Epidemiology

Poliomyelitis, an infection caused by a group of enteroviruses, leads to significant striated muscle paralysis in less than 5 percent of infected patients. 23 Prior to mass immunization, virtually all children were exposed to and developed antibodies to this enterovirus infection by age 4 years. A 1952 polio epidemic caused 14 new cases of paralytic polio per 100,000 persons, a rate similar to that seen in developing countries that are unable to provide mass immunization. Polio rates as high as 600 cases per 100,000 were seen as recently as the early 1980s in India. Mass immunization with the inactivated poliovirus vaccine (IPV) or attenuated oral poliovirus (OPV) has dramatically reduced the incidence of polio, which was 0.003 cases per 100,000 persons in the United States in 1981. Polio outbreaks still occur in populations that are not consistently immunized, such as was seen in the 1979 U.S. Amish epidemic, which was caused by the wild poliovirus type 1. Outbreaks also have been seen in association with inadequate immunization with modified poliovirus type 3, as was seen in the Finland outbreak in 1984-1985. With the use of OPV, some immunized children will develop polio, as will some young adults who are exposed to children who have been vaccinated with the OPV vaccine. Immunocompromised patients are at greater risk for contracting polio after exposure to children who were vaccinated with the OPV vaccine. In developing countries, recent IM injections, tonsillectomy, and strenuous exercise all have been associated with increased polio infection severity.

The prognosis for most enteroviral CNS infections is favorable with the exception of paralytic poliomyelitis and those enteroviral infections which occur during the first year of life.

The postpolio syndrome (PPMA) is expected to afflict up to 100,000 of the 250,000 U.S. adults with a prior history of polio. Since the majority of these polio cases occurred prior to mass immunization, these patients most likely will be older than age 50. There in no gender predisposition for PPMA. Although PPMA most often occurs after a stable, disease-free period of 20 to 30 years, there are risk factors that predict an earlier onset of postpolio syndrome. These include greater age at the time of initial polio infection, greater residual motor disability, residual bulbar or respiratory signs, and the occurrence of recent injuries that require limb immobilization.

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