Mohammed T. Abou-Saleh

72 PRINCIPLES AND PRACTICE OF GERIATRIC PSYCHIATRYrelationship between disease and disability is therefore valuable indeveloping the concept of functional aging. The World HealthOrganization has proposed a theoretical pathway progressingfrom disease to impairment to disability to handicap 5 . Analternative pathway, proposed by Nagi 6 and utilized by the USInstitute of Medicine 7 , progresses from diseases and conditions toimpairment to functional limitation to disability. An effort tooperationalize this latter pathway defines ‘‘impairment’’ asdysfunction and structural abnormalities in specific body systems,‘‘functional limitation’’ as restriction in basic physical and mentalactions such as ambulating, grasping and stepping up, and‘‘disability’’ as difficulty in doing activities in daily life such aspersonal care, household management, job and hobbies 8 .Although these pathways have remained essentially theoretical,increasing efforts are now under way to use empirical data todocument important steps along the pathway 9 .Disease severity and the co-occurrence of multiple diseases (comorbidity)play an important role in the process of disablement.In a study using data representative of the older US population, itwas found that the prevalence of disability increased withincreasing number of chronic conditions, after adjusting for ageand sex 10 . The synergistic effect of specific pairs of diseases ondisability has also been demonstrated. It is also clear thatintervening behavioral, environmental and social factors play animportant part in modifying the pathway along its entire courseand need to be understood more fully 11,12 .DOMAINS OF FUNCTIONING AND MEASURES OFPOPULATION DISABILITYFunctional aging is a multidimensional concept for which severaldomains must be considered to adequately characterize the totalolder person. Domains central to aging include physical, cognitive,psychological, sensory and social functioning. In the olderpopulation it is of value to separate the cognitive and psychologicaldomains, although at times it may be difficult to ascertainwhether cognitive impairment is related to a dementing disease ordepression. The importance of sensory impairments in limitingoverall functioning is receiving increasing attention in ongoinggerontological research. Social functioning, critically important inthe lives of older persons, reflects the impact of the other domainson interactions with family, friends and the community.Physical functioning, to be the focus of the remainder of thischapter, has traditionally been assessed through self-report of theability to perform specific tasks, including self-care activities suchas bathing and dressing (activities of daily living) and activitiesnecessary to maintain independence in the community, such asshopping and food preparation (instrumental activities of dailyliving) 13–15 . In cases where individuals have severe physical orcognitive impairment, proxies have been successfully employed toassess functional status, although proxies who have limitedcontact with the subject tend to provide less valid information 16 .Recently, performance measures of functioning, in which theindividual is asked to actually perform standardized tasks, havebeen employed 17 .A number of national surveys have estimated the prevalence offunctional disability in the US population 18 . The prevalence ofdisability in activities of daily living (ADLs) in the non-institutionalizedpopulation rises steeply with increasing age and is slightlyhigher for women compared with men at the older ages (Figure13.1) 19 . These rates do not reflect that portion of the populationresiding in nursing homes, where it is estimated that over 90% ofresidents require help with ADLs 20 . It is important to keep in mindthat, although chronological age is strongly related to disabilityprevalence, many other factors, such as socio-economic status, havelarge impacts on disability that are independent of age 12 .Figure 13.1. Percentage of persons have difficulty or being unable toperform one or more activities of daily living by age and sex, USA, 1995.Activities of daily living include: bathing or showering, dressing, using thetoilet, getting in and out of bed or chairs, and eating, walking and gettingoutside. Source of data: Second Supplement on Aging to the 1995National Health Interview SurveyCOMPRESSION OF MORBIDITY AND THEMEASUREMENT OF ACTIVE LIFE EXPECTANCYAn important issue related to functional aging is the relationshipbetween length of life and the amount of time spent in the disabledstate. Life expectancy has increased very substantially in thiscentury. A consequence of this, which is just beginning to beappreciated, however, is that escaping death during the early yearsfrom infectious diseases and other causes may mean that manymore people survive to ages where they suffer from chronicdiseases, which can lead to long-term disability and loss ofindependence. A major goal of gerontology is to increaselongevity without increasing the number of years spent in thedisabled or dependent state. Although the recent increase inlongevity is well documented, it is not now clear whether theseadded years of life have been accompanied by years of health andvigor or disease and disability. This question is of particularconcern in the coming century, when it is projected that there willbe continued increases in life expectancy and unprecedentednumbers of old and very old persons. The theory of compressionof morbidity predicts a future decrease in the number of yearswith severe disease and disability 21 .An important tool for evaluating compression of morbidity iswhat has been termed ‘‘active life expectancy’’ or ‘‘disability-freelife expectancy’’ 22 . Active life expectancy is defined as the averagenumber of years an individual at a given age will survive andremain in the active, or non-disabled, state. Most analyses ofactive life expectancy have employed the ADLs to definedisability, with active life expectancy calculated using life tabletechniques which consider transitions from the active, nondisabledstate to both death and disability. The original analysisof active life expectancy considered the transitions to both deathand disability as irreversible 22 .However, recent longitudinal studies of aging populations haverevealed that a substantial proportion of disabled persons makethe transition back to the non-disabled state. Methods to calculateactive life expectancy based on these kinds of changes, usingmultistate life tables, have been developed 23 .The relationship over time between life expectancy and activelife expectancy can be used to assess the occurrence of acompression of morbidity. Three possible scenarios for populationmorbidity in women are illustrated schematically in Figure