- Clinical science
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. The main clinical manifestations of poliomyelitis are weakness, decreased muscle tone, and hyporeflexia. The diagnosis is established based on polymerase chain reaction (PCR) findings of poliovirus RNA in cerebrospinal fluid. Poliomyelitis has been eradicated in most parts of the world thanks to the success 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 do not regain their full strength after recovery. Some patients go on to develop post-polio syndrome, a poorly understood syndrome characterized by progressive disability and muscle weakness decades after the initial polio infection.
- Causative agent
- Transmission route
- Incubation time: 7–14 days 
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: 
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
- 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 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
- Other viral infections: Other enteroviruses may also cause flaccid paralysis.
- : Paralysis is typically symmetrical in Guillain-Barré syndrome. In addition, it would not manifest with CSF pleocytosis.
- : may also present with muscle weakness and bulbar paralysis
- : Paralysis is typically symmetrical.
The differential diagnoses listed here are not exhaustive.
- 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.
- The inactivated poliovirus vaccine (IPV) is recommended for childhood immunization in the US and most high-income countries.
- In the US, children receive 4 doses of IPV, at 2, 4, and 6–18 months; , followed by a booster dose at 4–6 years; . See “.”
- The live attenuated oral poliovirus vaccine (OPV) is used for childhood immunization in resource-limited countries because it is less costly. It has a slightly increased risk of vaccine-associated paralytic polio and is being gradually replaced in favor of the IPV worldwide.