Global Programme on Evidence for Health Policy, WHO
Department of Psychiatry, Universidad Autónoma de Madrid, Spain
Global Programme on Evidence for Health Policy, WHO
Correspondence: T. B. Üstün, Classification, Assessment, Surveys and Terminology Team, World Health Organization, Avenue Appia 20, CH-1211, Geneva 27, Switzerland. Tel: +41 22 7913609; fax: +41 22 7914885/4160; e-mail: ustunb{at}who.int
See pp.
393403 and
editorial, pp.
379380, this
issue.
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ABSTRACT |
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Aims To presentthe new estimates of depression burden for the year 2000.
Method DALYs for depressive disorders in each world region were calculated, based on new estimates of mortality, prevalence, incidence, average age at onset, duration and disability severity.
Results Depression is the fourth leading cause of disease burden, accounting for 4.4% of total DALYs in the year 2000, and it causes the largest amount of non-fatal burden, accounting for almost 12% of all total years lived with disability worldwide.
Conclusions These data on the burden of depression worldwide represent a major public health problem that affects patients and society.
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INTRODUCTION |
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METHOD |
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A major focus of the GBD 2000 project was to improve the comparability, validity and reliability of the descriptive epidemiology for mortality and non-fatal health outcomes attributed to various diseases, injuries and risk factors. The GBD 2000 working group identified the need to review the epidemiological estimates and disease models developed for neuropsychiatric disorders that had been used in the original GBD projects assessment of disability. As a consequence of this review process, major changes have been made in the GBD 2000 study in the conceptualisation of depression, and new epidemiological estimates have been used for estimating DALYs.
Case definition
The definitions of depressive episodes established by the ICD10 were
used (codes F 32, F 33) (World Health
Organization, 1992). Major depressive episodes as defined in
DSMIV (American Psychiatric
Association, 1994) and DSMIIIR
(American Psychiatric Association,
1987) were considered alternative definitions, so that we would be
able to use epidemiological data from survey studies employing either of these
two standard psychiatric classification systems.
Regions
For geographic disaggregation of the GBD 2000, the six WHO regions of the
world (i.e. Africa, Americas, Eastern Mediterranean, Europe, South-East Asia,
Western Pacific) were further divided into 17 epidemiological subregions,
based on levels of child (under 5 years) and adult (1559 years)
mortality for WHO member states. Five mortality strata were defined in terms
of quintiles of the distribution of child and adult mortality (both genders
combined). When these mortality strata are applied to the six WHO regions,
they produce 14 mortality subregions. For the purposes of burden of disease
epidemiological analyses, two of these regions were further subdivided: EurB
into EurB1 and EurB2, the latter including the central Asian states; and WprB
into WprB1 (mainly China), WprB2 (South-East Asian countries) and WprB3
(Pacific Islands). Additionally, some member states have been reclassified
into subregions with similar epidemiological/geographical/ethnic patterns in
order to maximise the epidemiological homogeneity of the subregions for the
purposes of epidemiological analysis. A detailed table of these
epidemiological subregions can be downloaded from the WHO website at
http://www.who.int/whr/2003/en/member_states_182-184_en.pdf.
Years of life lost to mortality due to depression
The first analytical step in the GBD 2000 study was to estimate the
age-specific death rates, by gender, for the GBD subregions for the year 2000.
The number of deaths, by age and gender, provides an essential
envelope which constrains individual disease and injury
estimates of deaths. Competing claims for the magnitude of deaths from various
causes must be reconciled within this envelope. From the estimated
age-specific mortality rates, life tables for the populations of the
subregions can be derived using standard methods. The sources of mortality
data for GBD 2000 estimates are described elsewhere
(Mathers et al,
2002). Table 1
presents world deaths related to depression and other neuropsychiatric
conditions by gender and cause for the year 2000 that were used for the
estimation of years of life lost due to premature mortality (YLLs) as a result
of depression in the GBD 2000 analysis.
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Episode duration
The GBD 2000 study assumes a 6-month duration for episodes of depression.
This figure is based on recent studies and is consistent with classical
descriptions from the pre-antidepressant era. The same duration was used in
GBD 1990 and the US Burden of Disease Study. The length of depressive episodes
has been the subject of many investigations, most of which were carried out
with inadequate methodologies. The duration of episodes had a log-normal
distribution; therefore, the best way to report this variable is in terms of
logarithmic values or non-parametric parameters, such as medians and
quartiles. However, in the GBD project, the arithmetic mean has been used for
the estimation of the YLD across all the conditions. Therefore, an effort was
required to obtain information on duration of depressive episodes reported as
an arithmetic mean, to be consistent with the methods in the GBD project.
Data gathered as part of more recent epidemiological studies were re-analysed to establish the mean duration of episodes in subjects with depression in the community. The first study was the Baltimore Epidemiologic Catchment Area Follow-up (Eaton et al, 1989). In 1981, 3481 residents of Baltimore, from a probabilistic sample of 4238 residents, completed a psychiatric evaluation with the National Institute of Mental Health Diagnostic Interview Schedule (DIS). In 1993, these 3481 people were targeted for follow-up and interviewed again with the DIS. A specific analysis of this database was conducted to obtain estimates for the different parameters of the epidemiologic queuing formula: P=IxD, where P is the point prevalence, I is the incidence density and D is the mean duration of episodes. A total of 1725 respondents were interviewed and 536 episodes of depression were evaluated. The mean duration of episodes in weeks was 26.57 for both genders (male=25.7; female=26.85).
The second psychiatric database that was analysed to obtain estimates of the duration of episodes was the National Comorbidity Survey (Kessler et al, 1994). This survey did not include a longitudinal follow-up of cases identified in the community, but the diagnostic interview included questions related to the duration of lifetime episodes of DSMIIIR depression. For the purpose of this analysis, 3% of the subjects were excluded for having especially long durations. For the remaining 97% of the sample with episodes of depression, the overall mean duration was 22.6 weeks (Üstün & Kessler, 2002).
Finally, the NEMESIS study, a major epidemiological survey carried out in a national representative sample from The Netherlands, was also used to address the question of episode duration. This was a prospective study on the prevalence of psychiatric disorders in the Dutch population aged 1864. A total of 7076 people were interviewed personally in 1996 with the Composite International Diagnostic Interview (CIDI), and re-interviewed later as part of an incidence study. The survival analysis of the 250 persons with newly originated episodes during the follow-up period found a mean duration of 8.4 months (Spijker et al, 2002).
Prevalence estimates
We completed a systematic review of all available published and
non-published papers of meaningful population studies on depressive disorders
in order to create the most up-to-date data-set on the epidemiology of
depressive disorders. Criteria for studies to be included in this review were
as follows:
For depression, prevalence estimates were made by experts on the basis of published and unpublished studies. In addition, prevalence figures for ICD depressive episodes were derived from the CIDI interviews (World Health Organization, 1997) carried out as part of the WHO multicountry survey 20002001. This was a household survey carried out with the objective of assessing health states, disability and provision of health services in nationally representative samples (n=500010 000) of adults living in rural and urban areas of different WHO regions: China, Colombia, Egypt, Georgia, India, Indonesia, Mexico, Nigeria, the Slovak Republic and Turkey. Where no data for a region were available, experts on psychiatric epidemiology were encouraged to make informed estimates. Frequently, age patterns of incidence and prevalence were based on the assumption that some regions have similar epidemiological patterns but might differ in the level of incidence or prevalence. In the worst cases, where no information whatsoever was available, estimates were based exclusively on data or information from other regions. Table 2 presents a summary of the data sources and assumptions regarding the estimates for depressive disorders for all the WHO regions used in the burden calculations. Table 3 features age-standardised prevalence rate estimates for depressive episodes in the WHO epidemiological subregions. Comorbidity is common with psychiatric conditions, especially between anxiety and depressive disorders. For comorbidity between depression and anxiety disorders, comorbid states were attributed to the depressive illness more severe of the two conditions in terms of the disability weight assigned to the condition. The GBD estimates of anxiety disorder prevalence, incidence and severity, therefore, reflect no comorbidity with depression. This approach translates to lower burden estimates for anxiety disorders.
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Incidence estimates
For the epidemiology of depression, the primary sources of information are
cross-sectional population prevalence surveys. Incidence rates need to be
estimated based on these observed prevalence rates. Other sources of
information, such as small-scale follow-up studies, could be useful in
suggesting a credible range of incidence.
In GBD 2000, incidence estimates for depressive episodes were derived from prevalence and duration with the epidemiologic queuing formula P=IxD. The incidence estimates of depressive episodes used are presented in Table 3, broken down by gender. We compared these theoretical incidence estimates with the published results of the few studies that provide data on the incidence of depressive disorders in community samples. Murphy (2000) reviewed this issue and found an interval effect which has a major impact on the final incidence figures found in follow-up studies. Short-interval studies, such as the Epidemiologic Catchment Area follow-up study (Eaton et al, 1989), found lower annual incidence rates than the long-interval studies, such as the Stirling County study (Murphy, 2000). We considered it more appropriate to use short-interval studies in order to check the consistency of our epidemiological estimates. The incidence figure of depressive episodes will be closer to the incidence of depressive disorder obtained in short-interval studies than that obtained in long-interval studies.
Disability weights and severity breakdown
The YLD estimates of the GBD 2000 are based largely on the GBD 1990
disability weights or on the Dutch disability weights
(Stouthard et al,
1997). Three different severity levels of depressive episodes have
been considered in the disease model used for the estimation of the burden of
depression: mild, moderate and severe, with a disability weight of 0.14, 0.35
and 0.76, respectively.
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RESULTS |
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There is a marked contrast in the epidemiological patterns between rich and poor regions of the world. Thus, in the more developed countries, the share of disease burden from communicable, maternal, perinatal and nutritional conditions is typically around 5%, compared with 7075% in Africa. The contribution of depression to the total disease burden in Africa in 2000 was 1.2%, ranking in 13th position; in the Americas it was the leading cause, representing 8% of the total burden. Overall, in high-income countries the burden of depressive disorders was 8.9%, whereas in middle- and low-income countries the burden was 4.1% of the total DALYs.
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DISCUSSION |
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Comparison of GBD 1990 and GBD 2000
The original Global Burden of Disease study, GBD 1990, highlighted the
public health significance of depressive disorders, providing the tool for
comparative assessment in a general health context. However, in assessing the
burden of depressive disorders, the GBD 1990 study had certain shortfalls that
GBD 2000 tried to overcome. The first was that the epidemiological data used
as input for the original GBD study to calculate the burden due to depressive
disorders remain debatable: episode incidence was modelled as 29 per 100 000
per year for women, and 16 per 100 000 per year for men. Average age at onset
was taken as 37.1 years and the average episode duration was considered to be
6 months (Murray & Lopez,
1996). These incidence estimates are very low in comparison with
the recent findings from epidemiological surveys. In addition, depression is
now known to occur in younger age groups, often between 20 and 25 years, as
compared with the estimate of 37.1 used in the GBD analysis. This, in fact,
means that the degrees of burden estimated from the GBD results were actually
underestimates for depressive disorders. Moreover, the GBD study considered
depression as only an adult disease. There is overwhelming evidence at present
that depression occurs with considerable frequency in childhood and
adolescence (Costello et al,
1996). In the GBD 2000 the incidence estimates used were higher
(49 per 100 000 per year for women and 31 per 100 000 per year for men) and
with incident cases of depressive episodes appearing at younger ages, than in
the GBD 1990. Finally, in the GBD 1990 study, the disability weight for
depressive disorder was taken as 0.6 for untreated cases, irrespective of
severity of depression (i.e. mild, moderate or severe). However, it is evident
that various levels of illness severity are associated with different degrees
of disability. Different disability weights were assigned for the different
levels of severity in the GBD 2000.
Limitations
There are varying degrees of uncertainty in GBD 2000 estimates for
depressive disorders, reflecting uncertainty in the prevalence of depression
in different regions of the world and uncertainty in the variation of their
severity distribution. Despite intense efforts to obtain information on
prevalence estimates of this frequent condition in all the WHO regions, there
are still extensive and highly populated areas of the world where the
epidemiology of depression is largely unknown because of a lack of data. There
is a tendency in descriptive epidemiology to refuse to make estimates where
data are sparse, uncertain or based on studies that do not reach certain
methodological standards. To the contrary, disciplines such as demography and
economics often aim to make the best possible estimates using any available
data, employing a range of techniques, depending on the type and quality of
evidence. Thus, the GBD 1990 has been criticised by some epidemiologists for
using estimates rather than actual data. This is
not a relevant discussion for comparative burden estimates because all
epidemiological data relating to population are estimates of
varying degrees of precision or uncertainty. The GBD 2000 seeks to use all
available relevant data, to maximise the use of high-quality population-based
data, and, even for regions and conditions where data are sparse, to use the
available evidence and the best available methods to make inferences.
Otherwise, limitations on the evidence base for the epidemiology of diseases,
including depression, in some areas of the world translate to no
burden rather than the best achievable (even if uncertain) estimates of
burden, thus presenting health decision makers with a picture that is highly
misleading.
The WHO has initiated the World Mental Health Surveys, which will implement and analyse general population epidemiological surveys of mental, substance use and behavioural disorders in at least 18 countries throughout all WHO regions. This initiative will yield needed epidemiological parameters for many regions of the world and will improve the accuracy of the prevalence and incidence figures used for the estimation of the burden of depressive disorders in the future.
Depression as a public health priority
The fact that depressive disorders rank fourth as a source of DALYs, even
though they cause few deaths, underscores how assessment of both fatal and
non-fatal health outcomes affects the ranking of disease burden. Until
recently, counting deaths was the only way to determine the priorities for
public health actions and to control whether public health programmes were
succeeding. Mental disorders have never been ranked on the top 10 priority
list of public health significance when mortality indicators alone were used.
Once the mortality and disability effects of disease were combined into a
single metric, such as the DALYs, the magnitude of the burden of mental
disorders in general, and of depression in particular, became apparent. With
this new approach, depressive disorders are properly classified as being
priority health problems. This high burden is a result of a combination of a
high prevalence of depression, high impact on functioning and early age of
onset.
The importance of depression worldwide was one of the key findings of the original Global Burden of Disease study, which has been confirmed for the analysis of the year 2000. The results of the GBD 2000 study have shown variations by regions, but patterns and trends are remarkably similar worldwide. Depressive disorders constitute a large proportion in the global burden of disease, both in the developed and developing countries.
The Global Burden of Disease results have attracted the attention of policy makers and public health experts alike, because they provide a common metric for evaluating and priority-setting across a wide range of health problems. DALYs, being based on a universal measure of time, life-years, provide a trans-professional currency to determine priorities for health and human services and to evaluate their effectiveness.
There is a strong interest among policy makers to monitor the impact of health care reforms and other interventions, using a common cost-effectiveness measure. The appeal of the DALY measure is that it provides a potentially useful tool for health policy purposes: the transformation of epidemiological data into informed decisions about resource allocation for health care. DALYs can be used in different ways. They can be used to set priorities for service provision, as an outcome measure to monitor/evaluate performance of services in terms of consumer outcomes and to compare cost-effectiveness of different interventions.
These results of the Global Burden of Disease study have provided the most powerful scientific and advocacy support for mental health to date. It is now time to see how these findings and these tools can be applied to policy-making, planning and programme implementation.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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The views expressed are those of the authors and not necessarily those of the organisations they represent.
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Received for publication July 28, 2003. Revision received November 13, 2003. Accepted for publication December 15, 2003.
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