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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 (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.

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The commonly 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, as reflected by a relative lack of wallerian or secondary degeneration of fiber tracts; (2) infiltration of inflammatory cells in a perivascular and particularly paravenous distribution; (3) a distribution of lesions that is primarily in white matter, either in multiple small disseminated foci or in larger foci spreading from one or more centers.

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Table 36-1 presents a classification of the demyelinating diseases. The diseases included in this classification conform approximately to the criteria enumerated above. Like all classifications that are not based on etiology, this one has its shortcomings in that it is somewhat arbitrary. In some of the diseases here classified as demyelinating, notably, in necrotizing hemorrhagic leukoencephalitis and even in multiple sclerosis, there can be a severe degree of damage to axis cylinders and vascular structures as well as to myelin. In contrast, a number of diseases in which demyelination is a prominent feature are not included. 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 also occur in some small ischemic foci as a result of vascular occlusion or in larger confluent areas, as is the case in Binswanger disease (see Chap. 34). In subacute combined degeneration (SCD) of the spinal cord associated with pernicious anemia and in tropical spastic paraparesis (TSP), a demyelinating spinal cord disease, myelin may be affected earlier and to a greater extent than axis cylinder. The same is true of progressive multifocal leukoencephalopathy (PML), osmotic demyelination (also known as central pontine myelinolysis), and Marchiafava-Bignami disease. Some of these disorders and several others are no longer classified as demyelinating because the effects of the process are not primarily on myelin. In addition, for reasons that will become clear in subsequent discussion, the chronic progressive leukodystrophies of childhood and adolescence (e.g., globoid body, metachromatic, and adrenal leukodystrophies), although clearly diseases of myelin, are set apart and called dysmyelinating because of their unique genetic and morphologic features and are discussed in Chap. 37. Occupying an uncertain place in this ...

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