In the case of postexanthem encephalomyelitis, the syndrome generally begins 2 to 4 days after the appearance of the rash. Usually the rash is fading and other symptoms are improving when the patient, typically a child, suddenly develops a recrudescence of fever, convulsions, stupor, and sometimes coma. Less commonly, the patient may develop hemiplegia or a virtually pure cerebellar syndrome, as noted below (particularly after chickenpox), and occasionally a transverse myelitis, sphincteric disturbance, or other signs of spinal cord involvement. Choreoathetotic movements are seen infrequently. Likewise, optic neuritis is uncommon, but it does occur.
A possibly related variant of postinfectious encephalomyelitis that involves solely or predominantly the cerebellum deserves special comment. Typically, a mild ataxia with variable corticospinal or other signs appears within days of one of the childhood exanthems as well as after Epstein-Barr virus, Mycoplasma, Legionella, and cytomegalovirus infections, and after a number of vaccinations and nondescript respiratory infections. It is described in detail in Chap. 32 because it has a close relationship to certain viruses, particularly varicella, suggesting that some, if not most, cases are caused by an infectious meningoencephalitis. Others—for example, following mycoplasmal infection—occur after a long latency and show pathologic changes that are consistent with a postinfectious demyelination. Thus it is possible that there may be 2 types of acute cerebellitis, one paraor postinfectious and the other caused by a direct infection of the brain and meninges. The benign nature of the illness has precluded adequate pathologic examination; hence some of these statements are speculative.
Not all the neurologic complications of measles and other exanthems and acute viral infections are examples of postinfectious encephalomyelitis. As already noted, the illness is at times difficult to distinguish from viral meningoencephalitis. Infectious mononucleosis, herpes simplex, mycoplasmal infection, and other forms of encephalitis may all mimic the postinfectious variety. The Reye syndrome is usually not difficult to separate from postinfectious encephalomyelitis, even when it follows chickenpox or viral influenza, because of the normal CSF and high serum concentrations of liver enzymes and ammonia (see Chap. 32).
The MRI finding of large or multifocal areas of white matter damage can also be produced by intravascular lymphoma and progressive multifocal leukoencephalopathy (Chap. 31) and a rare leukoencephalopathy that follows inhaling heroin vapor (Chap. 41).
Postvaccinal ADEM Since late in the nineteenth century, it has been known that a severe form of encephalomyelitis may complicate the injection of rabies vaccine (“neuroparalytic accident”). Until quite recently, the rabies vaccine in common use consisted of killed virus that had been grown in rabbit brain tissue. Encephalomyelitis occurred in about 1 in 750 patients inoculated with this vaccine, and approximately 25 percent of cases with this complication proved fatal. Alternative vaccines, made from embryonated duck eggs (and later from human diploid cells) infected with fixed viruses, contain very little or no nerve tissue and are almost free of neurologic complications. In developing countries, where less-expensive brain-based vaccines are still in use, neuroparalytic accidents continue to occur. The observations of Hemachudha and colleagues indicate that the altered immune mechanism that is operative in the neuroparalytic accident is the same as that in postmeasles encephalomyelitis and experimental allergic encephalomyelitis.