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The adjective degenerative has no great appeal to the modern neurologist. It is also not an entirely satisfactory term medically, as it implies an inexplicable decline from a previous level of normalcy to a lower level of function—an ambiguous conceptualization of disease that satisfies neither clinician nor scientist. Moreover, it gives no hint as to the fundamental causation of a process and in all likelihood combines a number of mechanisms under one nondescript term. It would be tempting to attribute all progressive disease of the nervous system that are of unknown cause to degeneration. The problem is that many degenerative diseases of mundane type are caused in a proportion of cases by germ line genetic changes. All are currently called degenerative, but this nosology may be a transitional method of holding a place while awaiting more refined understanding. What is lacking at the moment is a precise subcellular mechanism for cellular loss; i.e., knowing the mutation that is associated with a disease is not equivalent to understanding the cause of an illness. In future editions of this book, it is very likely that an increasing number of degenerative diseases will be relegated to different and perhaps new categories.

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It is becoming increasingly evident that many of the diseases included in this category depend on genetic factors, or at least they appear in more than one member of the same family, in which case they are more properly designated as heredodegenerative. Even more diseases, not differing in any fundamental way from the heredodegenerative ones, occur sporadically, i.e., as isolated instances in given families. For diseases of this type, Gowers in 1902 suggested the term abiotrophy, by which he meant a lack of “vital endurance” of the affected neurons, resulting in their premature death. This concept embodies an unproven hypothesis—that aging and degenerative changes of cells are based on the same process. Understandably, contemporary neuropathologists are reluctant to attribute to simple aging the diverse processes of cellular diseases that are constantly being revealed by ultrastructural and molecular genetic techniques.

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The reader may be perplexed by the inconsistent use of the terms atrophy and degeneration, both of which are applied to diseases of this category. Spatz argued they are different on purely histopathologic grounds. Atrophy is an anatomical term that specifies a gradual wasting and loss of a system of neurons, leaving in their wake no degradative products and only a sparsely cellular gliosis. Degeneration is a clinical and pathologic term that refers to a process of neuronal, myelin, or tissue breakdown, the degradative products of which evoke a reaction of phagocytosis and cellular astrogliosis. There are many examples of diseases that were formerly classed as degenerative but are now known to have a genetic, metabolic, toxic, or nutritional basis, or to be caused by a “slow virus” or a nonviral transmissible agent. It seems reasonable to expect that an increasing number of diseases whose causes are now unknown will ...

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