The previous three chapters discussed motor neuron diseases (MNDs) that are inherited or degenerative in etiology. This chapter will focus on the less common (of this era), largely acquired motor neuron syndromes including the acute and delayed effects of poliomyelitis and other neurotropic viral infections. In addition, other less common causes of lower motor neuron (LMN) disease such as the potential association with malignancy and radiation exposure will be discussed.
POLIOVIRUS AND OTHER INFECTIOUS CAUSES OF MOTOR NEURON DISEASE
Technically, poliomyelitis implies inflammation of spinal cord grey matter regardless of cause resulting in a phenotype of acute flaccid paralysis (AFP). In this chapter, in order to avoid confusion, we will refer to poliomyelitis as the myelopathy associated with infection with the three strains of the poliovirus, distinguishing it from other infectious myelopathies that may also be dominated by LMN weakness. Other infectious myelopathies or myeloradiculopathies with signs and symptoms of notable sensory or long-tract involvement will not be addressed.
Poliomyelitis dates to antiquity. Endemic polio continues to occur in countries where vaccination programs and public health measures are suboptimal. In these cultures, individuals are likely to be exposed early in life. Cases occur more randomly than in epidemic disease and paralytic disease tends to be less severe in this population typically affected at an earlier age. Epidemic polio is a disorder of considerable historical interest. Epidemics tended to occur in the summer and early fall when people were more likely to be in contact with common water sources and each other. Individuals were typically exposed at an older age frequently resulting in more severe disease.
The Salk vaccine, a killed injectable product, became available in 1955. In the early 1960s, the live, attenuated oral vaccine (Sabin) was introduced providing two notable advantages, ease of delivery and long-term immunity. The disadvantage, however, was the potential for infection in vaccinated individuals, or those coming in contact with the vaccinated individuals who were themselves inadequately immunized. People in the latter category most at risk were those who had emigrated from countries without adequate vaccination programs, those whose religious or cultural beliefs opposed vaccination, or those whose immunity had lapsed from the exclusive use of the Salk vaccine. Since 2000, no cases of AFP from the polio virus have been reported in the United States coincident with withdrawal of Sabin vaccine usage.1
Although poliomyelitis is a disease of largely historical interest in the United States as a public health menace, it continues to have relevance on a number of levels. It is one of the earliest and best models of selective neuronal vulnerability from environmental cause. The development and distribution of effective polio vaccines represents one of the most notable triumphs of translational medicine in the 20th century. The relevance of poliomyelitis persists as well in that survivors of the paralytic polio epidemics of the late 1940s ...