WHO's Global Burden of Disease report provides important evidence for the relative effects of health disorders worldwide,2,29
affecting prioritisation for policy making and planning nationally, regionally, and internationally. However, the rank ordering of the contributions of chronic diseases to disability that were noted in this report differ in important respects from those estimated from results of the 10/66 population-based surveys (). In our studies, dementia is overwhelmingly and consistently the largest contributor to disability. Sensory impairment, both of eyesight and hearing, and heart disease contributed much less to disability than was suggested by the Global Burden of Disease estimates. According to our findings, stroke and arthritis merit a high ranking, especially since some of the effect of limb paralysis or weakness (with a median PAPF of 10·5%) almost certainly arises from these two diagnoses, which could well have been under-reported in our surveys.
Contributions of chronic diseases and disorders to disability according to Global Burden of Disease estimates of years lived with disability¶ and median population-attributable prevalence fractions from 10/66 population-based studies
Zero-inflated negative binomial regression was the most appropriate statistical method for modelling of WHODAS 2.0 disability scores, especially in a cross-cultural context. Score distributions were overdispersed and zero-inflated in all sites, and ZINB models fitted better than did either Poisson or negative binomial. The large between-site variation in zero inflation is an important finding, with the positive association with zero inflation in China and inverse association in India being most parsimoniously interpreted as a culturally determined predisposition to so-called nay-saying in China and yea-saying in the Indian setting. Having accounted for zero inflation, the count (negative binomial) part of the ZINB model provides the best, most culturally fair perspective on the contribution of chronic diseases to overall disability (). Unfortunately, since this method models the ratio of counts, we were unable to use these values to generate estimates of population effect (either PAPFs or components of variance explained). That our PAPFs had to be generated from a Poisson model () is therefore a slight limitation.
Our findings are consistent with a report from the Canadian Study of Health and Aging30
of a substantial excess disability attributable to dementia, after accounting, in multivariate models for comorbidity with physical, mental, and substance-use disorders. In population-based cohort studies of predictors of dependency31
in elderly people in the USA, multivariable analyses show that dementia and cognitive impairment are by far the most strongly and independently associated chronic health disorders. Psychiatric disorders and stroke also made an important independent contribution to dependency.31
Coronary heart disease, cancer, hypertension, lung disease, diabetes, and hip fracture did not predict dependency, and cardiovascular disease, arthritis, and lung disease were not associated with institutionalisation. Few such studies have been done in countries with low and middle incomes. However, in a cross-sectional study33
of elderly Chinese people living in Hong Kong, dementia (odds ratio 157·1), stroke (19·3), Parkinson's disease (14·2), and old fractures (2·5) were the chronic disorders most strongly associated with severe limitation. Finally, previous analyses of the same 10/66 Dementia Research Group dataset have shown that dementia is the largest independent chronic disease contributor to dependency, with a PAPF of 65% in Cuba34
and 44% in Dominican Republic.35
Some limitations need to be acknowledged. First, we did not include all chronic-disease domains in our analyses. Presence of cancer or endocrine, genitourinary, and oral disorders was not ascertained. However, according to the Global Burden of Disease report,2
these excluded disorders make a small contribution to disability, and those that were covered in our study generally accounted for a substantial proportion of disability. Second, and more importantly, not all disorders that were included were ascertained with equivalent rigour. Amartya Sen emphasised the problem of self-report, pointing out that “people in states that provide more education and better health facilities are in a better position to diagnose and perceive their own morbidities than are the people in less advantaged states, where there is less awareness of treatable conditions (to be distinguished from ‘natural’ states of being)”.36
Our data did not provide very strong evidence for this problem, other than, perhaps, with respect to the low prevalence of self-reported stroke in rural China and India, myocardial infarction or angina in rural Mexico, China, and India, and diabetes in rural India and rural China.
Prevalences of all self-reported impairments were strikingly low in rural compared with urban China, and eyesight problems were infrequently reported in urban India. We cannot exclude the possibility that had we diagnosed some of these disorders by more rigorous clinical assessment, we might have identified more morbidity. By extension, we might have underestimated their contribution to disability in the population with respect to that of dementia and depression, which were diagnosed through detailed clinical interviews. However, despite the fairly low prevalence of eyesight problems in rural China and in India, high PAPFs were recorded, presumably because of poor access to ophthalmic services, and because we selectively detected the most disabling cases.
Third, our data are cross-sectional. Therefore we cannot attribute causality from the recorded associations between health disorders and disability. Some associations might have been inflated by reverse causality, thus depression can be both a consequence and a cause of disability.37
Information bias could have occurred, since participants with disabilities might have been selectively more or less likely to have recalled impairments or to have been aware of diagnoses than were those without disabilities. Although population-attributable fractions are conventionally calculated from RRs for incident health outcomes, associations with prevalent disability might be more pertinent to our aims. Our PAPFs incorporate the effect of underlying health disorders on the incidence and duration of disability, and are hence analogous with the YLD approach that was used for the Global Burden of Disease report.2
The need to internationalise the disability research agenda is the subject of an important debate. Limitations arise from differences in disability definitions, study methods, and qualities of research across cultural contexts. The major strength of our study is the standard design and assessment procedures, in large representative samples, with high response rates, across three continents. From the outset, the 10/66 Dementia Research Group has been committed to careful cross-cultural validation,21
and we have now attempted to extend this approach to assessment of disability,11
in accordance with the evidence already assembled by WHO for the cross-cultural applicability of the International Classification of Functioning, Disability and Health1
and the WHODAS 2.0 assessment. Although the approach we have used to estimate the contribution of chronic diseases to disability could not replace that used for the Global Burden of Disease report,2
comparison of the results of these two exercises is illuminating. Such comparison does, at the least, raise important questions about the reasons for the large discrepancies, which would merit further exploration. One possibility is that societal preferences (the Global Burden of Disease disability weights) might not accurately show individual experiences of living with chronic disease (as assessed by the WHODAS 2.0).
Our findings should help to inform debates about priorities for health-service delivery and planning in countries with low and middle incomes. Our findings concur with those of the Global Burden of Disease report,2
to the extent that the leading causes of disability are very different from the causes of premature death, namely cancer and ischaemic heart disease. The chronic-disease agenda is dominated by prevention of avoidable deaths, and is hence skewed towards primary prevention of these disorders.38
Of course, prevention of chronic diseases also prevents disability. However, under the most optimistic of scenarios, the numbers of elderly people living with disabling chronic diseases will continue to rise, especially in countries with low and middle incomes. Our data suggest the need, in particular, for higher priority to be accorded to chronic diseases affecting the brain and mind. Aside from disability, these disorders are very likely to lead to dependency, and to present stressful, complex, long-term challenges to carers. The associated societal costs are enormous—those for dementia alone were estimated as US$315 billion per year worldwide.39
These individuals, and their families, are very poorly served by health services that remain focused on treatment of acute disorders, and do not provide outreach or continuing care; social protection for elderly people in many countries with low and middle incomes is also grossly inadequate.40
A comprehensive response to these challenges will need policies to: prevent disability through control of chronic diseases; reduce disability through active community-based rehabilitation; mitigate effects of disability on participation; and manage disability through universal access to support for family carers and other long-term care options. Such measures are already strongly advocated through international agreements, including the Madrid International Plan for Action on Ageing, and the UN Convention on the Rights of Persons with Disabilities enshrines participation, income, and access to health care as basic rights for all people with disabilities.