Understanding the Relationship Between Disease and Disability

The clinical assessment of disability in older persons began as an attempt to evaluate prognosis and service needs of persons with serious functional decrements in the rehabilitation setting. In the last 2 decades, the application of disability assessment has broadened considerably, and it now plays a role in activities ranging from national surveys of the older population to the determination of eligibility for informal and formal care. In population-based and clinical research studies, describing the functional status of older persons has added a very important dimension to the traditional medical approach, which characterizes persons mainly by the diseases from which they suffer. A great deal of progress in gerontology and geriatrics has undoubtedly been made because researchers turned away from the medical model of disease in order to understand how the overall functional problems of older persons affect the quality of their lives, their need for care, and their prognosis. Although this approach has been and will continue to be highly productive, we must now come full circle and begin to fit together a comprehensive picture that includes both disease and disability. And while we now believe that most disability is caused by diseases and injuries, the intricacies of the pathway from disease to disability, the specific diseases responsible for disability, and the mechanisms whereby disease causes disability all need elucidation. For a clinician who observes the functional consequences of a patient's stroke or hip fracture, the cause of that patient's disability is quite clear. But reversing the perspective and trying to piece together why an individual presenting with disability has reached this state is often not a simple matter. Taking this a step further and attempting to understand the diseases responsible for disability from a population perspective is particularly challenging. Research in this area is complex because multiple co-occurring conditions are common and can have a synergistic effect that is difficult to unravel, because disability may represent decline in several different aspects of functioning, each of which may be affected by a different disease, because the pathway from disease to disability may be influenced by many nondisease factors such as depression and social support and because of the potential for negative feedback loops, where disability leads to further disability or to new chronic medical conditions. Despite the methodologic obstacles, research has progressed on several fronts in recent years, and we are beginning to get a picture of how diseases and disability interrelate in the older population. Valuable research is being done on how specific impairments and functional limitations lead to disability in single diseases such as coronary heart disease,1 stroke,2,3 and arthritis.4,5 Cross-sectional and prospective studies have provided information about the association of disease and disability in representative populations. Boult and colleagues6 recently reviewed the findings from a number of prospective studies that evaluated the risk of incident disability according to disease status, and research continues to emerge that approaches the disease-disability relationship from various perspectives. This includes work on diseases predicting loss of higher level mobility,7 how the burden of disease affects disability in nursing home residents,8 the associations of clinically diagnosed disease with disability in the Framingham Study population,9 and the associations of diseases and physiologic impairments with disability in the Study of Osteoporotic Fractures10 and the Cardiovascular Health Study11 populations. The paper in this issue by Ettinger and colleagues12 adds both substantively and methodologically to this field. The authors provide evidence that if you want to know what condition is responsible for disability in a cognitively intact older person, just ask. A particular strength of their study is that it addresses specific disabilities. Global measures of disability would have obscured the consistent way that participants attributed both a disease and related symptom to a specific disability and the plausibility of the attributions (heart disease as a cause of disability in doing heavy housework, arthritis as a cause of disability in dressing). These findings make a good case that self-report in this area is valid, but, as the authors note, validity of this approach must ultimately be assessed by comparing self-report to an independent determination of the disease responsible for each specific disability. Participants attributed their disabilities to arthritis at a strikingly high rate. This important finding should be confirmed in research that employs clinical determinations of diseases responsible for disability. Arthritis is extremely common and might be the most obvious problem for individuals to report when their situation is complex. The methodology employed by Ettinger and colleagues12 is a departure from previous work in this area. Until now, evidence that a disease is a significant cause of disability in the population has come from cross-sectional and longitudinal studies that demonstrate that those with the disease are more likely to have or to develop disability than those without the disease. The limitation in this approach, however, is that it can not determine the specific disease actually responsible for disability in an individual, a critical piece of information that is the focus of the study by Ettinger et al. Disabled persons with both heart disease and arthritis are afflicted with two diseases shown in many studies to be associated with disability, but only one disease may be responsible for their disability. This determination may be made by individuals or their physicians or through more formalized research protocols. By knowing which disease is the underlying cause of a disability, new research in this field becomes possible. It would be of real interest, for example, to follow the progression of disability according to underlying cause in the population studied by Ettinger et al. The study of the natural history of disability would be greatly enhanced by being able to account for the principal diseases underlying individuals’ disabilities. Furthermore, our understanding of potentially modifiable factors that affect the onset and progression of disability could be considerably refined by knowing how these factors affect specific disease-disability combinations. The National Institute on Aging is now funding a research project, The Women's Health and Aging Study, that is studying factors related to change in functional status in disabled older women. A unique aspect of this study is the use by investigators of information from questionnaires, physical examinations, and medical records to adjudicate individuals’ underlying causes of disability. The reduction of disability in the older population must be approached with a mixture of strategies. Generic interventions that address a broad range of outcomes, such as exercise, are beginning to show real promise.13 But it is mainly disease that causes disability, and preventing disease or reducing its consequences is critical. Primary disease prevention, while the ultimate way to reduce disability, will never prevent all diseases of aging. Developing our understanding of the effect the major chronic diseases of aging have on disability and the many factors that influence the pathway from disease to disability is clearly a challenging but worthwhile pursuit. We still have a great deal to learn that may help reduce disability in those who have been lucky enough to survive to old age but have not had the good fortune to live to old age free of disease.