The Burden of Mental Disorders: A Comparison of Methods between the Australian Burden of Disease Studies and the Global Burden of Disease Study

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Voir page 436 le resume en francais. En la pagina 436 figura un resumen en espanol.

The prominent position of mental disorders, particularly depression, as a cause of disease burden is a widely quoted result of the Global Burden of Disease (GBD) study (1-5). However, there has been little discussion of the methods used to estimate the mental health burden at either the global or national level. The general debate about the use of the disability-adjusted life year (DALY) as a summary measure of population health has largely concentrated on the underlying assumptions of age-weighting and discounting and the relevance of burden of disease measurements to policy-making (6-11).

One report has challenged the severity weights for mental disorders used in the GBD study. Findings from a small community sample in New South Wales, Australia, led to a cautious conclusion that the GBD study may have overestimated the disability weights (DWs) for depression and substance disorders, while underestimating the level of disability associated with anxiety disorders (12). However, the epidemiological assumptions that fed into the GBD study's calculation of the burden of mental disorders have not been scrutinized in the literature. Only three of the intended ten volumes in the Global Burden of Disease and Injury Series have been published to date (13-15); and a description of methods used in calculating the burden of mental disorders has not yet appeared. The only epidemiological information is in vol. 2 of the series, detailing for each disease and world region the age-specific and sex-specific values of incidence, prevalence, average duration and mortality.

As part of the two recent burden of disease studies in Australia, an effort was made to examine critically the GBD estimates for mental disorders, to improve the methods and to apply them to the most appropriate information on the epidemiology of mental disorders in the country. The results of the national Australian study conducted by the Australian Institute of Health and Welfare and of an analysis of the burden of disease in Victoria carried out by the state's Department of Human Services are available as printed reports and on the internet (16-19). The two project teams worked closely together and shared methods and analyses.

The methods used to estimate the burden of mental disorder in Australia are discussed below, and departures from those of the GBD study are identified and justified. The consequences are described and discussed of the methodological changes on the estimates for the state of Victoria and the results are compared with those of the GBD estimates for the Established Market Economy (EME) region. Although burden was estimated for dementia and other neurological conditions in the Australian studies, these conditions were included in a separate category for nervous system disorders and they are not discussed here.

Methods

The disability-adjusted life year

Summary measures of population health combine information on mortality and non-fatal health outcomes to describe population health in a single number. The DALY was designed to provide a common measure for fatal and non-fatal health outcomes, to allow estimates of health impact to be mapped to causes, and to enable common values and health standards to be applied to all regions of the world (13).

DALYs for a disease are the sum of the years of life lost because of premature mortality in the population and the years lost because of disability for incident cases of the health condition in question. The DALY is a health gap measure that extends the concept of potential years of life lost because of premature death to include equivalent years of healthy life lost in states of less than full health, broadly termed disability (20).

The Australian burden of disease studies depart from the general methodology used in the GBD study in the following key areas.

* The GBD study used a standard life table (West level 26) with a life expectancy at birth of 82.5 years for women and 80 years for men, whereas the Australian studies used the Australian cohort life expectancy (taking declining mortality trends into account) for 1996. This resulted in a life expectancy at birth of 85.7 years for women and 81.5 years for men.

* In the GBD study, DALYs were discounted at 3% and age-weighted. In the Australian studies, DALYs were discounted but were not age-weighted. Age-weighting is intended to capture a greater social responsibility in young and mid-adult life for the very young and old. Age-weighting was not used in the Australian studies because it is perceived as inequitable by some people and because the GBD sensitivity analyses showed that it did not essentially change the overall estimates of burden (13).

* In addition to DWs developed for the GBD study, the Australian studies used those developed by Dutch researchers (21, 22) for many conditions because of their greater detail and their focus on the most common disabilities found in countries of low mortality.

* The GBD study did not account for the occurrence of comorbid health states, whereas the Australian studies made adjustments for the effects of comorbidity between highly prevalent physical conditions, between mental disorders, and between injuries.

* The Australian studies included a wider range of disease and injury categories than the GBD study and provided a more detailed age breakdown of the burden of disease.

Data sources

Apart from deaths associated with drug overdoses and, to a lesser extent, alcohol dependence, the number of deaths in Australia attributed to mental disorders is small. We included as heroin deaths a substantial number of deaths coded under accidental poisoning due to opioids in the International Classification of Diseases, ninth revision (ICD-9).

The estimation of the disability associated with mental disorders requires information on the incidence, average duration, and severity of each disease and its sequelae. The incidence of mental disorders is rarely measured; surveys tend to report one-year prevalence (the number of people who experienced relevant symptoms at any time during the preceding 12 months). To derive the incidence, we made extensive use of the DisMod software package developed by Harvard University to examine the consistency between estimates of incidence, prevalence, duration and mortality (23).

The data sources are summarized in Table 1. The main source of prevalence data for adults was the National Mental Health and Wellbeing Survey (MHS) of 1997 (24), in which information was collected on symptoms experienced in the preceding 12 months, 1 month, and 2 weeks for a representative sample of 10560 adults. A computerized version of the Composite International Diagnostic Interview was used in this work. Interviews were completed for 78% of the individuals approached. The unit record data of the survey contained information on the prevalence of mental disorders by ICD-10 and DSM-IV (Diagnostic and Statistical Manual of Mental Disorders) categories as well as a number of measures of disability, namely the abbreviated Short Form (SF-12), the General Health Questionnaire (GHQ), the Brief Disability Questionnaire (BDQ) and the Kessler psychological distress scale. We used the ICD-10 diagnoses for consistency with our other disease categories which were largely based on ICD-9 categories. The only exception was post-traumatic stress disorder (PTSD), for which we used the DSM-IV diagnosis, because the ICD-10 criteria were too broad and would have resulted in overestimation due to misclassification of other anxiety disorders (25). One of the modules of the Composite International Diagnostic Interview on mania was omitted from the survey, and this meant that estimates of bipolar disorder were inaccurate. Instead, we relied on estimates from international epidemiological studies (26).

Table 1. Data sources used in Australian burden of disease studies to estimate the incidence and/or prevalence of …