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In the language of neurology, the term demyelinating diseases has acquired a special meaning. To define these diseases precisely is difficult, for the simple reason that there is probably no disease in which myelin destruction is the exclusive pathologic change. The generally accepted pathologic criteria of a demyelinating disease are (1) destruction of the myelin sheaths of nerve fibers with relative sparing of the other elements of nervous tissue, that is, of axons, neuronal cell bodies, and supporting structures, which are less affected; (2) infiltration of inflammatory cells, particularly in a perivenous distribution; (3) lesions that are primarily in white matter, either in multiple small disseminated foci or in larger foci spreading from one or more centers. In most of the demyelinating diseases, it has been known since the early descriptions that there is some degree of neuronal and axonal degeneration, but it is the preferential effect on myelin that defines this group of disorders. The most common inflammatory demyelinating disease is multiple sclerosis (MS).
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A broad classification of the inflammatory demyelinating diseases is given in Table 35-1. Like all classifications that are not based on etiology, this one has its limitations. For example, in some of the diseases here classified as demyelinating, notably necrotizing hemorrhagic leukoencephalitis and some cases of multiple sclerosis, the inflammatory process may be so intense that there is destruction of all tissue in a region including vessels and axons.
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In contrast to demyelinating disease, there are a number of other conditions in which demyelination is prominent, but not considered the defining feature and are therefore not considered part of this category. In some cases of anoxic encephalopathy, for example, the myelin sheaths of the radiating nerve fibers in the deep layers of the cerebral cortex or in ill-defined patches in the convolutional and central white matter are destroyed, while axons, for the most part, are spared. A relatively selective degeneration of myelin may also occur as a result of vascular occlusion or in larger confluent areas of ischemia, as is the case in Binswanger disease (see Chap. 33). In subacute combined degeneration (SCD) of the spinal cord and in tropical spastic paraparesis (TSP), myelin may be affected earlier and to a greater extent than axons. The same is true of progressive multifocal leukoencephalopathy (PML), osmotic demyelination (also known as central pontine myelinolysis), ...