College of Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois, U.S.A.
This chapter describes aging changes in the structure and function of the human visual (see section entitled Vision) and auditory (see section entitled Hearing) systems. Aging-associated visual (Tables 1 and 2) and auditory impairments, disorders, and diseases are also presented in the above-mentioned sections, and current efforts for their treatment and rehabilitation are reviewed (Boxes 1-6). The basics of the aging changes in the senses of taste, smell, and somatic sensation are not included in the present chapter, mainly due to lack of space, but can be found in the previous editions of this chapter (1). In this fourth edition, some discussion of the aging of taste and smell is presented in Chapter 19 along with aging of the gastrointestinal system.
Sensory impairments in vision and hearing occur so commonly with aging that they often tend to characterize the aged and the aging process.
■ are due to intrinsic aging processes occurring in the sense organs and their neural and brain components,
■ may be caused by environmental effects, or
■ represent manifestations of aging diseases.
The study of the aging process in the human sensory systems, in addition to its importance and applicability to geriatrics, also provides some of the most interesting and challenging cases of gerontological investigation.
The elements comprising the various senses and their aging portray the entire spectrum of cellular, tissue, organ, and system aging: The peripheral receptor cells of the ear's cochlea and the eye's retina are permanently established at birth, with no turnover and regeneration in later life, in part contributing to the functional decrements in vision and hearing. The aging changes in the eye's lens provide another interesting model system for the study of the aging process, as they begin so early in life and lend themselves to a wide variety of investigations ranging from molecular biology to physiological optics.
■ IMPACT OF SENSORY IMPAIRMENTS ON ELDERLY LIFE AND HEALTH
The incidence of sensory impairments increases markedly in people with aging (Fig. 1). More than 25% of the population
85 years or older suffers from visual abnormalities; twice as many suffer from hearing impairments. Impaired vision and hearing reduce the capacity for social communication, one of humans' cardinal needs and functions, resulting in social isolation and deprivation. The impact of hearing and visual impairments on elderly health and mortality has been reviewed by Guralnik (3). Age-related vision and hearing impairments have a greater impact on long-term health than previously thought. The results of a 10-year study of 5000 subjects aged 55 to 74 years show that measured (not subjective) visual impairment was predictive of 10-year mortality; similarly, both measured and subjective visual impairments were significant in predicting certain aspects of functional disability.
Measured combined visual and auditory impairments lead to the highest risk of functional impairment (3). The synergistic effect of co-occurring impairments (4) may lead to effective future treatments of visual and hearing disorders of the elderly (3). The basic aspects of the effects of reduced capacities in somatic, olfactory, and gustatory senses on aging and physical and mental health of the elderly have been reviewed by Meisami (1). The reduced sensory abilities may lead to depression; in those already suffering from depression, they may hinder the progress of recovery. Because sensory losses in old age are so common and their consequences so widespread, an understanding of these impairments is now essential in geriatrics and elderly care (3).
The eye, with a structure that indicates exquisite adaptation for optical and nervous functions, is the sensory organ for vision (Fig. 2). The cornea, lens, pupil, the aqueous humor, and vitreous gel participate in the optical functions of the eye, while the retina carries out neural visual functions. Both the optical and neural compartments undergo aging changes, although those of the optical compartment are better known. Some of these changes in the eye's optical apparatus, such as those in the lens, start early in life. The changes in the optical compartment are probably the primary causes of decline in the visual capacities of the elderly, while the degenerative changes in the retina are one of the leading causes of old age blindness (Table 1) (6-13). In addition to the eye, aging changes in the visual pathways and central visual structures, such as the lateral geniculate bodies and the visual cortical areas, which
TABLE 1 Summary of the Aging Changes in the Human Eye
Anterior chamber Iris
Ciliary body and muscles
Pigment epithelium Functional changes
Corneal and lens functions
General optic functions Major pathologies
Increased thickness; decreased curvature; transparency loss; pigment and lipid accumulation; loss of epithelial cells; reduced epithelial regeneration Decreased volume and flow of aqueous humor Decreased dilator muscle cell number, pigment, and activity; mild increase in density of collagen fibers in stroma Increased anterior-posterior thickness; decreased curvature; increased pigment accumulation and opacity; decreased epithelial cell number; decreased new fiber formation and antioxidant levels; increased crossover in capsule collagens and lens crystallins; increased hardness in capsule and body Increased inclusion bodies; decreased water;
lesser support to globe and retina Decreased number of smooth muscles (radial and circular); increased hyaline substance and fiber in ciliary process; decreased ciliary pigment epithelial cells Decreased thickness in periphery; defects in rod outer segments, and regeneration of discs and rhodopsin; loss of rods and associated nerve cells; some cone loss; reduced cone pigment density; expansion of Muller cells; increased cyst formation; formation of Drusen-filled lesions; degeneration of macular region Loss of melanin; increased lipofuscin granules; atrophy and cell loss
Decreased accommodation power (presbyopia); increased accommodation reflex latency; increased near point-of-vision; increased lenticular light scattering; decreased refraction; decreased lens elasticity Decreased critical flicker frequency; decreased light sensitivity (increased light thresholds before and after dark adaptation); reduced color vision initially in yellow to blue range and later in the green range Increased pupillary constriction (senile miosis); reduced visual acuity; presbyopia
"Against the rule" astigmatism Cataract; hardening and loss of elasticity Senile macular degeneration; diabetes retinopathy Open angle and closed angle types have been found to be quite extensive in humans, may also be responsible for some of the visual impairments of the elderly, but the knowledge of these aspects is only recently developing.
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