Acute flaccid myelitis: How it is and is not like polio

Introduction

Acute flaccid myelitis and poliomyelitis (polio) share several clinical and epidemiological characteristics, prompting comparisons between these two neurologic conditions. Both diseases primarily affect the spinal cord, leading to muscle weakness and paralysis. However, despite these similarities, there are also significant differences in their etiology, epidemiology, and long-term outcomes. This entry explores the parallels and distinctions between acute flaccid myelitis and polio, shedding light on their implications for public health.

Clinical and pathological similarities

1. Neurologic presentation:
• Both acute flaccid myelitis and polio are characterized by acute flaccid paralysis, which is the sudden onset of muscle weakness or paralysis, typically affecting the limbs. In both conditions, the weakness is often asymmetrical and can progress rapidly.
• The paralysis in both diseases results from the destruction of motor neurons in the spinal cord. In polio, the poliovirus specifically targets the anterior horn cells, whereas in acute flaccid myelitis, inflammation and damage to these motor neurons occur, often associated with enterovirus D68 (EV-D68) and other viral infections.
2. Spinal cord involvement:
• Pathological examination of both diseases reveals similar findings, including inflammation of the gray matter in the spinal cord (myelitis) and damage to the anterior horn cells, which are crucial for motor function. This damage leads to the flaccid paralysis observed in both conditions.

Epidemiological parallels

1. Seasonal patterns:
• Both acute flaccid myelitis and polio show seasonal patterns in their incidence, with cases typically peaking during the late summer and early fall. This seasonal distribution is linked to the increased circulation of enteroviruses during these times, which are associated with both acute flaccid myelitis and polio.
2. Affecting primarily children:
• Acute flaccid myelitis and polio predominantly affect children. In polio, the most severe cases of paralysis often occur in young children, leading to the historical term "infantile paralysis." Similarly, acute flaccid myelitis primarily affects children, with the average age of affected individuals being around 5 to 7 years.

Key differences between acute flaccid myelitis and polio

1. Causative agents:
• Polio is caused by the poliovirus, which is a well-defined enterovirus with three serotypes (types 1, 2, and 3). Vaccination campaigns have been highly effective in reducing and nearly eradicating polio globally.
• Acute flaccid myelitis, on the other hand, is associated with several different viruses, particularly non-polio enteroviruses such as EV-D68 and EV-A71. Unlike polio, acute flaccid myelitis's precise causative agent is not always identified, and there is no specific vaccine or antiviral treatment available.
2. Global impact and eradication efforts:
• Poliomyelitis was once a major global health issue, causing widespread paralysis and death. The development of the inactivated polio vaccine and oral polio vaccine has been one of the greatest successes in public health, leading to near global eradication of the disease. There is no cure for polio, but vaccination is highly effective in preventing it, and polio is now endemic in only a few regions worldwide.
• Acute flaccid myelitis, while concerning, does not yet have the same global impact as polio did at its peak. It is relatively rare compared to polio, with outbreaks occurring sporadically and at a much lower frequency. However, the recurring outbreaks of acute flaccid myelitis, particularly since 2014, have raised public health concerns, especially because no preventive vaccine or specific treatment is available.
3. Long-term outcomes:
• Although both conditions can lead to long-term paralysis and disability, the outcomes in acute flaccid myelitis appear to be variable, with some children experiencing partial recovery while others have persistent deficits. Paralysis in acute flaccid myelitis can be severe, but fewer patients experience complete paralysis of large muscle groups compared to polio, and there is no known equivalent of post-polio syndrome in patients with acute flaccid myelitis. However, recovery from acute flaccid myelitis paralysis is often incomplete, and some children are left with significant long-term disabilities, requiring extensive rehabilitation.
• In contrast, polio often led to more predictable patterns of permanent paralysis in those severely affected. Historically, during polio outbreaks, a significant number of infected individuals developed permanent disabilities, including paralysis of the legs or even respiratory muscles, requiring life-long mechanical ventilation. Death can occur when respiratory muscles are paralyzed. Post-polio syndrome can also occur in those affected, leading to worsening muscle weakness, pain, and fatigue after decades of stability.
4. Which is more devastating?
• Polio is generally considered more devastating than acute flaccid myelitis due to its historical impact, higher rates of permanent paralysis, the potential for widespread outbreaks, and the lasting effects, such as post-polio syndrome. Polio caused large-scale epidemics with significant mortality and disability rates before the vaccine was introduced.
• Acute flaccid myelitis, although serious, is much less common than polio used to be. Though individual cases can be severe, with some patients experiencing long-term paralysis, acute flaccid myelitis’s impact is much smaller both on affected patients as well as the general public.

Public health implications

The parallels between acute flaccid myelitis and polio highlight the importance of ongoing surveillance, research, and public health preparedness. The experience with polio eradication offers valuable lessons for addressing acute flaccid myelitis, particularly in understanding the role of viral infections in causing neurologic damage and the importance of vaccine development.

Conclusion

Although acute flaccid myelitis and polio share several clinical and epidemiologic similarities, they also have significant differences that affect how they are managed and understood. Both conditions underscore the critical need for vigilance in public health, particularly in monitoring and responding to viral threats that can lead to neurologic damage. As research continues, the lessons learned from polio may guide efforts to combat acute flaccid myelitis and prevent its potentially devastating effects.

Related MedLink Neurology content:

• Podcasts: BrainWaves #38 Acute flaccid myelitis

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