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British Medical Bulletin 57:3-15 (2001)
© 2001 Oxford University Press

Classification, disability and the public health agenda

Depression and public health

Glyn Lewis and Ricardo Araya

Department of Psychological Medicine, University of Wales College of Medicine, Cardiff, UK

	Abstract

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
Depression is a common and disabling illness. For some time the disability has been relatively neglected by those interested in public health. Public health priorities have largely been determined by statistics on mortality. The World Health Organization has argued for some time that public health should be concerned with ‘adding life to years’ as well as ‘adding years to life’. This article will argue that depression should be a key priority for public health research.

	Classification

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
The classification and measurement of depression has attracted interest and controversy for many years. The great emphasis that has been given to measurement in psychiatry has often been a distraction, but accurate measurement and clarity about diagnostic issues is an essential prerequisite of any scientific process. Before discussing some of the more recent consensus about classification in depression, it is worth mentioning the general principles behind classification and measurement.

Classification has a purpose and is intended to help doctors with their work¹. There are three main functions: (i) communication; (ii) guiding treatment or prognosis; and (iii) informing research. Classifications have to be useful to survive in clinical practice, and will persist if they are used, even if they find little favour in the scientific journals. If these functions of classification are to be ‘effectively’ fulfilled, psychiatric diagnoses need to be reliable. Though the reliability of diagnosis tends to be largely a concern of the research community, we should not forget that clinicians also need to be able to make diagnoses with sufficient reliability in order to communicate with each other, their patients and to apply the results of research studies to their clinical work. In parallel with the discussion of reliability is that concerning the ‘validity’ of diagnostic categories. This expression is best thought of as utility in the absence of ‘gold standard’ measures. If diagnoses aid communication, treatment decisions and prognostic predictions then they are useful and in that sense ‘valid’.

There is now an international consensus over the diagnostic categories of depression and it is reassuring that both the major diagnostic manuals, DSM-IV and ICD-10, have the same diagnostic criteria for unipolar depression. Of particular note is that the criteria for major depressive disorder can be met even if a person complains of loss of interest rather than low mood. The criteria also allow for hypersomnia and increased appetite as well as the more conventional syndrome in which there is less sleep and poor appetite. Our current classification retains the opportunity to code the somatic syndrome of depression: early morning wakening, weight loss, diurnal variation, retardation or agitation and loss of libido.

There has been a trend to give less emphasis to the somatic syndrome of depression over recent years. In the past, these ‘biological’ symptoms were thought to reflect an illness more endogenous and less linked to adverse events in the environment in contrast to neurotic depression. Biological symptoms were also thought to indicate likely response to antidepressant medication. However, both of these statements have been challenged. There is evidence that life events are as common before depression with somatic symptoms as before depression without somatic symptoms². The link between a depressive episode and environmental adversity seems more important for the first episode of depression and subsequent episodes seem less linked to adverse events³. The antidepressants have also been successfully used in depressed patients without the somatic syndrome⁴.

The change in the perceived utility of the somatic syndrome has led to more emphasis on thinking of depression along a continuum of severity⁵,⁶. This is in tune with a long tradition within medical epidemiology that argues that almost all medical conditions in the community are most accurately viewed along a continuum⁷. For clinicians, categories are useful in order to guide decision making, but in the real world most illnesses including depression do not exist in simple categories but along continua. Kendell's classic study illustrated the continuum between the neurotic and endogenous forms of depression⁸. Likewise, community surveys illustrate that the key symptoms of depression are common in the community and exist across the whole range of severity⁹.

Most patients with depression are treated in primary care or its equivalent¹⁰,¹¹, and it is important to be aware that in primary care the whole range of depressive syndromes will be seen. Primary care physicians will see a large number of people in a ‘grey’ area where treatment decisions are difficult to make. One of the major challenges of research in this area is to help primary care physicians rapidly assess the severity of depression and link this with decisions about pharmacological and psychological treatment. There is increasing concern within primary care that patients with very mild depressive symptoms or problems of living are being medicalised and treated with antidepressants. Making the diagnosis of depression is at the heart of this controversy and in the context of primary care regarding depression along a continuum of severity seems particularly important.

	Co-morbidity

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
The overlap between the symptoms of anxiety, depression and other neurotic symptoms are legendary. This area has been given the new name of co-morbidity. The advent of operationalised criteria has highlighted the overlap between symptoms and syndromes when a categorical approach is taken towards diagnosis. A National Co-morbidity Survey has been carried out in the US¹². An alternative and probably simpler view is that depression and anxiety are two correlated dimensions¹³, though some might argue for a single dimension^14–16. One can certainly argue that a single dimension does describe neurotic symptoms in the community, even if a more complex classification is also of value.

The primary concern for clinicians and patients must be how classification and diagnosis is related to treatment. At present, antidepressants and psychotherapy, including cognitive behavioural therapy are the main options for both depressive and anxiety disorders and there is little guidance on which patients will benefit from which approach^17–21. These diagnostic issues will only be resolved by future research to investigate whether treatment response or outcome differs systematically according to clinical characteristics.

	Methods of measurement

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
There are a wide range of standardised methods for assessing depressive disorders with a confusing range of acronyms. It is useful to classify the methods into four different types: (i) rating scales; (ii) semi-structured interviews; (iii) fully structured interviews; and (iv) self-administered questionnaires.

Rating scales such as the Hamilton Rating Scales for Depression (HAMD)²² do not provide any guidance on the questions to be used though they provide (some) more guidance on how to code symptoms when present. They can, therefore, only be administered by trained clinicians. Though poorly standardised, the HAMD has become one of the most widely used assessments particularly in pharmaceutical trials. One suspects that its use will be superseded by more standardised methods of assessment. Semi-standardised interviews, such as the Schedules for Clinical Assessment in Neuropsychiatry (SCAN)²³, allow the interviewer some discretion in the use of questions but provide much more guidance about the conduct of the interviewer and a glossary of terms. This kind of methodology is probably essential for some of the most severe depressions seen in hospitals, for example those accompanied by delusions. However, for the more common, less severe depressions, more structured assessments have been developed.

In the UK, the revised clinical interview schedule (CIS-R)²⁴ has become widely used, including in the OPCS Psychiatric Morbidity Survey carried out in 1993, a household survey on 10,000 individuals representative of the UK population²⁵,²⁶. The CIS-R is a fully structured assessment, suitable for trained social survey interviewers and does not require any expert knowledge on the part of the interviewers. As such, it can also be administered using personal computers in which the subjects self-complete the questionnaire²⁷. The CIS-R elicits responses to 14 areas of symptoms including depression, anxiety, sleep and fatigue. It can be used to generate a total score, analogous to the single continuum approach, as well as diagnostic categories according to ICD-10. The World Health Organization has encouraged the development of the Composite International Diagnostic Interview (CIDI), a fully-structured interview based upon a US interview²⁸. The CIDI exists in a variety of versions (CIDI)²⁹ and adopts a similar approach to the CIS-R though it takes longer to administer and is based upon diagnostic criteria rather than structured around symptoms.

Self-administered questionnaires such as the General Health Questionnaire (GHQ)³⁰ and Beck Depression Inventory (BDI)³¹ have also been widely used. The GHQ asks general questions about symptoms and functioning though the BDI is more focused on the key symptoms of depression. The BDI has been frequently used as an outcome measure in studies of depression.

There are still some doubts about the ‘validity’ of the fully structured interviews such as the CIS-R and CIDI. A recent UK study found very low levels of agreement between the CIS-R, CIDI and the SCAN though the severity of depression for the SCAN and CISR showed better agreement³²,³³. Unfortunately, we still know little about the reliability of the SCAN assessment when used in the community though we know that clinical judgement alone is fairly unreliable. Most researchers now accept that fully structured interviews are the only feasible methodology for large scale community studies that often involve up to 200 interviewers. These assessments ensure that all subjects in a study are asked the same questions and this should reduce interobserver variability and ensure comparison with other studies.

These standardised methods are rarely used in clinical practice though many who use Beck's version of cognitive therapy³⁴ tend to assess progress using the BDI. It is important though that clinically relevant research keeps in touch with clinical practice and leads to results with practical implications. The research community is carrying out research that will hopefully benefit patients. If the criteria used in research are wildly different from those used by clinicians, the value of the research will be seriously undermined.

	Disability and depression

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
Several studies have shown a close association between depression and disability in both western and non-industrialised countries³⁵,³⁶. The association persists after statistical adjustment for the presence of physical illness and across time. Wells et al³⁷ studied more than 11,000 patients with different chronic diseases to find that the level of disability among patients with depressive symptoms, whether or not meeting diagnostic criteria for depression, was comparable or greater than that associated with the most frequent physical chronic diseases such as diabetes and hypertension.

Common mental disorders have also been linked to important indirect costs due to either diminished productivity or absence from work. Broadhead et al³⁵ found that people with depression had a 4.8 times higher risk than people without symptoms of having had sickness leave. More recently, an American study found that for every 100 workers, 6 days of work are lost for sickness leave and 31 days are lost for diminished productivity every month due to poor mental health³⁸.

These statistics will come as no surprise to clinicians used to treating those with depression or others familiar with the syndrome. Depression is often an exceptionally disabling condition, leading to difficulties in working and carrying out household tasks. Unlike many physical conditions, it can also lead to a profound deterioration of relationships with friends, family and work colleagues that can provide particular difficulties.

Despite the empirical and clinical evidence for a substantial population disability associated with depression, it has proved difficult to establish depression as a public health priority. Public health statistics have relied exclusively upon mortality rates to determine priorities and so conditions that lead to morbidity rather than mortality have been relatively neglected. The World Bank attempted to change this agenda by adopting a methodology that calculated the Disability Adjusted Life Years or DALYs lost to various diseases. This approach was designed to enable morbidity and mortality to be compared and, therefore, allow a rational setting of public health priorities.

	Disability Adjusted Life Years: the DALY

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
The novelty of this method was to try to adjust for the level of disability when calculating years lost as a result of ill health. If someone dies prematurely, then each year lost counts as one whole DALY. If someone has an illness, such as depression, then each year affected by illness will count as a proportion of a DALY. The controversial and difficult aspect of this method is in estimating what proportion should be used when calculating DALYs for non-fatal conditions. The higher the proportion, the more disabling the condition. Most criticism of the method has focused on the fact that two of the most important components of this indicator, the degree of disability produced by each specific disease and the relative value of each year of life achieved, are determined by a panel of ‘experts’ through an ill-defined and rather unsystematic process. Anecdotal accounts of ‘horse trading’ in relation to these estimates have also undermined confidence in the precise estimates that have been calculated. However, attempts are being made to base these estimates on a firmer empirical footing. A further limitation concerns the quality of the data on prevalence. The quality of much of the epidemiological information that has been used to estimate the burden of diseases around the world is of questionable reliability, particularly in relation to non-industrialised countries where few large scale surveys on representative samples have been carried out.

Despite these limitations, the World Bank Report and associated publications^39–41 have provided the first estimates that have allowed comparison between depression, other psychiatric disorders and physical illness leading to death. The report estimated that neuropsychiatric disorders led to 8% of the global burden of disease (GBD) measured in DALYs lost to illness (Table 1). For adults aged 15–44 years, psychiatric disorders are estimated to account for 12% of the GBD; if ‘self-inflicted, unintentional injuries’ are added, the proportion reaches 15.1% for women and 16.1% for men. In fact, mental disorders are projected to increase to 15% of the global disease burden and major depression is expected to become second only to ischaemic heart disease in terms of disease burden by the year 2020 (Table 2)⁴¹ .

View this table: [in this window] [in a new window]   Table 1 Global burden of disease (GBD) measured in disability adjusted life years (DALYs) lost in illness39–41

 

View this table: [in this window] [in a new window]   Table 2 The global burden of disease in 2020

 
Psychiatric disorder has received little priority in the non-industrialised world. Demographic transition and improved measures to combat infectious disease are leading to a change in the pattern of disease in many poor countries⁴². In Chile, for example, life expectancy is now over 70 years and, along with many other areas of the world, the burden of disease is largely produced by non-communicable diseases familiar to those in the West. These changes will contribute to the growing importance of depression and other psychiatric disorders in world health.

These figures, with all their caveats, have profound implications for public health and epidemiological research. Depression in particular, is as big a potential public health issue as ischaemic heart disease (IHD). For IHD, the major risk factors of high cholesterol, smoking and hypertension are well known and preventive strategies are proposed in relation to all these factors. In contrast, little is known about the aetiology of depression and, in particular, there is little evidence for strategies that would lead to primary prevention of depression.

	Severity and disability

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
For clinicians, the more severely affected individuals will always retain clinical priority. As a result, most secondary mental health services, at least in the UK, have a policy to target those with severe mental illness and most cases of depression are, therefore, treated within primary care. The same argument holds for those with depression of varying degrees of severity. Clinical priority, within primary and secondary care will give priority to those with the more severe illnesses who are more disabled by their illness. This priority changes when a public health perspective is taken. In a population, it is the aggregate disability that is important. There are many more people with mild illness than severe illness. As a result, the aggregate burden of disability associated with depression of mild severity may be greater than the disability associated with the smaller number of people with the more severe depressions. Broadhead et al³⁵ have illustrated this phenomenon using data from the Epidemiological Catchment Area Study in the US (Table 3). The population burden of disability is greater for those who fall below the DSM-III threshold than for those who meet the criteria for major depression. The more depressed individuals are more disabled, but there are fewer of them.

View this table: [in this window] [in a new window]   Table 3 Relationship between major depression, subthreshold affective disorders and disability days

 
This phenomenon is frequently seen in epidemiology as illustrated by Rose⁴³,⁴⁴. For example, most cerebrovascular disease occurs in those with average blood pressure even though individuals with high blood pressure are at greater individual risk. This observation has important implications for preventive strategies. Rose argued that reducing the mean diastolic blood pressure by 1 mmHg would save as many deaths from cerebrovascular disease as all the hypertension clinics then in the UK. The most effective way of preventing cerebrovascular disease is, therefore, to change the population mean blood pressure rather than trying to target those with the highest blood pressure. Individuals with hypertension need to have treatment because of their high individual level of risk. In most circumstances, though, targeting those at high individual risk will have little impact upon the burden of disease in the population.

It is likely that population-based approaches designed to reduce the prevalence of depression will be needed. Depressed individuals need treatment for their symptoms and associated disability, but this will do nothing to reduce the burden of disease attributable to depressive symptoms that fall below the usual criteria for defining major depression. Research to establish risk factors for depression is required which in turn will inform preventive strategies targeted at the whole population.

	Life events

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
The concept of life events has been an important one in the epidemiology of depression⁴⁵. In the UK, Brown's approach towards the measurement of life events has been particularly influential⁴⁶,⁴⁷. Research on life events has provided good evidence that environmental stresses increase the risk of developing depression⁴⁵. However, the life events methodology has not led to practical preventive strategies based upon a population approach.

The most commonly used method to measure life events in the UK has been the Life Events and Difficulties Schedule (LEDS). It provides a composite measure of life event plus context (i.e. the circumstances of that individual rated by a panel). Brown and colleagues have persuasively argued that the association between life events and depression is much stronger when such ratings of context have been taken into account⁴⁶. Indeed, one interpretation of the life events literature is that it demonstrates the importance of the psychosocial context in the aetiology of depression. Life events on their own are not that important. Low socio-economic status is clearly one important element of context that influences LEDS ratings and is effectively incorporated in the analysis by combining it within the life event measure.

The emphasis on life events has often been misinterpreted in the literature. For example, policy makers have often erroneously regarded the life events literature as supporting the idea that depression is caused by ‘acts of God’ and, therefore, nothing can be done to prevent them⁴⁸. This conclusion has led to an emphasis on providing social support to those who might experience life events in the hope that this will reduce the possible impact of an adverse occurrence. Providing support to those at high risk may be beneficial for those individuals at high risk. However, from a public health perspective, this kind of approach is unlikely to lead to a large population impact on the prevalence of depression.

	Socio-economic status and depression

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
What kind of research will help to establish risk factors for depression, that will lead on to practical preventive strategies? At present, the area of socio-economic inequalities might be a promising avenue for future research.

Most of the literature concerning socio-economic status has investigated associations with the common mental disorders of depression and anxiety. However, the associations are probably similar with depression when considered alone⁴⁹. The current literature on socio-economic status and common mental disorder is quite confusing. In part this is because socio-economic status has been measured in a variety of different ways. In the UK, there is now good evidence that the main association is between low standard of living and the prevalence of common mental disorder. This is independent of registrar general social class and educational attainment. For example, Lewis et al⁴⁹ in the OPCS Psychiatric Morbidity Survey found an independent effect of owning a car and owning rather than renting a home on the prevalence of common mental disorder assessed using the CIS-R.

Such cross sectional data cannot address two important issues. First, is there an association between socio-economic status and duration of disorder rather than incidence. Data from the BHPS suggest that socio-economic measures appear to delay recovery rather than increase incidence⁵⁰. There is also a possibility that those with poor mental health have a reduced capacity to earn more. This is usually called social selection. Unsurprisingly, there is some evidence for social selection^51–53, but it does not appear to be able to explain the whole socio-economic gradient. There is evidence from the US that low income is associated with incidence⁵⁴.

All the data referred to above have been collected in industrialised countries. There is now data emerging from non-industrialised countries where similar analyses have been undertaken in which the various measurable aspects of socio-economic status have been studied. For example, Ludermir and Lewis⁵⁵ in a community survey in a poor area of Recife, Brazil found that years in education were independently associated with prevalence of common mental disorder. Araya et al⁵⁶, in a larger community study in Santiago, have described a similar finding. At present, it is not clear why these Latin American countries should have found different results from studies carried out in the UK. There is certainly more variation in the level of education in non-industrialised countries. Perhaps, education is a more important determinant of life opportunity in the industrialised world? In a non-industrialised country, education might reflect the socio-economic status of the person's own upbringing more accurately than in western countries.

It is likely that low socio-economic status is an important determinant of rates of depression in the community. If this evidence is confirmed and strengthened, it might provide a route to preventive policies directed at the whole population. Future research will need to put more emphasis on longitudinal studies and begin to work out the possible mechanisms that link poverty, education and depression.

	Depression and public health

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
Depression is a major public health problem. Very soon it will lead to almost as much disability as ischaemic heart disease, but, in contrast, very little is know about the risk factors for depression that could inform preventive approaches. Epidemiological research is needed in order to establish risk factors for depression. Most people expect that the risk factors will involve matters such as socio-economic status as well as family upbringing and the influence of these on personality. If this is true, preventive policies will need to influence some fundamental aspects of government social and economic policy. Income distribution, the care of young children and policies with relevance to the family may all have an impact on the rates of depression. If we are to influence this debate, we will need robust evidence from well designed and conducted studies to convince politicians and their advisers. From the perspective of public health, depression must, therefore, be a major priority for research.

	Footnotes

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 
Correspondence to: Professor Glyn Lewis, Department of Psychological Medicine, Monmouth House, University of Wales College of Medicine, Heath Park, Cardiff CF14 4XN, UK

	References

Top Footnotes Abstract Classification Co-morbidity Methods of measurement Disability and depression Disability Adjusted Life Years:... Severity and disability Life events Socio-economic status and... Depression and public health References

 

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