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.

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, shuffling 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.

Critchley, in 1931 and 1934, drew attention to a number of neurologic abnormalities that he had observed in octogenarians and for which no cause could be discerned other than the effects of aging itself. Several reviews of this subject have appeared subsequently (see especially those of Jenkyn, of Benassi, and of Kokmen [1977] and their associates). The most consistent of the neurologic signs of aging are the following:

• Neuroophthalmic signs: progressive smallness of pupils, decreased reactions to light, and near farsightedness (hyperopia) as a result of impairment of accommodation (presbyopia), insufficiency of convergence, restricted range of upward conjugate gaze, frequent loss of the Bell phenomenon, diminished dark adaptation, and increased sensitivity to glare.
  
• Progressive hearing loss (presbycusis), especially for high tones, and commensurate decline in speech discrimination. Mainly these changes are a result of a diminution in the number of hair cells in the organ of Corti.
  
• Diminution in the sense of smell and, to a lesser extent, of taste (see Chap. 12).
  
• Motor signs: reduced speed and amount of motor activity, slowed reaction time, impairment of fine coordination and agility, reduced muscular power (legs more than arms and proximal muscles more than distal ones) and thinness of muscles (sarcopenia), particularly the dorsal interossei, thenar, and anterior tibial muscles. A progressive decrease in the number of anterior horn cells is partially responsible for these changes, as described further on.
  
• Changes in tendon and frontal reflexes: A depression of tendon reflexes at the ankles in comparison with those at the knees is observed frequently in persons older than 70 years of age, as is a loss of Achilles reflexes in those older than 80 years of age. The snout or palmomental reflexes, which can be detected in mild form in a small proportion of healthy adults, are frequent findings in the elderly (in as many as half of normal subjects older than 60 years of age, according to Olney). However, other so-called cortical release signs, such as suck and grasp reflexes, when prominent, are indicative of frontal lobe disease but sometimes are expected simply as a result of aging.
  
• Impairment or loss of vibratory sense in the toes and ankles. Proprioception, however, is impaired very little or not at all. Thresholds for the perception of cutaneous stimuli increase with age but require the use of refined methods of testing for their detection. These changes correlate with a loss of sensory fibers on sural nerve biopsy, reduced amplitude of sensory nerve action potentials, probably as a result of loss of dorsal root ganglion cells.
  
• The most obvious neurologic aging changes—those of stance, posture, and gait—are fully described in Chap. 7 and further on in this chapter.

Jenkyn and colleagues, based on their examinations of 2,029 individuals aged 50 to 93 years, have determined the incidence of certain common neurologic signs of aging. Notable again is the high frequency of snout and glabellar responses, but also limited downgaze and upgaze in approximately one-third of persons older than age 80 years. Table 29-3 summarizes these data.

With regard to the interesting population of the “oldest old,” those older than 85 years of age, Kaye and colleagues reported that deficits in balance, olfaction, and visual pursuit are distinctly worse than in younger elderly persons. Also of interest is the observation by van Exel and colleagues that women in this age group perform better than men on cognitive tests.

Effects of Aging on Memory and Other Cognitive Functions

Probably the most detailed information as to the effects of age on the nervous system comes from the measurement of cognitive functions. In the course of standardization of the original Wechsler-Bellevue Intelligence Scale (1955), cross-sectional studies of large samples of the population indicated that there was a steady decline in cognitive function starting at 30 years of age and progressing into old age. Apparently all forms of cognitive function partake of this decline—although in general certain elements of the verbal scale (vocabulary, fund of information, and comprehension) withstand the effects of aging better than those of the performance scale (block design, reversal of digits, picture arrangement, object assembly, and the digit symbol task).

However, the concept of a linear regression of cognitive function with aging has had to be modified in the light of subsequent longitudinal studies. If the same individual is examined over a period of many years, there is virtually no decline in his performance, as measured by tests of verbal function, until 60 years of age. Beyond this age, verbal intelligence does decline, but very slowly—by an average of less than 5 percent through the seventh decade and by less than 10 percent through the eighth decade (Schaie and Hertzog). Also, in a series of 460 community-dwelling individuals (55 to 95 years of age) studied by Smith and coworkers (1992), there was no significant decline with age in verbal memory and in registration–attention; similar results were found by Petersen and colleagues in 161 normal, community-dwelling individuals 62 to 100 years of age. The most definite effects of age were in learning and memory and in problem solving—cognitive impairments probably attributable to a progressive reduction in the speed of processing information. The latter may be reflected in the slowing of event-related evoked potentials and by a number of special psychologic tests (see Verhaeghen et al).

As regards these cognitive functions, the ability to memorize, acquire, and retain new information, recall of names, and avoidance of distraction once set on a course of action, diminishes with advancing age, particularly in those older than 70 years of age. Moreover, memory function may be disturbed in this way despite the relative retention of other intellectual abilities. Characteristically, there is difficulty with recall of a name or the specific date of an experience (“episodic” memory) despite a preservation of memory for the experience itself or for the many features of a person whose name is momentarily elusive (“tip-of-the-tongue syndrome”). Also characteristic is an inconsistent retrieval of the lost name or information at a later date. It has been found, however, that if older persons are allowed to learn new material very well, until no errors are made, they forget this information at a rate similar to that of younger individuals.

Kral, who first wrote informatively on this type of memory disturbance 50 years ago, referred to it as benign senescent forgetfulness. He pointed out that such a memory disturbance, in distinction to that of Alzheimer disease, worsens very little or not at all over a period of many years and does not interfere significantly with the person’s work performance or activities of daily living. Crook and coworkers

Related posts:

Disorders of the Special Senses Chapter 20. Delirium and Other Acute Confusional States Chapter 24. Fatigue, Asthenia, Anxiety, and Depression Chapter 26. Disorders of the Autonomic Nervous System, Respiration, and Swallowing Chapter 39. Degenerative Diseases of the Nervous System Chapter 38. Developmental Diseases of the Nervous System

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