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It has long been the practice to set apart a group of diseases of the brain and spinal cord in which destruction of myelin, termed demyelination, is a prominent feature. 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 idea of a demyelinating disease is an abstraction that serves primarily to focus attention on one of the more striking and distinctive features of one group of pathologic processes. Another unifying feature of most of these processes is the participation of an inflammatory reaction in proximity to demyelination. The most common and important inflammatory demyelinating disease is multiple sclerosis (MS).
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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, i.e., of axons, nerve cells, 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 disorder. In the language of neurology, therefore, the term demyelination has acquired a special meaning.
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A broad classification of the inflammatory demyelinating diseases is given in Table 36-1. 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, in necrotizing hemorrhagic leukoencephalitis and even in some cases of multiple sclerosis, the inflammatory process may be sufficiently intense so much so that there is destruction of all tissue in a region including vessels and axons.
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In contrast, a number of diseases in which demyelination is a prominent feature are considered part of this category, as mentioned earlier. 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 most of the axis cylinders are spared. A relatively selective degeneration of myelin may ...