerve conduction and electromyography [NCS/EMG]) can also be helpful in differentiating AFM from neuromuscular disorders, like botulism and Guillain Barre.
Finally, respiratory samples should be collected and sent for viral testing. The collection of other samples, like stool, can also help determine the presence of a viral causative agent.
Close monitoring during the initial days of the illness is essential, as children may develop severe respiratory disease, as well as neurological compromise of respiratory muscles, heart rate, and blood pressure regulation. Because of this, an important number of affected individuals will need to be admitted to the intensive care unit to receive cardiac, respiratory, bladder, and bowel support.
Unfortunately, no medical therapies have proven efficacy in AFM. Some treatments generally used are intravenous immunoglobulin (IVIG), IV steroids, and plasma exchange (PLEX), as they are commonly used for inflammatory disorders of the spinal cord. The purpose of these treatments is to attempt to reduce inflammation in the spinal cord and further prevent the individual’s immune system from causing damage. IVIG products have demonstrated to contain neutralizing factors against the enterovirus D68, and its more frequently used in the setting of AFM. The use of steroids and PLEX is controversial, as they might have a negative impact on the immune system’s ability to fight viral infections.
Early initiation of physical, occupational, and speech therapy services in the intensive care units is essential. Additionally, engaging family members in the process of recovery since the acute setting is also very important.
AFM seems to be a monophasic disorder, but the grade of recovery is extremely variable. Unfortunately, only around 10% of patients recover completely. The most affected muscle may be the least likely to recover, especially proximal limb muscles. Residual weakness is followed by ongoing muscle atrophy and functional impairment of the extremities. Other muscles, like facial and respiratory muscles, appear to recover in a greater proportion, however, some affected individuals might develop the need for chronic ventilatory and feeding support.
Other long term complications in AFM include neuropathic pain, chronic constipation, joint and bone problems like scoliosis, bone mass loss, joint contractures, as well as psychological and social effects. Multimodal rehabilitation is key in promoting continued recovery.
Rehabilitation & Recovery
Appropriate rehabilitation is the best treatment for AFM.
Physical, occupational and speech therapy should be started as early as possible, even during the intensive care unit, to begin the process of restoration of functionality and prevention of secondary complications that come with immobility. It is important to remember that support is not only physical, but also psychological, and both the child and the family will need all the support available as early as possible to assist in this adjustment.
Inpatient and outpatient rehabilitation
After the child is medically stable, the main goal is to transition to an inpatient program and eventually an outpatient rehabilitation program. It has been shown that significant improvements can be achieved with a structured, multidisciplinary, and individualized rehabilitation program, even when initiated late after limb paralysis. Short term goals should focus on facilitating functional independence according to their age, as well as the use of compensatory devices, and long term goals directed towards the prevention of musculoskeletal complications.
Activity-based restorative therapy (ABRT), that should be tailored to each individual, includes weight-bearing exercise, functional electrical stimulation (FES), locomotor gait training, task-specific practice, and massed practice with high count repetitions. Activity-based therapy can also be applied in the aquatic setting.
Additional rehabilitation interventions to consider are pulmonary management for those with ventilator dependence, otolaryngology care, and speech and language pathologists for those children with difficulty swallowing (dysphagia) and talking (dysphonia).
The rehabilitation plan should also include evaluation for orthotic devices, adaptive equipment, identification of home and school needs, a plan to facilitate community reentry, psychosocial support, and training for the family as well.
Because AFM mainly affects growing children, continued rehabilitation with periodic rounds of activity-based therapy and ongoing specialty care including neurology and physiatry is key. If you are interested in learning more about rehabilitation interventions, please visit our Resource Library.
Nerve and tendon transfers
Children with a poor recovery in an affected muscle group or diaphragm can be considered for potential nerve transfer surgery.
The idea is that healthy nerves are taken from one area of the body and then transferred to a denervated nerve. This procedure is derived from previous experiences from other causes of nerve injuries (for example, obstetric brachial plexus injury). We currently have anecdotal data on a series of individuals with AFM, with generally positive results, mainly with upper extremity surgery.
In general, for nerve transfer to be successful, it must occur before irreversible degeneration of the nerve-muscle connection. However, in practice, the correct timing for when surgical intervention should occur remains controversial and every case should be individualized. On the other hand, tendon transfer surgery is not time-sensitive and can be considered even years after the onset of AFM.
In a subset of for patients with involvement of their respiratory muscles, additional strategies like nerve transfer to the phrenic nerve and diaphragmatic pacing have also been explored, but there is not enough data for a validated demonstration of its efficacy.