Poliomyelitis

Poliomyelitis, or polio, is a disease caused by poliovirus, a type of human Enterovirus. Poliovirus spreads via fecal-oral transmission. Although the majority of poliovirus infections are asymptomatic, some may invade the central nervous system, leading to motor neuron death and irreversible paralysis that can cause respiratory insufficiency and death. Symptomatic poliomyelitis most commonly manifests as a flu-like illness. In rare cases, patients develop paralytic poliomyelitis, which manifests with weakness, decreased muscle tone, and hyporeflexia. The diagnosis is established based on polymerase chain reaction (PCR) findings of poliovirus RNA in body fluid samples (e.g., stool, oropharyngeal swab, cerebrospinal fluid). Poliomyelitis has been eradicated in most parts of the world as a result of global vaccination initiatives. The treatment of poliomyelitis is mainly supportive. Patients with brainstem or respiratory muscle involvement may require intubation and mechanical ventilation. The majority of patients with paralytic poliomyelitis do not regain their full strength after recovery. Some patients go on to develop post-polio syndrome, which is characterized by progressive disability and muscle weakness decades after the initial infection.

• As a result of widespread vaccination measures, poliomyelitis has largely been eradicated in most countries worldwide.
• In 2022, paralytic poliomyelitis due to vaccine-derived poliovirus was confirmed in an unvaccinated individual in New York. ^[1]
• Poliovirus is still endemic in Afghanistan and Pakistan. ^[2]

Epidemiological data refers to the US, unless otherwise specified.

Causative agent

• Poliovirus is an RNA virus in the family Picornaviridae, genus Enterovirus.
• Wild poliovirus type 1 causes most paralytic manifestations of poliomyelitis. ^[2]
• Vaccine-derived poliovirus type 2 (VDPV2): oral polio vaccine (OPV) replicates in a recently vaccinated person → viral reversion to neurovirulence → poliomyelitis in unvaccinated or immunocompromised individuals exposed to the recently vaccinated person ^[1]
• Humans are the only hosts.

Route of transmission ^[2]

• Fecal-oral: absorption of poliovirus in the intestinal tract
• Droplet (rare)

Incubation time

• 7–14 days ^[3]

Risk factors for poliovirus infection ^[4]

• Traveling to endemic regions
• Working with polioviruses specimens (e.g., laboratory workers, researchers)
• Caring for individuals likely to be infected with poliovirus (e.g., healthcare personnel, refugee camp workers)
• Underimmunized and/or immunosuppressed individuals ^[5]
  • Working or living in endemic regions
  • In close contact with international children receiving the oral poliovirus vaccine (e.g., international adoptees, refugees)

• The virus replicates in the gastrointestinal tract (oropharynx and small intestine) following oral ingestion → enters the bloodstream → potential invasion of the gray matter of the spinal cord (particularly the lower motor neurons of the anterior horn) → myelitis

Over 72% of infections with poliovirus are asymptomatic. Clinical infection (mostly flu-like symptoms) is observed in less than 24% of cases and less than 1% of infected individuals develop paralysis. The clinical forms of poliomyelitis are: ^[3][6]

Poliomyelitis without CNS involvement (abortive poliomyelitis)

• Nonspecific symptoms: gastroenteritis, fever, nausea, sore throat, myalgia, and headaches for 1–3 days
• Complete recovery without complications or transition to poliomyelitis with CNS involvement

Poliomyelitis with CNS involvement

Nonparalytic poliomyelitis: aseptic meningitic form

• Begins several days following abortive poliomyelitis (often temporary, symptom-free interval)
• Fever, neck stiffness, headache, vomiting, muscle pain
• Neck muscle weakness (head drop sign: head falls back when placed in a supine position)
• No paresis

Paralytic poliomyelitis

• Occurs 2–3 days following the meningitic form after a brief symptom-free interval
• Fever, malaise, headache, nausea
• Severe back, neck, and muscle pain
• Asymmetric acute flaccid paralysis worsens over hours to days.
  • Most commonly affects the leg muscles, although the arms, abdomen, trunk, thorax, and eyes may be affected
  • Paralysis is usually more severe in proximal muscles than in distal muscles.
  • Ascending paralysis with diaphragmatic involvement → respiratory failure
  • Bulbar form with brain stem involvement (rare): damage to the cerebral or autonomic nerve centers (cranial nerves and respiratory center) → central respiratory paralysis
• Diminished deep tendon reflexes
• Muscle atrophy
• Hypotonia
• Fasciculations

• Post-polio syndrome (PPS) ^[7]
  • Most frequent complication observed following poliovirus infection (up to 40% of survivors)
  • Occurs decades after infection
  • Manifests with progressive muscle weakness and pain, even in areas that were not affected by the initial infection

• Consider poliomyelitis in unvaccinated patients with nonspecific viral symptoms who: ^[8]
  • Live in areas with known community spread
  • Have recently traveled to a region where polio has not been eradicated or OPV is still utilized
• Initial test: PCR of stool or oropharyngeal samples to test for enterovirus ^[9]
• Confirmatory test: PCR of CSF, stool, or oropharyngeal samples to test for poliovirus RNA ^[8]
• CSF will show:
  • High protein levels
  • Pleocytosis with either neutrophils (early infection) or lymphocytes (late infection)
  • Normal glucose levels

• Other viral infections: Other enteroviruses may also cause flaccid paralysis.
• Guillain-Barré syndrome: Paralysis is typically symmetrical in Guillain-Barré syndrome. In addition, it would not manifest with CSF pleocytosis.
• Acute intermittent porphyria: may also present with muscle weakness and bulbar paralysis
• Spinal muscular atrophy: Paralysis is typically symmetrical.

The differential diagnoses listed here are not exhaustive.

• Pain relief
• Airway management and mechanical ventilation if needed (e.g., signs of airway compromise, respiratory failure)
• Close monitoring of blood pressure and respiratory function

• The prognosis is dependent on the form of poliomyelitis.
• In the abortive and nonparalytic forms, complete recovery without complications occurs within 2 weeks.
• In the paralytic form, the most serious short-term risk is death due to diaphragmatic involvement. After recovery, most patients have residual deficits, including muscle weakness or contractures and muscle pain.
• Post-polio syndrome may lead to muscle weakness and pain decades after polio infection.

Vaccination ^{[10][11][12][13]}

• Available vaccines
  • Inactivated poliovirus vaccine; (IPV)
    • Subcutaneous or intramuscular injection
    • Can be administered individually, with other vaccines, or as a combination vaccine.
  • Live-attenuated oral polio vaccine (OPV): discontinued in the US, but may be used in resource-limited countries ^[14]
• Immunization schedule ^[14]
  • IPV is recommended for all individuals; see the ACIP immunization schedule for details.
  • Individuals with risk factors for poliovirus infection may require additional doses of vaccination. For more details, see: ^[4][5][12]
    • “Immunization schedule by medical indication”
    • “Vaccines before travel”