Infectious Demyelinating Diseases

This chapter focuses on three human CNS demyelinating diseases—namely, acute disseminated encephalomyelitis (ADEM), also known as postinfectious encephalomyelitis, progressive multifocal leukoencephalopathy (PML), and multiple sclerosis (MS). These three diseases have very different clinical courses and distinctive pathological features, although all share the essential element of demyelination. ADEM and PML have antithetic modes of pathogenesis. ADEM is a predominantly extraneural infection resulting in a virus-induced host autoimmune response, whereas PML is a direct lytic infection of oligodendrocytes in an immunocompromised host. It is clear that a diverse array of viruses can infect the human central nervous system. The viral infections can result in a wide variety of clinical and pathological symptoms. While some viruses may cause widespread inflammation and neurodegeneration, other viruses may remain latent in the CNS and only produce pathological changes during reactivation. Viral induced demyelination can occur both from direct infection of the myelin-producing oligodendrocytes, as in PML, or by indirect mechanisms that have yet to be determined. The outcome of a viral CNS infection depends not only on viral characteristics, but also on the interaction between virus and host cell.