Our own experience with this form of poliomyelitis has consisted of several patients who were referred over the years for paralyzing illnesses initially thought to be Guillain-Barré syndrome (Gorson and Ropper). In each case, the illness began with fever and aseptic meningitis (50 to 150 lymphocytes/mm3 in the CSF), followed by backache and widespread, relatively symmetrical paralysis, including the oropharyngeal muscles in two cases and asymmetrical weakness limited to the arms in two patients. There were no sensory changes. One patient had a mild concurrent encephalitic illness and died months later. The evolving electromyographic changes indicated that the paralysis was caused by a loss of anterior horn cells rather than by a motor neuropathy or a purely motor radiculopathy, but this distinction was not always certain. MRI was remarkable in showing distinct changes in the gray matter of the cord, mainly ventrally (Fig. 32-3). No virus could be isolated from the CSF and serologic tests in our patients failed to implicate any of the usual encephalitic RNA viruses, including poliovirus. The patients had been immunized against the poliomyelitis viruses.
The concept that viral infections may lead to chronic disease, especially of the nervous system, had been entertained since the 1920s, but only many decades later was it firmly established. This group is identified by a long latency between an initial infection and the reemergence of the virus to cause clinical effects and a slow evolution once symptoms begin. Indirect and direct evidence supported this view: (1) the demonstration of a slowly progressive noninflammatory degeneration of nigral neurons long after an attack of encephalitis lethargica; (2) the finding of inclusion bodies in cases of subacute and chronic sclerosing encephalitis; (3) the discovery of chronic neurologic disease in sheep caused by a conventional RNA virus (visna)—it was in relation to this disease in sheep that Sigurdsson and Rida first used the term slow virus infection to describe long incubation periods during which the animals appeared well; and (4) the demonstration by electron microscopy of viral particles in the lesions of progressive multifocal leukoencephalopathy and, later, isolation of virus from the lesions. The suggestion that the late onset of progressive weakness after poliomyelitis (“postpolio syndrome”) might represent a slow infection has never been verified. Claims have also been made numerous times over the years for a viral causation of multiple sclerosis, amyotrophic lateral sclerosis, and other degenerative diseases, but the evidence in all instances has been questionable.
The established human slow infections of the nervous system caused by conventional viruses include subacute sclerosing panencephalitis (measles virus), progressive rubella panencephalitis, and progressive multifocal leukoencephalopathy (JC virus). These diseases, except for PML, are decidedly rare. They are caused by viruses and are not to be confused with a group of subacute and chronic neurologic diseases that are instead the result of prions, entirely distinct transmissible agents. These are accorded a separate section later in this chapter.