Poliomyelitis

Etiology

Poliovirus, the virus causing acute polio and postpolio syndrome, is a member of the Picornaviridae family and belongs to the Enterovirus C species. There are 3 serotypes of wild poliovirus: types 1, 2, and 3.[4] Wild poliovirus 1 was the primary cause of most of the world’s paralytic polio cases until vaccines became widespread. Wild types 2 and 3 are considered eradicated as of 2015.[1]

There are cases of paralytic poliomyelitis due to a loss of viral attenuation in the oral polio vaccine (OPV), known as vaccine-associated paralytic poliovirus (VAPP). VAPP is extremely rare, occurring approximately 3.8 times per million cases in countries using the oral poliovirus vaccine. VAPP is associated with serotype 3 and is most common in patients with immunodeficiency. There is a theory that circulating maternal antibodies and the timing of the first OPV dose affect the risk of developing VAPP.[5]

Another potential paralytic poliomyelitis is vaccine-derived; this is caused by mutations of the attenuated virus within the OPV that can lead to increased virulence and allow for naturally selective proliferation in populations with low herd immunity. Due to this risk, a worldwide movement has been made to increase the use of inactivated polio vaccine (IPV) and remove attenuated type 2 polio within the OPV formulation to create a bivalent OPV (containing only types 1 and 3).[6] In the United States, IPV is the only vaccine available, with the CDC recommending a 4-dose schedule (see Treatment/Management section).

Epidemiology

Before global health initiatives, nearly 1 in 200 patients with poliomyelitis developed permanent paralysis. In 1988, 350,000 cases of endemic poliomyelitis spread across 125 countries.[7] That same year, the World Health Assembly set goals for completely eradicating wild-type polio. Since then, type 1 has been the only circulating wild type and is endemic only to Pakistan and Afghanistan, and sporadic outbreaks continue to occur. In late 2021 into early 2022, wild-type polio type 1 caused paralytic polio in 9 patients in Southeastern Africa. The outbreak was related to a virus traced back to Pakistan.[8] Recent cases of type 2 polioviruses derived from oral vaccines are also increasing in sub-Saharan Africa and Asia.[1] Some barriers to complete eradication include poor program compliance and access to healthcare in remote endemic areas. Routine childhood vaccination has been successful against the spread of the disease globally, with a 99% reduction in cases from the earlier stated 350,000 in 1988 to just 33 in 2018.[7]

Pathophysiology

Poliovirus primarily spreads via fecal-oral contamination, but oral-oral spread is also possible. Primary infection can lead to viral replication in oropharyngeal and gastrointestinal lymphatic tissues. Maximum viral excretion begins 2 to 3 days before symptoms start and continues for an additional week.[9] In up to 95% of cases, infections are non-paralytic, presenting as a flu-like illness. In approximately 5% of cases, pure motor paralysis can occur.[10] The spread of the virus to the central nervous system is poorly understood. If this spread occurs, the virus may cause anterior horn neuronal death, resulting in a physical exam consistent with intact sensation and pure motor deficits.[11]

Histopathology

Enlarged motor neurons are characteristic but not pathognomonic of poliovirus. The original innervating neurons are destroyed in the muscle fibers affected by acute poliomyelitis. Neighboring neurons attempt to reinnervate the denervated tissue; this leaves the surrounding motor units innervating more muscle fibers than intended, enlarging the affected neuron, and is suspected to be part of the fatigue associated with postpolio syndrome later in life.[12]

History and Physical

During patient evaluation, a pure motor deficit (sensation intact) alongside a viral prodrome is suggestive of paralytic poliomyelitis. Seventy-five percent of infections are subclinical. Clinically, the prodrome may consist of headaches, myalgias, fatigue, nausea, neck stiffness, or pharyngitis. Up to 95% of poliovirus infections present with this constellation of symptoms and remain non-paralytic. Five percent may develop aseptic meningitis. In infections that progress into the spinal cord, severe muscle spasms with myalgias typically present before an asymmetrical flaccid paralysis. The paralysis tends to have a lower limb predominance. Acute flaccid myelitis with viral prodrome can also be caused by other viral infections (see Differential Diagnosis section). Most other enterovirus-related acute flaccid myelitis conditions have an upper extremity predilection.[11]

Poliomyelitis can present in stages, such as the acute stage, recovery stage, and residual paralysis stage. The acute stage is marked by symptoms such as fever, neck stiffness, profound muscular weakness, paraparesis, and autonomic dysfunction. In the convalescent or recovery stage, the acute features disappear, and the recovery of paralyzed muscles begins. This stage can last up to 2 years, with maximal improvement occurring in 6 months. In the last stage, the patient is left with residual paralysis, imbalance of muscle power, and poor posture. Sixty percent of patients have some form of residual deficit. 

Evaluation

Clinicians should have a high suspicion of viral meningitis when acute flaccid paralysis is present. Testing should include blood, cerebral spinal fluid, respiratory, stool viral cultures, and polymerase chain reaction to detect poliovirus or other viruses in the differential diagnosis. Electromyograms and brain/spinal cord magnetic resonance imaging should be obtained to rule out other pathologies. There are no definitive biomarkers widely accepted regarding postpolio syndrome; diagnosis is made by exclusion. There is currently early research on potential biomarkers of postpolio syndrome in the cerebral spinal fluid using polymerase chain reaction.[13]

Treatment / Management

Supportive care is the mainstay of treatment, as no approved antiviral poliomyelitis medications exist. In patients affected by paralytic polio, most (60%) do not regain full strength. The more severe the acute phase of the disease is, the greater the likelihood of residual deficits and the development of postpolio syndrome in the future.[14](B2)

Polio immunity is accomplished by administering inactivated polio vaccine (IPV) or live-attenuated oral polio vaccine (OPV, Sabin). In developing countries, the OPV is still used primarily because of cost and ease of administration. However, due to the increased possibility of VAPP with its use, IPV is preferred in most vaccination schedules. The OPV formulation is changing to remove type 2 to reduce the chance of strain mutations, as type 2 is the most common strain of VAPP. Type 2 is being phased out because its wild-type has been eradicated as of 1999, yet it continues to cause outbreaks of VAPP in countries previously completely free of polio.[5] In the United States, IPV is the only vaccine available. Children typically receive 4 doses of IPV at 2 and 4 months, then between 6 and 18 months, followed by a booster between 4 and 6 years of age. This vaccine schedule confers greater than 95% efficacy.[15] A single lifetime IPV booster is recommended for adults who have completed the primary series but may be traveling to areas with circulating viruses.[16](A1)

The release of joint contractures is also an important management step. Many surgical interventions can be considered to reestablish balance in the muscles around the joint to prevent deformities. Surgical management aims to get the patient walking and correct factors that will lead to deformity with age.

In the acute stage, treatment involves general supportive measures for fever and irritation, preventing respiratory tract infections, and managing respiratory paralysis. The paralyzed legs are supported by splints to relieve pain and spasms; splints also play an imperative role in the prevention of the development of deformities.

In the convalescent stage, the goal should be to achieve an acceptable physical status for the patients to be integrated into their communities. Nonfatiguing exercise programs are the most appropriate to deal with muscle overuse weakness. Orthoses are helpful in the conservative and definitive treatment of many deformities and are available for all body parts.

Differential Diagnosis

Other viruses can cause acute flaccid paralysis, including enterovirus A71, enterovirus D68, and coxsackievirus A.[3] Many healthcare professionals have advocated for continual surveillance of novel enteroviruses that may be associated with paralysis.[17] Other viruses that may cause similarly presenting signs and symptoms include the West Nile virus, varicella-zoster virus, Japanese encephalitis virus, and rabies virus. Syndromes, such as Guillain-Barré, should also be ruled out.[18][19] 

Noninfectious considerations include spinal cord infarctions, acquired axonal neuropathies, myasthenia gravis, Lambert-Eaton myasthenic syndrome, or rhabdomyolysis. Clinical clues such as injury to an affected limb compared to prodromal illness will help to differentiate between diagnoses.

Prognosis

The presentation of polio is variable, ranging from viral symptoms without paralysis to quadriplegia and even respiratory failure. Patients with polio who present with only viral prodromes are likely to see complete resolution of symptoms. For those who experience acute paralysis, the degree of paralysis often remains static. Approximately 30% to 40% of patients with polio will develop postpolio syndrome decades after the primary infection. This multifactorial progression depends on factors including severity of acute paralysis, age of onset, and even socioeconomic status.[14][20][21]

Complications

The most significant complications of polio infection include paralysis with bulbar involvement, fatal respiratory and cardiovascular collapse, and postpolio syndrome; postpolio syndrome is characterized by new-onset or progressive muscle weakness in a patient previously diagnosed with poliomyelitis.[14][20] There is an estimation that up to 40% of the 15 to 20 million known polio survivors worldwide develop some form of postpolio syndrome. The classic symptom is new or progressive muscle weakness decades after the acute infection. Other symptoms include myalgias, respiratory distress, joint pain, atrophy, dysphagia, and generalized fatigue.[22] The exact pathophysiology of PPS is unknown; this condition is believed to result from the overuse of remaining motor units, inflammatory changes in the central nervous system, or deteriorating neuromuscular junctions.[23] Complications of this syndrome include respiratory failure, fractures, or failure to thrive.