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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 59:M86-M93 (2004)
© 2004 The Gerontological Society of America

Advancing the Taxonomy of Disability in Older Adults

Suzanne G. Leveille1,2, Linda P. Fried3, William McMullen1 and Jack M. Guralnik4

1 Hebrew Rehabilitation Center for Aged, Boston, Massachusetts.
2 Harvard Medical School, Boston, Massachusetts.
3 Schools of Medicine and Public Health, Johns Hopkins University, Baltimore, Maryland.
4 Laboratory of Epidemiology, Demography, and Biometry, National Institute on Aging, Bethesda, Maryland.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 Appendix
 References
 
Background. Refinement of the traditional task-based classification of disability is needed to advance the field of disability research and to inform clinical practice. We propose an enhancement of the taxonomy of disability that incorporates information about symptoms and impairments that directly contribute to disability. In the first step of this new development, we present evidence to support five categories of disability related to pain, balance, weakness, endurance, and other symptoms.

Methods. The Women's Health and Aging Study, begun in 1992, was a population-based study of older disabled women living in the Eastern region of Baltimore and surrounds. In-home examinations were conducted every 6 months for 3 years. During the second examination, the 879 participants were asked the main symptom cause of their disability for each activity with which they had difficulty. Symptom causes of disability in activities of daily living were grouped into five nonmutually exclusive categories as stated above. Descriptive analyses were used to compare groups according to sociodemographic, health, disease, and physical performance information.

Results. Women with pain-related disability were younger, more obese, and had high prevalence of widespread pain and symptomatic osteoarthritis. Balance-related disability was related to having an age of 85 years or older, being Caucasian, and poor performance in tests of standing balance and gait. Women with weakness-related disability were older, more often African American and sedentary, had high prevalence of stroke and diabetes, and performed poorly in chair-stands and knee strength tests. Endurance-related disability was associated with low self-rated energy, depressive symptoms, smoking history, and lung and cardiovascular diseases.

Conclusions. The proposed refinement of the taxonomy of disability describes a set of empirically derived symptom and impairment-related disability groupings that have criterion and face validity. Further research about symptom and impairment-related disability in other populations of older persons is warranted.

THE term disability refers to a broad range of chronic restrictions to routine life maintenance and self-care activities that are essential for independent living. For three decades, researchers and clinicians have typically classified age-associated physical disability within two domains, basic self care activities known as activities of daily living (ADL) (1) and instrumental activities of daily living (IADL) (2). The former generally refers to bathing, dressing, transferring from a bed or chair, using the toilet, and eating. The latter, IADL, refers to more complex or demanding tasks including housework, preparing meals, shopping, using the telephone, and managing money. These global task-based classifications continue to provide a useful nomenclature across health and social services and research. However, advances in our understanding of the causes and dimensions of disability have readied the field of disability research for further refinement of the taxonomy of disability.

The multifactorial nature of age-associated disability is generally accepted by aging researchers. Because of the profound compensatory ability of the human organism, disability often cannot be definitively attributed to a single deficit, except in the case of a catastrophic problem such as stroke or hip fracture. In general, people do not readily succumb to singular deficits unless other preexisting conditions have already reduced their functional ability. In spite of this, most older disabled persons can point to a single main cause of their disability in a specific task such as bathing, walking, or shopping (3,4). The research evidence supports the validity of self-reported causes of disability, particularly self-reported symptoms and impairments (3,4).

Years ago, the establishment of measures of IADL and ADL were the early steps in defining functional status (1,2). Nagi, and later, Verbrugge and Jette, moved the field forward in their delineation of the theoretical pathway from pathology to disability (5,6). In this article, we propose that a classification of disability based on symptoms and impairments that are major causes of disability will provide the next step in the development of the disablement taxonomy. Our proposed classification identifies five general categories of disability related to pain, weakness, balance, endurance, and other symptoms. This classification is based on evidence supporting the disablement pathway and provides a more-defined focus for research and clinical practice in the area of disability prevention and management. As a first step in this conceptual development, we present a descriptive analysis of self-report and physical performance information from a population of older disabled women. Our initial focus is on ADL because they are the most essential for independent living and are widely used in clinical practice.


   METHODS
Top
 Abstract
 Methods
Results
 Discussion
 Appendix
 References
 
Study participants were 879 disabled women who participated in the 6-month exam (round 2) of the Women's Health and Aging Study (WHAS), a 3-year follow-up study of the course and consequences of disability in older women. The 6-month follow-up interview was the first exam during which participants were queried about symptoms they thought caused their disability, the focus of this report. Participants in the study were aged 65 and older and lived in the community in the Eastern region of the City and County of Baltimore, Maryland. Eligibility was based on mild-to-severe disability in at least two of four domains of functioning (ADL, IADL, upper extremity function, and lower extremity mobility). Women were excluded who had moderate or severe cognitive impairment based on a Mini-Mental State Examination (MMSE) (7) score below 18, a cut-point that is valid for dementia screening in low income and minority populations (8,9). Of the 5316 women sampled from the Medicare enrollment files who were alive and living at home, 4137 (78%) participated in the home-based screening. Of these, 1409 were eligible and 1002 (71%) agreed to participate and signed the informed consent, approved by the Institutional Review Board of The Johns Hopkins Medical Institutions. Interviews and tests of physical performance were conducted in participants' homes every 6 months for 3 years. Details of the methods and more detailed descriptions of the cohort have been published previously (10,11).

Participants were asked whether or not they had difficulty or were unable to perform tasks within the four domains of functioning mentioned above. In this report, we address ADL disability (bathing, dressing, transferring from bed or chair, eating, and using the toilet). In health care, clinicians, social service providers, and researchers assess ADL ability as a key indicator of functional status and need for supportive services including long-term care. For each task, participants were asked the main symptom that caused them to have difficulty or inability, and were shown a list of 34 symptoms and impairments. We used an empirical approach to group symptoms and impairments into five general impairment categories: pain, balance, endurance, weakness, and other symptoms (see Appendix). The reference category included women who did not report any ADL difficulty. Because participants could report more than one cause if they reported disability in more than one task, the categories were not mutually exclusive, with the exception of the non-ADL disabled reference category. There were 19 women (3.2%) who identified main causes of their ADL disability that were not on the list of 34 symptoms and impairments. These included recent fractures (wearing casts) and a variety of vague conditions related to mobility limitations. This set of causes was included in the "other symptoms" category.


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  		Categories of Impairment-Related Disability and Possible Main Symptom or Impairment Causes of Disability.

 
Sociodemographic and health characteristics measured at baseline that were relevant for the present analyses included age, race, height, weight, cognitive functioning, smoking status, and prevalent chronic conditions. Body mass index (BMI) was calculated as measured weight in kilograms divided by measured height in squared meters. Measures from the 6-month follow-up examination, concurrent with the ADL measures, included pain, activity, psychosocial factors, and physical performance. Participants were asked to rate their pain at several joint sites and in their back using a numeric rating scale, with 0 indicating no pain and 10 indicating excruciating pain. Pain was categorized hierarchically according to location and severity, as follows: widespread pain (upper extremity, lower extremity, and axial pain), moderate pain in one or more sites (rated >=4), mild pain at one or more sites (rated 1–3), and no pain (rated 0). Previous WHAS studies showed that pain categorized in this way predicted incident falls and progression of disability (12,13). Physical activity was categorized using a three-level variable based on a summary score of amount of walking, stair-climbing, heavy housework, regular exercise, dancing, bowling, and outdoor chores (14). Moderate or severe depression was based on a score of 14 or above on the Geriatric Depression Scale (15). To measure self-perceived energy, participants were asked to rate their energy level in the previous month on a scale from 0 to 10, with 0 indicating no energy and 10, the "most you have ever had." Energy ratings were grouped into three levels as follows: low [0–3], medium [4–6], and high [7–10].

Validation of the presence of major chronic conditions at baseline (angina, hip fracture, osteoarthritis of the hand, knee, and hip, stroke, pulmonary disease, peripheral arterial disease, diabetes mellitus) was determined using complex clinical decision algorithms incorporating information, as indicated, from in-home interviews, physician diagnoses, physical examinations, medication inventories, and hand photos, as well as hip and knee radiographs, hospital records, physician questionnaires, and blood tests performed for the study (16). Symptomatic hip, knee, and hand osteoarthritis were assessed using the American College of Rheumatology criteria for osteoarthritis (17). Interrater agreement by three epidemiologist-clinicians using the algorithms for these three conditions ranged from {kappa} = 0.93–1.0 (16).

Physical performance tests included measures of gait, balance, strength, and endurance (10). Slow gait (<0.43 m/s) was determined from a timed usual-pace, 4-meter walk. The chair-stand test measured the time required to stand 5 times as quickly as possible from a straight-backed chair with arms folded across the chest. Balance was tested using three 10-second stands: standing with feet touching side-by-side, semitandem stand with the side of the heel of one foot touching the side of the big toe of the other foot, and full tandem stand with the heel of one foot touching the toes of the other foot (18). Balance impairment was defined as a score of 0, indicating inability to stand unassisted.

The Short Physical Performance Battery (SPPB) score was a summed score based on 6-month follow-up results of the gait, balance, and chair-stands tests, where each component was scored 0 to 4, for a total score of 0 to 12. The SPPB score, a measure of lower extremity function, has been shown to predict disability, hospitalization, and death in several elderly cohorts (19–22). Maximal knee extension and hip flexion strength, measured at the baseline examination, were tested using a hand-held dynamometer (Nichols Manual Muscle Tester, Fred Sammons, Inc., Burr Ridge, IL), with two trials for each leg. Low knee or hip strength, or the lowest tertile of strength, was determined based on the average of the maximum strength for each leg as a proportion of kilogram of body weight.

Statistical Analysis
Analyses were cross-sectional and primarily used descriptive statistics. Sociodemographic and health characteristics and physical performance measurements were presented using frequency percents within categories of impairment-specific disability. Differences between the non-ADL disabled women and women in the ADL disability categories were tested using the chi-square test.


   RESULTS
Top
 Abstract
 Methods
 Results
Discussion
 Appendix
 References
 
Of the 88% of WHAS participants who completed the first follow-up examination (n = 879), more than one quarter were African American (29%) and the rest were nearly all Caucasians. Reflecting their low socioeconomic status, only one third of the women completed high school and about half lived alone (49%). In general, women with ADL disabilities were older than women who did not have ADL disability (mean ages, 79.2 ± 8.2 years and 75.4 ± 7.1 years, respectively). Among the 598 women (68%) who reported difficulty with one or more ADLs, pain was the most commonly identified cause of their disability, reported by 58% of the ADL-disabled women (Figure 1). The next most commonly reported symptomatic cause of disability was weakness (31%).



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  		Figure 1. Percent of women with activities of daily living disability within categories of causes of disability, first 6-month follow-up, Women's Health and Aging Study. Symptom-related disability categories are not mutually exclusive; the proportions that reported more than one main cause of disability (overlapping) are indicated in the shaded bar segments. ADL = Activities of Daily Living

 
Among the disabled women, symptoms associated with disability varied with age (Table 1). Of the women who had ADL disability, those with pain- or endurance-related disability were somewhat younger than women with balance-related disability. The majority of women who reported balance-related ADL disability (52%) were aged 85 years and older, compared with 30% of women with pain-related disability. In general, women with pain-related disability were more overweight than other ADL-disabled women, with an average BMI of 30.5 ± 7.3, compared with 26.9 ± 6.6 for other women.


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  		Table 1. Demographic and Health Characteristics According to Category of Reported Symptom Causes of ADL Disability Among 281 Non-ADL-Disabled and 598 ADL-Disabled Women, 6-Month Interview, Women's Health and Aging Study, 1992–1995.

 
As expected, women with ADL disability were less physically active than non-ADL disabled women, but women with pain-related disability and those in the "other" symptoms category were somewhat more active than those with ADL disability attributed to balance, weakness, or endurance symptoms (Table 1). Prevalence of depressive symptoms was substantially higher among ADL-disabled women compared to other women. However, among ADL-disabled women, those with pain-related disability had the lowest prevalence of depressive symptoms (22.4%), though only slightly lower than other ADL-disabled groups. The average physical performance score, a measure of lower extremity mobility, was 4.0 to 5.5 in ADL disability groups, compared to an average score of 7.9 in non-ADL disabled women. Half of women with balance- or weakness-related disability scored in the lowest category of the performance battery (scores of 0–3), contrasted with about one third of women with ADL disability attributed to other causes.

Prevalence of low energy and musculoskeletal pain were as expected within the disability groupings. Nearly one third of women with endurance-related disability had low energy (self-rated as 0–3 on a 0–10 scale), a much greater proportion than any other ADL-disabled or non-ADL disabled group (Table 2). Also, moderate-to-severe pain and widespread pain were most common among women who reported that pain was the main cause of their disability.


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  		Table 2. Symptoms and Chronic Conditions According to Category of Reported Symptom Causes of ADL Disability Among 879 Women, 6-Month Interview, Women's Health and Aging Study, 1992–1995.

 
There were substantial differences between groups in the proportions with confirmed major chronic diseases. Symptomatic osteoarthritis (OA) of the hip, knee, or hand was most common in women who reported pain-related disability. More than half of women with pain-related disability had symptomatic knee OA compared with one quarter to one third of women in other disability categories. Lung disease was most common in women with endurance-related disability (56.4%), and stroke was most common in women who had weakness-related disability (16.7%). Women with weakness- or endurance-related disability had the highest prevalence of diabetes (24% and 26%, respectively).

Compared to WHAS participants with no ADL disability, women with ADL disability had markedly higher prevalence of physical impairments, determined from physical performance testing (Table 3). Differences in impairments between the disability categories were evident but often modest. Approximately half of women with balance- or weakness-related disability had slow gait or were unable to perform the chair-stand test, compared with one quarter to one third of other ADL-disabled women. Also, those with balance-related disability had the highest prevalence of impairments in standing balance. One third to one half of all ADL-disabled groups exhibited hip or knee strength impairments, but those who reported weakness-related disability had, in general, poor performance on both strength measures.


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  		Table 3. Percent With Physical Impairments According to Category of Reported Symptom Causes of ADL Disability Among 879 Women, 6-Month Interview, Women's Health and Aging Study, 1992–1995.

 
We conducted a set of analyses to better understand the composition of the disability categories. Because there were five ADL tasks studied, women potentially could report up to five different causes of their ADL disability if they had disability in all five ADL tasks. Of the 369 women who had disability in two or more ADL tasks, 181 (49%) reported more than one main cause of disability. Across the symptom–disability categories, there were minimal differences in the average number of ADL task difficulties, ranging 2.4–2.8 across ADL categories. Among those who reported pain-related ADL disability, one third also reported disabilities due to other impairments, the lowest proportion with mixed causes of disability of the five impairment categories (Figure 1). In all other categories, more than half of women had more than one main cause of disability. Women with endurance-related disability were the most likely to report more than one cause of their disability (64%).


   DISCUSSION
Top
 Abstract
 Methods
 Results
 Discussion
Appendix
 References
 
This research presents a modification of the traditional approach to conceptualizing disability in older persons. Our results provide evidence that a symptom and impairment-based classification of disability is valid and feasible. Disability in daily self-care activities in older women is largely due to a limited set of symptoms and impairments within five categories: pain, balance, weakness, endurance, or other symptoms. From a list of 34 possible causes of disability, there were only 17 symptoms or impairments identified by at least 1% of older disabled women as main causes of their disability (4). To date, a long list of risk factors, impairments, and limitations have been identified in epidemiologic studies as causes of disability in the older population (23). However, our findings suggest that if we listen to reports by disabled older women themselves, we may gain important information about some of the most disabling factors affecting their daily lives. This is a critical step if we are to refine functional assessment and develop more targeted approaches to treat and manage disability in older adults.

We are proposing that, in the same way that patient's self report of symptoms is essential in diagnosing chronic diseases, self-report of the symptomatic causes of disability is informative to our understanding disability, for both clinical and research purposes. Unless they are cognitively impaired, people are likely to know why they have stopped or why they are experiencing difficulties in performing essential self-care activities. These questions have face validity. Our earlier findings showed that women were better able to identify symptoms than conditions that caused their disability (4). The main cause of disability that a person reports is likely to be the most apparent symptom or impairment that interferes with her or his activity. In other words, other impairments may predispose an individual to disability such as a progressive loss of muscle strength, but factors such as pain, impaired balance, or fatigue are likely to have an immediate and noticeable impact on daily functioning and, therefore, are the most likely to be reported. In terms of the disablement pathway, these impairments are generally proximal to disability end-points (6).

This study showed a number of associations that support the criterion validity of self-reported causes of ADL disability. We found that moderate or worse pain was more prevalent among those reporting pain-related disability. Symptomatic osteoarthritis was common across groups but most common in those with pain-related disability. The greatest prevalence of lung disease was found among women who reported endurance-related disability. This latter group also rated their energy level lower than any other group. Women who reported balance-related disability performed worse than others on the balance performance tests. Those who reported symptoms of weakness as the main cause of disability were the least able to perform the chair-stands test and had the poorest knee strength. Despite the high prevalence of physical impairments in this group of older disabled women, when asked, they were able to identify a single main cause of their disability that was consistent with objective performance measures and other symptom assessment information.

These findings provide clues about underlying causes of disability. Women with balance or weakness-related disability tended to be older, had lower BMI, and performed worse on the physical performance battery than other women, suggesting that frailty or sarcopenia may be contributors to their disability. Pain-related disability was more of a problem among younger women, consistent with epidemiologic evidence that pain prevalence peaks in the 65-to-85-year-old age group then decreases in advanced ages (12,24). African-American women were less likely than Caucasians to report balance-related disability but more likely to report that weakness caused their disability, possibly related to higher prevalence of stroke and diabetes in African Americans (25,26). Diabetes often leads to comorbid conditions and can contribute to disability through a variety of pathways (27–29). More women with pain-related disability were obese, compared with any other group, in part related to greater risk for OA with obesity (30), and possibly also related to fat-associated increases in inflammation (31–33).

The categories of symptom and impairment-related disability described in this article are intended to serve as a guide for future research. We are not proposing that the symptom and impairment categories are fixed, distinct, or comprehensive. Findings from Table 3 confirm that women with ADL disability have a high prevalence of several types of impairments regardless of the main cause of their disability. Earlier WHAS research showed that coimpairments, for example, impairments in both strength and balance, are very common and disabling in older women (18,34). Coimpairments could not be determined using the self-report data because participants were not given the option to identify more than one symptom or impairment cause of their disability in a specific task. Also, it is possible that the main cause of disability at one time point may not be the main cause at a later time, since impairments are likely to evolve and accumulate over time. For example, limited movement due to pain could lead to reduced strength and possibly also balance problems in subsequent years. However, better assessment and management of pain-related disability in a 70-year-old could potentially prevent a weakness-related disability later when that same person is aged 80 years or older. Also, treatment of balance-related disability in an 85-year-old often will require comprehensive physical rehabilitation to enhance strength, endurance, and balance. More research is needed to understand the evolution of physical impairments, including coimpairments, and their relationship to disability with the goal of improving treatment and prevention.

Another consideration is the role of frailty in the development of disability. The term frailty generally refers to a state of biological decline associated with aging that is marked by increased vulnerability to disability, disease, and mortality (35–37). Efforts to characterize frailty have targeted pathological aging changes as well as physical impairments. Biomarkers of inflammation, clotting abnormalities, and metabolic dysregulation (37–39), and measures of gait, balance, and strength impairments have all been included in proposed definitions of frailty (36,40). It is generally agreed that frailty is a pathological state that leads to physical impairments, functional limitations, and disability, and, as such, frailty belongs at the origin of the Nagi disablement pathway. Our impairment-related classification will enhance efforts to treat disability whether it is due to frailty or disease pathology.

Although several studies have found that cognitive and emotional problems contribute to risk for disability (23,41–44), no WHAS participants attributed their ADL disability to an emotional problem, and fewer than 3% reported a cognitive factor as a main cause of difficulty, and only for shopping activity. The list of symptom causes of disability (Appendix) included cognitive and emotional problems. However, because of the low prevalence of these reported causes, cognitive or emotional problems were not included in the proposed classification of disability. In general, study participants may be more willing to identify physical problems than emotional or cognitive problems as main causes of their disability. Also, women with moderate-to-severe cognitive impairment were excluded from WHAS, limiting the prevalence of cognition-related disability. Future work in the development of the classification may determine valid cognitive or psychological impairment-related categories of disability.

Our findings are derived from one of the only population-based studies of disabled older women and may be generalizable to similar urban populations in the United States. Participants in the WHAS represented the one third of older women living in the community with disability (10). Another population-based study of older adults, the Cardiovascular Health Study, showed the validity of self-reported symptom information, and also found that musculoskeletal pain was the most commonly reported symptom cause of disability, followed by shortness of breath, fatigue, and weakness (3). Further studies are warranted to confirm these findings in other populations including men, other minority groups, and persons with incident disability.

Our classification of self-reported main causes of ADL disability in older women introduces the next step in the taxonomy of disability. Symptom and impairment-based disability goes beyond measurement of task difficulty to identify proximal causes of disability for further investigation by clinicians and researchers. Clinicians may consider asking older patients about their perceived causes of their activity difficulties to develop treatment plans that address symptom- and impairment-related disability as well as underlying pathology. Research is needed to evaluate and further refine the proposed categorization of disability across domains of functioning. Additional validation of the classification against objective performance measures in other populations is needed. Ultimately, new approaches to prevention and treatment of disability that incorporate the identified disability categories could be developed and tested.


   Appendix
Top
 Abstract
 Methods
 Results
 Discussion
 Appendix
References
 


   Acknowledgments
 
Our work was supported by an Arthritis Investigator Award from the Arthritis Foundation and by National Institute on Aging contract PSC10.11.93004. The Women's Health and Aging Study was supported by National Institute on Aging contract NO1-AG12112.

Address correspondence to Suzanne G. Leveille, PhD, HRCA Research and Training Institute, 1200 Centre Street, Boston, MA 02131-1097. E-mail: leveille{at}mail.hrca.harvard.edu

Received June 23, 2003

Accepted July 30, 2003


   References
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 Abstract
 Methods
 Results
 Discussion
 Appendix
 References
 

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