Paragraph: TABLE 40.1 Proposed Mechanisms of Virus-Induced CNS Demyelination CNS infection Infection of oligodendrocytes Direct destruction Pathogenic immune response to viral antigens on cell membranes Introduction of cell membranes into systematic circulation Infection of other CNS cells Release of cytokines or viral proteins toxic to myelin supporting cells or myelin membranes Extraneural infection Molecular mimicry (virus proteins and myelin proteins) Disruption of immune responses 954 40. INFECTIOUS DEMYELINATING DISEASES Definition ADEM is an acute, inXammatory, demyelinating disease of the brain and spinal cord. In most patients it has an abrupt onset days to several weeks after a viral exanthem or viruslike illness. But the disease is not speciWc to viruses and has been reported after several bacterial illnesses, immunizations, and drug and serum administration. The nosology is confusing, since the disease has been described under a remarkable variety of names. Post-infectious, parainfectious, post-exanthematous, post-vaccinal, postmeasles, and post-inXuenzal encephalomyelitis have been applied to describe the clinical settings. Acute disseminated encephalomyelitis, perivascular myelinoclasis, perivenous encephalitis, and acute demyelinating encephalomyelitis have been coined to describe the pathological features. Allergic encephalomyelitis, immune-mediated encephalomyelitis, hyperergic encephalomyelitis, and disseminated vasculomyelinopathy imply knowledge of pathogenetic mechanisms . Since the essential features for diagnosis are the neuropathological changes, ADEM will be used here except in speciWc cases such as post-measles and post-vaccinal encephalomyelitis. Acute hemorrhagic necrotizing leukoencephalits is generally regarded as a more intense, ''hyperacute'' form of ADEM. However, this rare acute demyelinating disease has distinct clinical and pathological features and is associated with a diVerent spectrum of antecedent infections. ADEM was a common disease in the mid-20th century, representing about one-third of all cases of encephalitis. The most common cause of ADEM was measles, which, along with ADEM cases following rubella and mumps, has been largely eliminated in regions of the world that have adequately protected children with the measles-mumps-rubella (MMR) vaccine. The second major cause of ADEM was a vaccine, vaccinia virus, which was discontinued after the worldwide eradication of natural smallpox in 1977. More recently, the varicella vaccine has further reduced the risk of ADEM. Now in countries with active childhood vaccination programs, ADEM makes up less than 10% of the cases of encephalitis, and the most common antecedent illnesses are nonspeciWc respiratory infections (Johnson, 1998) . The incidence of ADEM after clinically distinctive virus illnesses is highly variable (Tab. 40.2); the incidence after Epstein-Barr (EB) virus, Mycoplasma pneumoniae, inXuenza, and nonspeciWc upper respiratory infections are uncertain. The clinical Wndings after speciWc infections show some distinctive features, and the mortality and morbidity rates are quite diVerent. Despite the common pathology, the pathogenesis may vary. Most data on pathogenesis relate to post-measles encephalomyelitis. In acute fatal cases the brain may be congested and swollen. On gross sections, vessels are prominent in white matter with discoloration along the veins. 
 Title: Infectious Demyelinating Diseases
Output: True