Introduction 

Do you know how poliovirus, well-known for its lifelong disability, affects our nervous system and causes paralysis?

Polio (poliomyelitis), is caused by poliovirus infection. While most people generally have mild or no symptoms, polio’s several subtypes are well-known for its major cause of mortality, acute paralysis and lifelong disabilities.^1,2 With its profound effect on the central nervous system (CNS), poliovirus infiltration particularly targets motor neurons among the brainstem and spinal cord; it intricately disrupts the delicate (neuromuscular) signaling balances between the brain and voluntary muscles. Hence, result in varying degrees of weakness, even paralysis in severe cases. In light of polio’s detrimental effect to one’s mobility and quality of life, figuring out its underlying neurological mechanisms is crucial to develop effective treatment and preventive measures to battle against its neurological aftermath. 

Overview of the nervous system 

Before diving deep into the neurological mechanisms of polio infection and paralysis, it is important to know about our nervous system a bit better. 

The nervous system is a complex network crucial for communication and coordination, comprising the CNS and peripheral nervous system (PNS). 

Central nervous system (CNS)

The CNS, as the commanding center of our body, consists of the brain and spinal cord. Works similarly as a computer, CNS is vital for information processing, initiating responses, while relaying messages to other body parts for sensory input integration, movement regulation, and governing cognitive functions like thinking, memory, and emotions.³  

Peripheral nervous system (PNS) 

The PNS, includes the cranial and spinal nerves that branch out of the brain and spinal cord, communicating CNS to the limbs and organs. The PNS is further subdivided into somatic and autonomic nervous systems. The former are connected to the skin and muscles via sensory and motor neurons for conscious movements and activities. The latter consists of nerves bridging the CNS and visceral organs, such as the heart, stomach, and intestines regulating unconscious physiological responses.⁴   

Motor neurons 

Motor neurons (MNs) are specialized nerve cells in the CNS, responsible for conveying motor commands from CNS to muscle and glands. Hence, they are vitally responsible for voluntary (skeletal) muscle contractions, as well as the activation control of movement. Motor neuron lesions lead to conditions such as motor neuron disease (MND) or polio, resulting in muscle weakness and voluntary movement impairments.⁵ 

Poliovirus transmission and spreading 

Polio transmission 

With humans as the only known reservoir, poliovirus is primarily transmitted via faeco-oral, oral-oral route, or person-to-person contact; this includes transmission through contaminated food, water, utensils, or droplets and aerosols from the throat or feces of infected individuals. While only one of the three wild poliovirus serotypes (WPV1) remains contagious (WPV2 and WPV3 have been eradicated), it can still be easily spread among areas with high population density, poor sanitation, healthcare infrastructures, and oral vaccine coverage, when individuals ingest contaminated food or water.² 

Polio disseminary routes 

Once enters the body and multiplies in the mouth, throat or gastrointestinal (GI) tract, poliovirus typically travels to various organs and tissues through three different pathways:^1,6 

Alimentary mucosa to blood

Poliovirus enters the bloodstream through the alimentary mucosa after initial oral ingestion. It breaks through the digestive tract mucosal barrier, meanwhile infecting lymph nodules and along the intestinal epithelium, and passes into the bloodstream through circulation. Hence, facilitating poliovirus spread to various organs and tissues throughout the body.⁶   

Permeation through blood-brain-barrier to brain

In certain cases, poliovirus infects the brain and spinal cord via perneration to the blood-brain barrier (BBB). As poliovirus infiltrates through the BBB, it can potentially destroy the dedicated neuronal machinery, resulting in neurological complications.⁶ 

Skeletal muscles to brain through lower motor neurons  

Poliovirus can spread to the brain through voluntary muscles and lower motor neurons (LMNs). As poliovirus invades the CNS, it travels along the afferent neuronal pathway to the CNS. Thereby, reaching the brain and spinal cord.⁶  

Mechanism of nervous system damage 

The neural mechanism of poliovirus in damaging motor neurons is a multifaceted process, involving a series of intricate interactions between virus and the host nervous system.  

Viral entry and replication

Due to the notable neurotropism of poliovirus (i.e., its ability to infect nervous system tissue), with a strong preference invading motor neurons, particularly LMNs (anterior horn cells) among the spinal cord (or brainstem) being the primary affinity target. Poliovirus binds to specific receptors on the surface of LMNs and penetrates into them. Once inside the LMNs, poliovirus hijacks into the intracellular machinery and subsequently replicates within them; resulting in a surge of viral load. This extensive polio viral multiplication then overwhelms the cellular machinery, inducing stress and destruction to the neurons.^6,7 

Neuronal damage 

When the affected LMNs are under stress, this disrupts the vital cellular processes such as protein synthesis, energy metabolism, and transport mechanisms. Intracellular functioning then becomes unable to sustain, resulting in breaking down of essential cellular components and motor neuronal dysfunction. Poliovirus continues to replicate and spread within the motor neurons, the accumulated cell damage triggers a further cascade of events that cumulate in   

lower motor neuronal degeneration or death (neuronophagia). These motor neuronal diminishes impairs LMNs abilities in transmitting motor commands (signaling) to muscles, resulting in contractility reduction or failure. Hence, weakness and paralysis among the various polio subtypes.^6,7

Immune response and inflammation 

The presence of poliovirus in the nervous system would elicit an immune response in clearing the virus from external. However, as immune cells may mistakenly attack the infected neurons in contribution to neuronal damage. This could result in further inflammation, injury, and loss in surrounding neuronal tissues. 

Axonal transport 

Without vaccinations or timely interventions, poliovirus can continuously propagate further within the CNS along the transport system of insulated tracts (axons) to distal neurons elsewhere in the CNS (e.g., posterior horn cells and dorsal root ganglia), as well as the adjacent motor neurons. This allows the spreading of poliovirus from one neuron or neuronal region to another, cultivating the increased neuronal damage extent, thereby serving various severe neurological symptoms among polio subtypes.^6,7

Development of paralysis 

The development of polio paralysis involves a complex interplay between poliovirus, the host’s immune and nervous system, resulting in a spectrum of outcome that requires timely interventions and management to minimize its long-term effect.  

Initial symptoms

Polio typically starts with a range of non-specific flu-like and intestinal symptoms, such as headache, sore throat, fever, vomiting, diarrhea, and fatigue. While they usually subsides in a few days, within a week. However, poliovirus invades the nervous system in a small percentage, leading to more serious complications like meningitis or paralysis.^1,2

Onset of paralysis

Paralytic polio happens when PV invades and attacks the CNS. It usually begins a few days to weeks after the initial non-specific flu-like symptoms. By which, weakness usually comes suddenly after one to two days and peaks at 48 hours after onset.^1,2

Types of paralysis

Paralytic polio is generally classified as acute flaccid paralysis (AFP), due to its rapid spread and specified LMNs destruction that affected neuronal regions, hence corresponding skeletal muscles no longer receive signals from the brain, resulting in asymmetric weakness and paralysis, whilst the proximal ones are more affected than distal.   

Spinal polio

As the most common paralytic polio targeting LMNs in the spinal cord, it usually begins with meningitis symptoms, following severe myalgia with localized sensory and motor deficits, such as hyperaesthesia, paraesthesia, spasms, and fasciculations, resulting in muscle weakness and paralysis, particularly among lower limbs.²

Bulbar polio

This is a serious subtype resulting from brainstem LMNs destruction, leading to weakness and paralysis of bulbar muscles, responsible for speaking, swallowing, and even breathing. Manifest as nasal speech, dysphagia, pooling of secretions, and dyspnoea.²   

Bulbospinal polio

A combination of both spinal and bulbar polio, resulting in paralysis in both the limbs and bulbar muscles. 

Factors influencing polio paralysis severity 

Polio paralysis could varies from (mild partially) to severe (completely) due to several factors: 

Viral subtype and virulence

The strain of poliovirus infected could affect the severity of paralysis. While wild PV are more virulent and harder to control than vaccine-derived PV. The former tends to cause more severe neurological symptoms. Hence more severe polio-paralysis.⁸

Immune response

Due to the vaccine-administered minimal or non-viral strains, vaccinated individuals tend to have higher alimentary tract and intestinal resistance, plus antibodies in the bloodstream. Thereby, a more robust immune response and limited paralytic extent.⁷ 

Age 

Children under five are more susceptible to more severe polio paralysis, probably due to their not-yet-mature immune system.  

Nutritional status 

Malnourished populations may be more vulnerable to severe polio complications. 

Summary 

Polio occurs when poliovirus attacks the nervous system by entering, replicating within and destructing the motor neurons. As motor neuronal intracellular functioning is destroyed, signal transmissions from the brain to skeletal muscles are disrupted, resulting in muscular weakness and paralysis of affected neuronal regions. While poliovirus-induced inflammation and the immune system could further exacerbate neuronal damages in severe cases. As motor neurons typically do not regenerate, this could result in more serious irreversible neurological complications and paralysis. Despite being partially eradicated, effective vaccines and public health measures are vital to prevent poliovirus spread and infection, thus reducing the potential adverse effect on mobility and quality of life among the infected population.    

FAQs 

How is polio paralysis related to post-polio syndrome?

Polio paralysis and post-polio syndrome are two interrelated conditions affecting individuals with histories of poliovirus infection, typically due to residual effect and neuromuscular strain. 

While polio paralysis are the immediate muscle weakness or paralysis resulting from acute polio infection, post-polio syndrome (PPS) often develop years to decades after initial polio-infection with new symptoms emerging.    

What are the typical rehabilitation management methods for polio paralysis and post-polio? 

Tailored strategies such as non-fatigue exercises, cardiorespiratory endurance training physiotherapy, mobility aids, occupational therapy, speech therapy and potentially respiratory therapy.