Figure 27-4. Gaussian or bell-shaped curve of intelligence and its skewing by the group of intellectually delayed individuals with diseases of the brain. The shaded areas indicate the two groups of persons who have delays in this sphere. The population on the extreme left is largely comprised of those with overt cerebral pathology and is intended to illustrate a slight overlap with those whose intelligence falls in the lower end of the normal distribution. The latter group is considered not to have a pathologic basis of low intelligence, in the past called “subcultural” as discussed in the text and in Chap. 37.
Chapter  27    Normal Development and Deviations in Development of the Nervous System
The Neurology of Aging
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. 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 28-1 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 that result from the later stages of aging. 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, to which may be added the deteriorations in vision and hearing that occur to varying degrees in the aged. A working definition of frailty has been given by Fried and is summarized in Table 28-2. In the past, this was referred to as “failure to thrive,” a term adopted from pediatrics. A simplified approach has been given the British Geriatrics Society. They have identified slowed walking speed, meaning the inability to cover 4 m in less than 5 s, the inability to stand from a chair and walk 3 m and return to sit down again in under 10 s, a score of 3 or more on a questionnaire called PRISMA 7, which focuses on age over 85, health problems that require the individual to stay at home, and the need for a cane, walker, or wheelchair. The British Geriatrics Society paper and the review by Clegg and colleagues are recommended on this subject.
Effects of Aging on the Nervous System
Of all the age-related changes, those in the nervous system are of paramount importance. Actors portray old people as being feeble, idle, obstinate, given to reminiscing and having tremulous hands, quavering voices, stooped posture and slow, shortened steps. In so doing, they have selected some of the most obvious effects of aging on the nervous system. The lay observer, as well as the medical one, often speaks glibly of the changes of advanced age as a kind of second childhood. “Old men are boys again,” said Aristophanes.