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As indicated in the preceding chapter, standards of growth, development, and maturation provide a frame of reference against which every pathologic process in early life must be viewed. It has been less appreciated, however, that at the other end of the life cycle, neurologic deficits must be judged in a similar way, against a background of normal aging changes (senescence). The earliest of these changes begins long before the acknowledged period of senescence and continues throughout the remainder of life. Most authors use the terms aging and senescence interchangeably, but some draw a fine semantic distinction between the purely passive and chronologic process of aging and the composite of bodily changes that characterize this process (senescence).

Biologists have measured many of these changes. Table 29-1 gives estimates of the structural and functional decline that accompanies aging between ages 30 and 80 years. It appears that all structures and functions share in the aging process. Some persons withstand the onslaught of aging far better than others, and this constitutional resistance to the effects of aging seems to be familial. It can also be said that such changes are unrelated to Alzheimer disease and other degenerative diseases but that in general, the changes of aging reduce the capacity to recover from virtually any illness or trauma. An entity of "frailty" has been conceived to encompass the sum of breakdown in multiple organ systems as a result of aging. The review by Clegg and colleagues is recommended on this subject. With respect to the nervous system, it entails loss of muscle mass, strength and endurance, decreased appetite, unintentional weight loss, and reduced mobility and balance. A working definition of frailty has been given by Fried and is summarized in Table 29-2. In the past, this was referred to as "failure to thrive," a term adopted from pediatrics.

Table 29-1 Physiologic and Anatomic Deterioration at 80 Years of Age
Table 29-2 Criteria for Frailty (the Presence of 3 or More of the 5 Features May Be Used to Define Frailty)

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