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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 a clinician nor a scientist. Moreover, it gives no hint as to the fundamental causation of a process and in all likelihood combines a number of mechanisms under 1 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; that is knowledge that a protein aggregates within or between cells is not equivalent to understanding the cause of an illness.
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Gowers in 1902 suggested the term abiotrophy to encompass the degenerative diseases, 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. It is increasingly evident that many of the diseases included in this category depend on genetic factors. Some appear in more than one member of the same family, in which case they may be properly designated as heredodegenerative. Even more diseases, not differing in any fundamental way from the heredodegenerative ones, occur sporadically, that is as isolated instances but still, genetic factors such as single nucleotide polymorphisms and copy number variations are often involved in pathogenesis.
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Degeneration is nonetheless used as 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. What characterizes the degenerative disease as much as the loss of cells is the concentration of damage in functionally related cells, or systems; for example the cerebral cortex, motor system, extrapyramidal apparatus, or cerebellum, which are representative of the structures that are the targets of damage in this class of disease.
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The basis of aging changes is also explainable at the neuronal level, but the nature of these alterations is not understood. A fundamental problem is the distinction of these aging deteriorations from degenerative disease. When a degenerative neurological disease appears in adult life, one must assume that the clinical presentation is modified to some extent by life-cycle phenomena—the patient's function being a sum of both processes. However, their separation is of fundamental importance in diagnosis ...