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Disability in America: Toward a National Agenda for Prevention 3 A Model for Disability and Disability Prevention A common understanding of such terms as injury, impairment, handicap, functional limitation, and disability is essential to building effective, coherent prevention programs. Several frameworks have been advanced to describe disability-related concepts, but none has been universally adopted. The lack of a uniformly accepted conceptual foundation is an obstacle to epidemiological research and surveillance and to other elements critical to effective disability prevention programs. This chapter describes a conceptual framework of disability that is derived primarily from the works of Saad Nagi (1965; Appendix A, this volume) and the World Health Organization (1980). The framework is used as the basis upon which to build a model of the interacting influences involved in a stagelike disabling process that can lead to disability and that includes risk factors and quality of life. CONCEPTUAL FRAMEWORK There are two major conceptual frameworks in the field of disability: the International Classification of Impairments, Disabilities, and Handicaps (ICIDH), and the "functional limitation," or Nagi, framework, which is not accompanied by a classification system. The ICIDH is a trial supplement to the World Health Organization's International Classification of Diseases; it has stimulated extensive discussions of disability concepts, received both positive and negative reviews in the literature, and is used widely around the world. Several European countries including France and the Netherlands have adopted the ICIDH and use it extensively in administrative systems and clinical settings. As a classification system that has received broad international sponsorship the ICIDH deserves considerable attention, and the WHO is to be commended for its efforts in developing a system that has met with such success. As has
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Disability in America: Toward a National Agenda for Prevention been pointed out in the literature, however, the ICIDH is neither a classification of persons nor a research tool. The original intent of the ICIDH was to provide a framework to organize information about the consequences of disease (Haber, 1990). As such, the ICIDH has been considered by some as an intrusion of the medical profession into the social aspects of life—as a "medicalization of disablement" (Badley, 1987). The WHO is planning a revision of the ICIDH in the near future, however, which will provide opportunities for significant improvements. Both frameworks (i.e., the ICIDH and the Nagi) have four basic concepts. In the ICIDH the four concepts are disease, impairment, disability, and handicap. In the Nagi framework the four concepts are pathology, impairment, functional limitation, and disability. Several authors have compared the two frameworks, and most have noted similarities, particularly between Nagi's concept of pathology and ICIDH's concept of disease and between the two frameworks' characterizations of impairment (Nagi, Appendix A, this volume; Duckworth, 1984; Frey, 1984; Granger, 1984; Haber, 1990). The more important distinctions between the Nagi framework and the ICIDH occur in the last two conceptual categories and go beyond simple terminology. The ICIDH concept of disability seems to correspond to Nagi's concept of functional limitation, or "activities of daily living" (as used in the National Health Interview Survey), and the ICIDH concept of handicap (which subsumes role limitations) seems to correspond to Nagi's concept of disability. Both frameworks recognize that whether a person performs a socially expected activity depends not simply on the characteristics of the person, but also on the larger context of social and physical environments. Conceptual clarity, however, seems to be a problem with some of the classifications in the ICIDH. As Haber (1990) points out, for example, some of the classifications in the ICIDH are confusing, such as classifying certain social role limitations (e.g., family role, occupational role) under "behavior disabilities," instead of "occupation handicaps'' or "social integration handicaps." Another example (Haber, 1990) is the distinction between "orientation handicaps" and disabilities associated with self-awareness, postural, or environmental problems. In considering the options for a conceptual framework, the committee was faced with the fact that the ICIDH includes the term handicap in its classification. Traditionally, handicap has meant limitations in performance, placing an individual at a disadvantage. Handicap sometimes has been used to imply an absolute limitation that does not require for its actualization any interaction with external social circumstances. In recent years, the term has fallen into disuse in the United States, primarily as a result of a feeling on the part of people with disabling conditions that handicap is a negative term. Although the term handicap is used often as a synonym for disability in American legislation, at least three federal agencies have changed their
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Disability in America: Toward a National Agenda for Prevention names to use the term disability instead of handicap: the former National Council on the Handicapped became the National Council on Disability in January 1989, the National Institute of Handicapped Research was redesignated the National Institute on Disability and Rehabilitation Research in 1986, and the President's Committee on Employment of the Handicapped was renamed the President's Committee on Employment of People with Disabilities in 1988. Mostly out of deference to those who feel that handicap is a denigrating term when used to describe a person, this committee decided not to use it. Yet the shadow of handicap as a commonly used term hovers behind the concept of "quality of life" and has the effect of reducing quality of life even though impairment, functional limitation, and even disability do not necessarily do so. Much as the term "cripple" has gone out of style, "handicap" seems to be approaching obsolescence, at least among people with disabilities in the United States. The committee concurs with those who have noted internal inconsistencies and lack of clarity in the ICIDH concepts of disability and handicap (Nagi, Appendix A, this volume; Haber, 1990). It notes the opportunity and calls attention to the need for its pending revision, prefers not to use the term handicap in this context, and offers an alternative framework that does not focus on the consequences of disease. The committee's alternative framework draws on the widespread acceptance and success of the ICIDH and the conceptual clarity and terminology of the Nagi framework, and then adds risk factors and quality of life into a model of the disabling process. The committee found this framework and model to be useful in understanding and describing the relationships that exist among and between components of the disabling process as well as in identifying strategic points for preventive intervention. It is hoped that this will be considered as a viable alternative in the revisions of the WHO/ICIDH. The conceptual framework used in this report is composed of four related but distinct stages: pathology, impairment, functional limitation, and disability. In the course of a chronic disorder, one stage can progress to the next. But depending on the circumstances, progressively greater loss of function need not occur, and the progression can be halted or reversed. Thus disability prevention efforts can be directed at any of the three stages that precede disability, as well as at the disability stage itself, where efforts can focus on reversal of disability, restoration of function, or prevention of complications (secondary conditions) that can greatly exacerbate existing limitations and lead to new ones. The four stages of the framework are summarized in Figure 3-1 and are briefly discussed below. A more detailed discussion and description of Nagi's concepts and terminology, vis-à-vis the alternative approach developed by the WHO (1980), appear in Appendix A. A recent editorial by Mervyn Susser (1990) adds considerable insight into the historical development of related concepts.
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Disability in America: Toward a National Agenda for Prevention PATHOLOGY → IMPAIRMENT → FUNCTIONAL LIMITATION → DISABILITY Interruption or interference of normal bodily processes or structures Loss and/or abnormality of mental, emotional, physiological, or anatomical structure or function; includes all losses or abnormalities, not just those attributable to active pathology; also includes pain Restriction or lack of ability to perform an action or activity in the manner or within the range considered normal that results from impairment Inability or limitation in performing socially defined activities and roles expected of individuals within a social and physical environment Level of reference Cells and tissues Organs and organ systems Organism—action or activity performance (consistent with the purpose or function of the organ or organ system) Society—task performance within the social and cultural context Example Denervated muscle in arm due to trauma Atrophy of muscle Cannot pull with arm Change of job; can no longer swim recreationally FIGURE 3-1 An overview of the concepts of pathology, impairment, functional limitation, and disability. Pathology Pathology refers to cellular and tissue and changes caused by disease, infection, trauma, congenital conditions, or other agents. Much pathology is a reflection of the mobilization of the body's defenses against abnormalities. In the case of acute diseases, destruction of the normal cell architecture may result in particular manifestations (some combination of signs and symptoms) that aid identification of the underlying cause, or etiology. Many chronic diseases have multiple or uncertain etiologies. High serum cholesterol, hypertension, and smoking, for example, all increase the risk of heart disease, but not all people with these traits develop heart disease.
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Disability in America: Toward a National Agenda for Prevention Predisposing factors that can lead to pathology are called risk factors. In the committee's model, risk factors can be biological, lifestyle and behavioral, or environmental (physical or social). Risk factors are discussed in greater detail later in the chapter. Impairment Impairment is defined as a discrete loss or abnormality of mental, physiological, or biochemical function. Impairment includes losses caused by all forms of pathology. A specific impairment might have different etiologies and different types of pathology. All pathologies, however, are accompanied by impairments (Figure 3-2). Impairments include anomalies, defects, or losses and relate to the specific functioning of an organ or organ system but not to the organism as a whole. Examples of impairments are absence or displacement of body parts, reduced blood flow, mechanical problems of joints, paralysis, stiffness, and numbness. The severity of impairment varies by condition, by the tissues and organs affected, and by the extent to which tissues and organs are damaged. For example, the human immunodeficiency virus (HIV) attacks T-cells, compromising the immunity of the infected person. Compromised immune function is but one impairment associated with HIV exposure. Depending on the extent of immune system suppression, several other conditions and impairments may occur. In contrast, other diseases such as arthritis are more specific in terms of the type and location of impairments they cause. Functional Limitation Functional limitation is the term proposed by Nagi to describe effects manifested in the performance or performance capacity of the person as a whole. An example of a functional limitation is the inability to lift a 25-pound box and carry it 25 feet. This type of limitation may be caused by impairment of any one of several body systems, including reduction of pulmonary function (emphysema), denervation of muscle tissue (amyotrophic lateral sclerosis), or restriction in range of joint motion (arthritis). All functional limitations result from impairments, but not all impairments lead to functional limitation (Figure 3-2). Several factors other than the nature and degree of impairment affect functional performance. For example, of two individuals with the same level of pulmonary function, one may be able to complete an activity such as walking upstairs, whereas the other cannot. Only the latter individual has a functional limitation as a result of this particular impairment. Such variation may be related to the capacities of the individual's other body systems (e.g., cardiovascular fitness, muscular strength, or pain tolerance).
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Disability in America: Toward a National Agenda for Prevention FIGURE 3-2 According to the Nagi framework, all pathology is associated with impairment, but not all impairments lead to functional limitations. Similarly, all functional limitation and disability is associated with impairment, but not all functional limitations lead to disability. Disability can also exist in the absence of functional limitation (e.g., disfigurement). (This diagram serves to illustrate the conceptual relationship among the categories in Nagi's framework; the sizes of the boxes do not reflect the relative size of that category in the U.S. population.) Disability Disability is the expression of a physical or mental limitation in a social context—the gap between a person's capabilities and the demands of the environment. People with such functional limitations are not inherently disabled, that is, incapable of carrying out their personal, familial, and social responsibilities. It is the interaction of their physical or mental limitations with social and environmental factors that determines whether they have a disability. Most disability is thus preventable, which not only will significantly improve the quality of life for millions of Americans but also could save many billions of dollars in costs resulting from dependence, lost productivity, and medical care. Pathology, impairment, and functional limitation all involve different levels of organismic function. Disability, however, refers to social rather than organismic function. According to Nagi (Appendix A, this volume): [Disability is a] limitation in performing socially defined roles and tasks expected of an individual within a sociocultural and physical environment. These roles and tasks are organized in spheres of life activities such as those of the family or other interpersonal relations; work, employment, and other economic pursuits; and education, recreation, and self-care. Not all impairments or functional limitations precipitate disability, and similar patterns of disability may result from different types
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Disability in America: Toward a National Agenda for Prevention of impairments and limitations in function. Furthermore, identical types of impairments and similar functional limitations may result in different patterns of disability. Several other factors contribute to shaping the dimensions and severity of disability. These include (a) the individual's definition of the situation and reactions, which at times compound the limitations; (b) the definition of the situation by others, and their reactions and expectations—especially those who are significant in the lives of the person with the disabling condition (e.g., family members, friends and associates, employers and co-workers, and organizations and professions that provide services and benefits); and (c) characteristics of the environment and the degree to which it is free from, or encumbered with, physical and sociocultural barriers. Thus one way in which disability differs from pathology, impairment, and functional limitation is in the role of factors external to the individual. Disability is defined by the attributes and interactions of the individual and the environment, whereas the preceding stages are defined solely by characteristics of the individual. For example, whether a person with an impairment is able to work depends not only on the nature and severity of his or her impairment and resulting functional limitation but also on such factors as the state of the economy, characteristics of the workplace, availability of transportation, and the individual's particular work skills and training. Whether a person with a functional limitation lives independently may be determined by supportive social contacts and the architectural features of his or her home. Pathology, impairment, and functional limitation can be determined by examination and testing of the individual, but disability is a relational attribute—the interaction of an individual's functional limitation with the demands of expected tasks and roles and with the environmental conditions under which roles and tasks are to be performed. Referring to specific pathologies or impairments as disabilities ignores the interactive nature of the process that can lead to disability. To understand disability as it is defined here, one must also understand the concepts of roles and tasks, and how they relate to each other. The concept of task is best understood in relation to the concept of role. Simply put, roles—such as being a teacher, researcher, parent, or civic leader—are organized according to how individuals participate in a social system (Parsons, 1958). Tasks are specific physical and mental actions through which an individual (not a subsystem of an individual, which would be at the impairment level) interacts with the physical and social world and performs his or her roles. One task does not define a role; roles are made up of many tasks, which are modifiable and somewhat interchangeable. Finally, although disability can be prevented by improving the functional capacity of the individual—the traditional aim of rehabilitation—this is not
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Disability in America: Toward a National Agenda for Prevention the only nor perhaps even the most effective method. Disability can be prevented by changing societal attitudes that now restrict employment opportunities for persons with functional limitations, by modifying the buildings in which such people work, or by providing accessible modes of transportation (all of which are components of the Americans with Disabilities Act). Disability can be prevented by building living quarters, parks, and other facilities with fewer obstacles restricting access and use by persons with functional limitations. The opportunity to prevent disability by manipulating characteristics external to the individual greatly expands the traditional medical notions of disability and the consequent approaches to treatment and services, and reflects more of a public health approach. Personally and socially expected activities can be accomplished by changing the means to the ends. Capacities are the means; expected activities are the ends. One reason why impairments and functional limitations do not necessarily lead to disability is that individuals with a given impairment may overcome specific functional limitations by compensating with other functional capacities to avoid disability. Installing ramps in buildings, for example, enables people with mobility limitations to perform activities that would otherwise be denied to them. In summary, disability begins with physical or mental health conditions that limit the performance of individuals in personally, socially, and culturally expected roles. The limitation may be total, rendering an activity unperformable, or it may be partial, restricting the amount or kind of an activity a person can perform. Although conceptually distinct, disability is often confused with disease and impairment. For example, specific diagnostic conditions and impairments, such as mental retardation, cerebral palsy, or multiple sclerosis, are erroneously referred to as disabilities. But depending on various factors, these conditions may or may not lead to disability (although the risk of disability is high for each of the examples given). Moreover, the scope and severity of limitation that follows even the most physiologically damaging disorders—those that pose the greatest risk of physical disability—vary among individuals, including those with the same condition. MODEL OF DISABILITY Building on the conceptual frameworks of Nagi and the WHO, and placing disability within the appropriate context of health and social issues, the committee developed a model for disability. The model, shown in Figure 3-3, depicts the interactive effects of biological, environmental (physical and social), and lifestyle and behavioral risk factors that influence each stage of the disabling process; the relationship of the disabling process to quality of life; and the stages of the disabling process that often precede disability. Each component
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Disability in America: Toward a National Agenda for Prevention of the model (i.e., risk factors, quality of life, and the disabling process) is discussed below. Risk Factors Risk factors are biological, environmental (social and physical), and lifestyle or behavioral characteristics that are causally associated with health-related conditions (Lalonde, 1974; Last, 1988). They can be identified by comparing the frequency of a condition's occurrence, such as disability, in a group having some specific trait with the frequency of the same condition in another group without that trait. Identifying such factors can be a first step toward identifying a mechanism of action, and then developing preventive interventions. For example, workers in a factory where there is high exposure to dust may have higher rates of respiratory disease than other factory workers. In this case, exposure to dust-borne hazardous particles may be identified as a cause, the mode of biological action elucidated, and appropriate preventive measures identified. Some risk factors are implicated in a variety of chronic diseases, resulting in what has been termed general susceptibility (Syme and Berkman, 1976). Socioeconomic status is important among these risk factors. Epidemiologists have also called attention to changes in the nature and distribution of disease as nations develop economically and standards of living change accordingly (Omran, 1979). Such changes have engendered debate on the relative importance of lifestyle, sanitation, nutrition, and public health in the changing incidence and prevalence of chronic diseases. Similarly, there are many risk factors and causal routes associated with disability. Marge (1988) lists the following 16 causes of disabling conditions: • Genetic disorders • Perinatal complications • Acute and chronic illness • Unintentional and intentional injuries • Violence • Environmental quality problems • Lack of physical fitness • Alcohol and drug abuse • Tobacco use • Nutritional disorders • Educational deficiency • Deleterious child-rearing practices • Familial-cultural deleterious beliefs • Unsanitary living conditions • Inaccessibility to adequate health care • Stress Whether through injury, disease, personal-choice behaviors, genetic traits, or some other causal mechanism, multiple risk factors of various types can converge to predispose an individual to the disabling process, as shown in Figure 3-3. In addition, risk factors interact at the different stages of the disabling process (note the circles between the stages that represent the
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Disability in America: Toward a National Agenda for Prevention FIGURE 3-3 Model of disability showing the interaction among the disabling process, quality of life, and risk factors. Three types of risk factors are included: biological (e.g., Rh type); environmental (e.g., lead paint [physical environment], access to care [social environment]); and lifestyle and behavior (e.g., tobacco consumption). Bidirectional arrows indicate the potential for "feedback." The potential for additional risk factors to affect the progression toward disability is shown between the stages of the model. These additional risk factors might include diagnosis, treatment, therapy, adequacy of rehabilitation, age of onset, financial resources, expectations, and environmental barriers, depending on the stage of the model.
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Disability in America: Toward a National Agenda for Prevention various risk factors), and these are often different risk factors than those that precipitate the initial condition. In addition, risk factors exist internally (e.g., through individual choices) and externally (e.g., through the physical and social environment). The disability research and service communities have not yet adopted a systematic, comprehensive conceptual model for understanding risk factors for disability. This committee, however, believes that the model described in this report, incorporating three risk factor categories—biological, environmental (physical and social), and lifestyle and behavioral—will help move the disability research and service communities closer to a more unified understanding of disability and disability prevention. Although many disability risks cannot be neatly categorized, and many occur at the intersection of two or three categories, this model presents an initial framework for exploring possible points for preventive interventions. The scope of each risk category is discussed briefly below. Biological Factors Biological risk factors are those that develop within the body as part of one's basic biology and organic makeup. They include genetic and other inborn or inherited characteristics as well as the metabolic aspects of maturation, growth, aging, and the interactions of the varied and complex systems of the body. Biological risk factors associated with disabling conditions are often the same as those associated with specific diseases because the disabling condition often results from the disease (e.g., arthritis, diabetes, atherosclerosis). Many biological risk factors are genetic, as in the case of Tay-Sachs disease, a condition that causes progressive retardation, paralysis, blindness, and death by age 3 or 4. Preventive strategies directed toward decreasing biological risk factors include pharmaceutical prophylaxis and treatment, nutritional modification, exercise, and prenatal care. Environmental Factors The defining characteristic of environmental risk factors is that they are health-related risks that exist outside the person and over which the individual has little or no control. There are two types: social and physical. The social-environmental risk factors overlap to some extent with the lifestyle and behavioral risk factors, but are primarily the product of societal structures. The physical-environmental risk factors are primarily the product of the built (i.e., human-made) environment. The social (i.e., social-environmental) risks for disability are a function
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Disability in America: Toward a National Agenda for Prevention the environment affect disability outcome, and does the effect of the environment depend on the nature of the impairment and functional limitation? The following sections discuss data and research needs that should be addressed to ensure that basic epidemiological elements of effective prevention efforts are in place. Risk Factors Research on biological, environmental (physical and social), and behavioral risk factors is one of the cornerstones of epidemiology and, consequently, of health promotion and disease prevention. As the model of the disabling process illustrates, knowledge of risk factors is central to disability prevention. Indeed, a comprehensive understanding of risks is critical to answering three fundamental questions: Given exposure to environmental agents or other provocations, why do some persons develop potentially disabling conditions and others do not? Given such exposure and the occurrence of pathology or injury, why does one person develop a disability and another does not? That is, what determines the progression toward functional limitation and disability? At the aggregate level, why do some population groups have higher rates of disability than others? At each stage in the disabling process, biological and behavioral characteristics and features of the social and physical environment have determinative effects on individual outcomes. The genetically determined healthy or unhealthy nature of an individual's body systems is not the sole factor in the development of disease or disability. For example, not all people with abnormal glucose levels develop diabetes, and not all diabetics develop functional limitations or disabilities. An epidemiology of disability requires an expanded perspective on risk factors because any specific type of disability can be the product of many different kinds of pathology, impairment, and functional limitations. Moreover, a complex array of variables, many of them outside the bounds of the usually emphasized biological risk factors, can speed, slow, halt, or reverse the stage-to-stage progression to disability. Such variables include the adequacy and availability of social and medical services, socioeconomic status, marital status, job experience, and amount of educational and vocational training. Research has demonstrated the importance of psychosocial risk factors in disability (Haan et al., 1989), but the findings thus far are largely in the form of leads for further research. Critically important details remain to be identified—for example, the influence of social support, a concept that refers to the quality and breadth of one's relationships with a mate, other
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Disability in America: Toward a National Agenda for Prevention family members, friends, and others. Lack of social support has been associated with an increased risk of heart disease, complications of pregnancy and delivery, suicide, and other conditions (Dutton and Levine, 1989; U.S. Department of Health, Education, and Welfare, 1979). An important question is, what underlying biological mechanisms are affected by social support? The answer to the question of underlying biological mechanisms may not emerge if the focus of investigation is limited to only one condition. Perhaps the most productive way to detect the underlying mechanism in this case is to study all health consequences associated with inadequate social support (Haan et al., 1989). Although the condition-specific approach of epidemiology has increased our understanding of diseases and their prevention, it may lead to overly narrow perspectives on prevention, corresponding to disease classifications (see Table 3-1). An alternative approach would be based on risk factors that predispose an individual to several disease conditions that can lead to disability, such as those shown in Table 3-2. From the viewpoint of public health, a classification scheme that identifies causative features common to several disabling conditions may foster more efficient prevention programs, focusing on risk factors implicated in multiple conditions that predispose an individual to disability. This strategy might offer opportunities to achieve benefits that are larger than the sum of the returns to individual disorder-specific initiatives. Although epidemiology is essential to disability prevention, very little epidemiological research on risk factors for disability or on disability per se has been done, and few studies have been conducted to identify populations at increased risk of disability. Most relevant data relate to clinical conditions, which correspond most closely to the pathology and impairment stages of the committee's model. Some functional limitation and disability information can be extrapolated from the NHIS data (see Chapter 2), but this methodology does not produce very precise measures. In addition, although potentially disabling conditions are dynamic and can improve as well as deteriorate, existing data systems can neither measure the dynamics of disability progression nor identify risk factors that accelerate progression from impairment to functional limitation to disability. More specific epidemiological data are needed on the incidence and prevalence of functional limitation and disability and their attendant risk factors. Populations at higher risk for disability need to be identified and their risk factors assessed to develop interventions to prevent disability. Longitudinal studies are needed to help define the dynamic nature of impairment, functional limitation, and disability and to describe the natural history of chronic conditions and aging in terms of these functional outcomes.
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Disability in America: Toward a National Agenda for Prevention TABLE 3-1 Major Causes of Death and Associated Risk Factors, United States, 1977 Cause Percentage of All Deaths Risk Factors Heart disease 37.8 Smoking, hypertension, elevated serum cholesterol, diet, lack of exercise, diabetes, stress, family history Malignant neoplasms 20.4 Smoking, work-site carcinogens, environmental carcinogens, alcohol, diet Stroke 9.6 Hypertension, smoking, elevated serum cholesterol, stress Non-vehicular injuries 2.8 Alcohol, drug abuse, smoking (fires), product design, handgun availability Influenza and pneumonia 2.7 Smoking, vaccination status Motor vehicle crashes 2.6 Alcohol, no seat belts, speed, roadway design, vehicle engineering Diabetes 1.7 Obesity Cirrhosis of the liver 1.6 Alcohol abuse Arteriosclerosis 1.5 Elevated serum cholesterol Suicide 1.5 Stress, alcohol and drug abuse, gun availability SOURCE: Matarazzo, 1984. Reprinted with permission. The Need for Surveillance The changing demographic profile of the U.S. population and the associated patterns of disability risk demonstrate the necessity of continued surveillance of the incidence and prevalence of chronic physical and mental health conditions, injury, and disability. Some research indicates that the risk of disability has been increasing for all population age cohorts, although there is considerable debate about the reasons for this trend. There has also been a noticeable increase in work disability rates (Chirikos, 1989). In addition, the aging of the population may bring increased risks of disability. Existing national data sets that track the prevalence of chronic conditions over time are useful for disability surveillance. The lack of data on incidence rates, however, is a serious void in disability surveillance and an impediment to fundamental understanding of the disabling process. Incidence data provide a measure of the rate at which a population develops a chronic
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Disability in America: Toward a National Agenda for Prevention condition, impairment, functional limitation, or disability and thereby yield estimates of the probability or risk of these events. Most existing data, however, provide information only on prevalence, not incidence. Prevalence is the net result of changes in incidence and the duration of time a person has a condition. Duration is determined by rates of recovery and mortality. When one compares population groups, only incidence data provide a clear picture of how risks differ among populations. Prevalence data, on the other hand, reflect not only these risks but also differences in rates of recovery and mortality. Thus populations with equal risks of developing TABLE 3-2 Risk Factors in Chronic Disease and Disability Risk Factor Some Related Conditions Smoking Lung cancer Emphysema Bronchitis Other respiratory diseases Coronary artery disease Burns (especially home fires) Alcohol abuse Injuries sustained in motor vehicle accidents, especially head injuries and pedestrian injuries Cirrhosis Fetal alcohol syndrome Lack of prenatal care Mental retardation Cerebral palsy Congenital heart abnormalities (via rubella) Various congenital anomalies (e.g., through failure to control blood sugar in pregnant diabetic women) Other developmental disabilities Socioeconomic status Low birthweight Injury Coronary heart disease Lung cancer Osteoarthritis Death Diabetes mellitus Cervical cancer
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Disability in America: Toward a National Agenda for Prevention disability may differ in prevalence because of differences in access to medical and rehabilitative care. Information on incidence is therefore critical to understanding the causes of disability. Data on duration, however, are useful to gauge rates of recovery and mortality. Only when incidence and duration are known can one understand what causes disability and what determines its course. Collecting data on the incidence and duration of pathology, impairment, and functional limitation as well as secondary conditions is an important component of the disability surveillance effort that is needed. Although the NHIS includes some disability-related questions, it is quite limited in scope because it is a general-purpose survey of the health of the nation and not designed to investigate efficiently the causes and risks of disability. To conduct such an investigation requires a comprehensive longitudinal survey that could address each path of the model displayed in Figure 3-3, particularly the biological, lifestyle and behavioral, and physical and social environmental factors influencing transitions from pathology to impairment and on to functional limitation and disability. A longitudinal survey of disability is needed to assist in determining the causes and rate of transition between pathology, impairment, functional limitation, and disability. The survey should make use of data linkages to existing agency data sets on need, use, and costs of services; be responsive as a policy development resource tool; and evaluate the causal relationship between socioeconomic status and disability. The development and implementation of this survey should be a collaborative effort involving the U.S. Census Bureau, the Centers for Disease Control, the National Center for Health Statistics, the National Institute on Disability and Rehabilitation Research, the National Institute on Aging, the Health Care Financing Administration, the Social Security Administration (SSA), and other relevant agencies. Before conducting a new survey, however, consideration should be given to the utility of longitudinal analysis of existing data sources such as the SSA 1971-1974 disability survey, the Boston University project of the Framingham Study, the SSA 1969-1970 Retirement History Survey, the Census Bureau's SIPP, and the Department of Labor manpower mobility surveys. The Need for Priorities In terms of goals and implementation, disability prevention is usually thought to mean primary prevention—averting the onset of a potentially disabling pathology or an impairment that leads to a disability (see section on primary prevention that follows). The model set forth in this chapter, however, underscores the fact that well after the onset of a potentially
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Disability in America: Toward a National Agenda for Prevention disabling condition, multiple points of intervention exist at which to prevent disability or diminish its severity. Although this model can help lead to many new opportunities for prevention, it does not specify what priorities to place on the possible points of intervention. Priority setting must include an analysis of epidemiological data pertaining to the causes and natural history of various disabling conditions. As noted in Chapter 2, some of the less prevalent potentially disabling conditions (e.g., spinal cord injury) have a high risk of disability, whereas some of the more prevalent conditions (e.g., arthritis) have a fairly low risk of disability. The inverse relationship between the prevalence of a condition and the risk of disability presents an enormous challenge in forging prevention strategies. Primary prevention strategies are normally targeted to higher-than-average risk groups in the general population, even though the overall risk of acquiring a disabling condition is very small. Secondary prevention strategies are targeted to those who have already acquired a condition but may not be experiencing its disabling effects. Neither course of action may be necessarily efficient or cost-effective. The committee considered several competing and overlapping principles and criteria on which priorities for prevention could be based. These included the following: prevalence of specific conditions that can cause disability; number of persons who are likely to experience some degree of limitation or disability associated with a particular condition; severity of disabling conditions and their probable impact on the individual, the family, and society; the number of expected disability years (not merely the prevalence of a condition or its limitations); and how the prevalence and severity of selected conditions are likely to grow in future years. As discussed in the recent National Research Council report on disability statistics (NRC, 1990), a study is needed in which a combination of the above-mentioned principles and criteria is used to conduct an objective analysis that will lead to alternative indexes of disability risk and public health impact. These indexes can then be used to set priorities for prevention efforts among all conditions. A disability index or group of indexes is needed to help establish priorities for disability prevention among conditions and to gauge and monitor the magnitude of disability as a public health issue. This index or group of indexes should include measures of independence, productive life expectancy (both paid and unpaid), and quality of life.
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Disability in America: Toward a National Agenda for Prevention In the absence of such an index, the committee is reluctant to recommend prevention strategies that favor one disabling condition over another. However, in succeeding chapters the committee cites some of the needs and issues related to several categories of individual disabling conditions. Major gaps exist in the data and knowledge about risk factors associated with disability. One reason for these gaps is that most disability-related data are oriented toward clinical categories or impairments. Such categories may have clinical utility for addressing the treatment needs of persons with specific impairments, but they are not useful in fostering an epidemiology based on risk factors such as those related to the social and physical environment. As discussed earlier (under the section on risk factors), the committee believes that specific conditions may not always be the most appropriate or effective means for setting priorities or identifying targets for the development of preventive intervention strategies. An alternative method for consideration is to focus on risk factors or causes that are generic to the etiology of several disabling conditions. Some examples include smoking, alcohol abuse, drug abuse, socioeconomic status, and lack of prenatal care (see Table 3-2). These risk factors are already associated with many of the nation's leading health problems. Less understood is their relationship to disability. Cause-oriented disability data need to be considered possible alternatives in the development of approaches to identifying priorities in disability prevention. APPLYING TRADITIONAL PREVENTION STRATEGIES TO DISABILITY The standard public health model delineates three categories of prevention efforts—primary, secondary, and tertiary—each one focusing on distinct stages in the natural history of diseases. This same model is applicable to the prevention of disability. And, as is true for all prevention programs, epidemiological data and analyses are the cornerstones of effective planning and evaluation. Thus the quality and quantity of the available epidemiological data, as discussed in the previous section, will be critical to the development of effective intervention strategies. Here, the committee briefly summarizes the primary, secondary, and tertiary approaches to prevention and how they might be applied to disability (see Patrick and Peach  for additional information). Prevention efforts that are specific to various disabling conditions are discussed in more detail in succeeding chapters. Primary Prevention Primary prevention focuses on healthy persons, seeking to avoid the onset of pathological processes by reducing susceptibility, controlling exposure
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Disability in America: Toward a National Agenda for Prevention to disease-causing agents, and eliminating or at least minimizing behaviors and environmental factors that increase the risk of illness, injury, or disability. Interventions include (1) health promotion and education, which are largely tailored to fostering adoption of healthy lifestyles; (2) health protection, such as measures designed to improve air quality or food safety; and (3) preventive health services, such as immunization or counseling. Most public health efforts fall into the category of primary prevention. Unfortunately, and incorrectly, people with potentially disabling conditions often are not recognized as a target population for primary prevention efforts, despite the fact that having a potentially disabling condition frequently increases the need for good health promotion and disease prevention practices. With respect to disability, primary prevention usually means preventing the initiation of a potentially disabling condition such as spinal cord injury. However, having a disabling condition does not preclude the need for other primary prevention activities such as exercise and immunization. Primary prevention of disabling conditions is a focus of attention in this report, but additional emphasis is focused on people who already have potentially disabling conditions, i.e., secondary and tertiary prevention. Health-promoting practices, appropriate medical care, and other measures that help ensure good health and a reasonable quality of life are as important to people with disabling conditions as they are to people without them. Similarly, they are as important to the elderly as they are to the young. It is never too late to benefit from quitting smoking, adopting good dietary practices, or engaging in regular exercise, as illustrated in the report on the benefits of smoking cessation for those with coronary heart disease who are over 55 years of age (Hermanson et al., 1988). These and other health-promoting measures pay health dividends to all. Indeed, health promotion directed toward older adults has great potential for impact because the benefits of healthy behaviors may be achieved relatively quickly. Given that the prevalence of chronic diseases rises sharply in this age group and that this segment of the population is growing rapidly, the societal benefits of health promotion and disability prevention during later life may be great (Institute of Medicine, 1990a). Moreover, the purpose of health promotion is not simply to extend life but also to improve the quality of life and to extend active life free of disability (Fries, 1988; Katz et al., 1983). Health promotion is applicable to all age groups and although the messages might change for different ages, the major themes with respect to exercise, diet, substance abuse, and injury prevention are often the same. Reinforcing messages in the community, at schools, at the workplace, and in the doctor's office provides the social context that can facilitate behavior change. Health promotion for children should help establish lifelong habits for maintaining health. For adults, the emphasis should be on modifying risk factors related to disease and disability and maintaining healthy behaviors (Institute of Medicine, 1990a; Keil et al., 1989; Pinsky et al., 1985).
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Disability in America: Toward a National Agenda for Prevention Secondary Prevention Secondary prevention activities include early detection and treatment of persons with early or asymptomatic disease, reduction in risk factors, vocational and educational counseling, and social interventions. Common approaches include periodic screening of high-risk individuals and subsequent treatment of the pathology. Secondary prevention can in many cases cure a specific pathology, but in other cases secondary prevention merely slows the progression of a pathology toward becoming a clinical condition. People with chronic diseases and those with disabling conditions can benefit significantly from secondary prevention efforts, and, as noted earlier, much of this report focuses on secondary and tertiary prevention. Not all diseases and disabling conditions, however, can be prevented. Examples include conditions that are strongly related to the process of aging (Fries and Crapo, 1981). Aging-related conditions include arteriosclerosis, non-insulin-dependent diabetes, cancer, osteoarthritis, emphysema, and cirrhosis, as well as numerous other conditions that are less prevalent. Prevention measures are applied differently to aging-related conditions because individuals are seldom observed to be totally free of pathogenic changes. Plaque deposits in arteries, for example, can be found in most individuals, even at very young ages. Fries and Crapo (1981) argue, therefore, that it is better to think of controlling (or eliminating) risk factors to affect the progression of these conditions rather than to prevent the onset of the underlying pathological process. They maintain that primary prevention of aging-related conditions is not possible because such conditions are a part of aging and occur in all individuals. However, the rate at which such universal conditions progress can be reduced so that clinically significant symptoms can be avoided or delayed. Thus prevention of many aging-related conditions begins with secondary prevention that aims to reduce the progression of these universal pathological processes. In some instances, although the condition may not disappear, secondary prevention is considered successful if from the standpoint of the affected individual the symptoms are not noticeable and do not require clinical treatment. In such cases the condition in essence has been prevented. What are often considered to be primary prevention activities, such as not smoking, are often secondary interventions for many aging-related, and potentially disabling, conditions because the condition has already been initiated. Tertiary Prevention Tertiary prevention strategies concentrate on arresting the progression of a condition and on preventing or limiting additional impairment, functional
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Disability in America: Toward a National Agenda for Prevention limitation, and disability. These strategies can be directed toward the person, his or her environment, or society as a whole. Rehabilitation efforts, which attempt to restore function and the capacity to perform one's roles, are in the domain of tertiary prevention. Rehabilitation can address not only the individual with a functional limitation or disability but also elements of the physical and social environments that preclude participation in the activities of society by people with disabling conditions. Modifying or eliminating social and physical obstacles to personal autonomy and societal participation present opportunities for prevention strategies that are not often enough accepted into the traditional province of public health. Measures designed to foster independent living and help ensure a reasonable quality of life should clearly be major elements of disability prevention policies and strategies. Tertiary prevention, as well as secondary prevention, has not received as much emphasis in public health as the health-promoting, disease-preventing measures of primary prevention. However, the fact that more than 35 million people already have some type of disabling condition underscores the need to develop and implement secondary and tertiary prevention strategies that are directed toward people with disabling conditions, and that will reduce the risks of additional limitation and prevent disability and secondary conditions. With the aging of the population there is growing interest in the prevention of age-related chronic disease and disability and the secondary and tertiary strategies that are designed to prevent them (Patrick and Peach, 1989). Given the dynamics of the disabling process and the variety of interacting risk factors, primary, secondary, and tertiary preventive measures will often be required in concert. To take AIDS as an example, primary prevention is needed in the form of educating individuals about high-risk behaviors. Testing for exposure to HIV, especially in high-risk populations, and treatment to postpone the progression of the disease to AIDS or the characteristic set of symptoms known as AIDS-related complex (ARC) constitute secondary prevention. Tertiary prevention includes rehabilitation programs and social services that seek to reduce the effects of AIDS or ARC so that affected people can perform desired roles and live independently. Another example is people who use wheelchairs and therefore have increased risk of developing pressure sores. Preventive intervention strategies would include passive restraints that prevent spinal cord injury in automobile crashes (primary prevention), modifying wheelchairs or teaching the individual who uses the wheelchair how to relieve pressure to reduce the likelihood of pressure sores (secondary prevention), and treating the sores to prevent infection and promote healing (tertiary prevention). In this, as in other cases, there are many opportunities to interrupt the disabling process and the progression toward disability.
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Disability in America: Toward a National Agenda for Prevention Finally, although tertiary prevention might be where most prevention of disability itself occurs, primary and secondary strategies are essential elements of disability prevention because they intervene in the disabling process to reduce the likelihood of progression of predisposing conditions toward disability. Thus the public health and medical aspects of disability prevention are important, but should not overshadow or undercut the essential understanding of the social context of disability, as described throughout this report. Given the existence of predisposing functional limitations, the predominant means of disability prevention and amelioration are often social and economic.
Representative terms from entire chapter: