Viral infections of the fetus, notably rubella, CMV, HIV, herpes zoster, Epstein-Barr, and HSV infection of the newborn are important causes of CNS abnormalities. This subject is covered in detail in “Intrauterine and Neonatal Infections” in Chap. 37.
In the past, this syndrome was almost invariably the result of infection of anterior horn cells by one of the three types of poliovirus (“polio” referring to grey matter). However, illnesses that clinically resemble poliovirus infections can be caused by other enteroviruses such as the Coxsackie groups A and B and Japanese encephalitis, as well as by West Nile virus. Epidemics of hemorrhagic conjunctivitis (caused by enterovirus 70 and formerly common in Asia and Africa) and probably enterovirus 68 can also be associated with a lower motor neuron paralysis resembling poliomyelitis (Wadia et al). In countries with successful poliomyelitis vaccination programs, these other viruses are now the most common causes of the anterior poliomyelitis syndrome. In some cases, the illnesses induced by these viruses are benign and the associated paralysis is insignificant. West Nile virus is an exception that has been associated with a severe and persistent asymmetrical flaccid poliomyelitis.
The important paralytic disease in this category nonetheless remains poliomyelitis. Although no longer a scourge in areas where vaccination is routine, its lethal and crippling effects are still fresh in the memory of physicians who practiced in the 1950s. In the summer of 1955, when New England experienced its last epidemic, 3,950 cases of acute poliomyelitis were reported in Massachusetts alone, and 2,771 were paralytic. The details of this epidemic described by Pope and colleagues are worth reviewing by any student of the disease. Polio has essentially vanished from the Americas, the only cases currently being imported ones. Live polio vaccine, which had been the source of approximately 150 cases in previous decades, is no longer used in the United States.
Of course, because it is highly communicable, acute poliomyelitis still occurs in regions of the world where large-scale and recurrent vaccination is not yet practical. In a recent year, there were fewer than 2,000 cases in the world but there are periodic small outbreaks. For these reasons and because it stands as a prototype of a neurotropic viral infection, the main features of the disease should be known to neurologists.
The paralytic residua of previous epidemics can still be seen. In these cases, a delayed progression of muscle weakness may appear many years after the acute paralytic illness—a condition termed postpolio syndrome.
The poliomyelitis agent is a small RNA virus that is a member of the enterovirus group of the picornavirus family. Three antigenically distinct types have been defined and infection with one does not protect against the others. Poliomyelitis is a highly communicable disease. The main reservoir of infection is the human intestinal tract (humans are the only known natural hosts), and the main route of infection is fecal-oral, that is, hand to mouth, as with other enteric pathogens. The virus multiplies in the pharynx and intestinal tract. During the incubation period, which is from 1 to 3 weeks, the virus can be recovered from both of these sites. In only a small fraction of infected patients is the nervous system invaded. Between 95 and 99 percent of infected patients are asymptomatic or experience only a nonspecific febrile or meningitic illness. It is the latter type of patient—the carrier with inapparent infection—who is most important in the spread of the virus from one person to another.