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The Nature of Depression

Martin Connor, Adrian Wells, and Peter L. Fisher

Introduction

Sadness and despair are common experiences for many people, historically based descriptions reflecting the cultural context. Historical accounts indicate that the cause of severe mood disturbance was attributable to a physical illness for which the sufferer bore no responsibility. Symptoms of severe mood disturbance or melancholia included extreme sadness, an inability to function, and the frequent presence of delusions (Daly, 2007). Melancholia was thought to be caused by an imbalance of the 'bodily humours' (Daly, 2007; Akiskal & Akiskal, 2007). Conversely, accounts of less severe mood problems implied that the sufferer was ultimately responsible. In early Christian monastic settings a constellation of undesirable feelings and behaviours that interfered with devotional duties was known as the 'sin' of acedia (Jackson, 1981). This state was attributed to laziness or a 'lack of care' and was characterized by apathy, loss of hope, drowsiness, and a desire to flee the monastery (LaMothe, 2007). However, acedia was not considered equivalent to normal sadness, since the fourth-century monk John Cassian described it as a 'dangerous foe' that was 'akin to sadness' (Daly, 2007, p. 34). These historical descriptions of the 'symptoms' of melancholia and acedia loosely correspond to those of major depression as defined in modern diagnostic systems, which will be discussed in the next section.

Diagnosing Major Depressive Disorder

Major depression is a common but clinically heterogeneous disorder that is frequently comorbid with others. Current diagnostic methods rely on identifying constellations of psychological and behavioural symptoms through structured clinical interviews (see chapter 2 for a detailed account of assessment measures and processes). Major depressive disorder (MDD) is diagnosed according to either the current (fifth) edition of the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM-V; APA, 2013) or the World Health Organization's International Classification of Diseases (ICD-10; WHO, 1993). Because major depression is a highly recurrent disorder (Boland & Keller, 2008), both systems operationalize it in terms of the occurrence of a single 'depressive episode' (WHO, 1992), also known as a 'major depressive episode' (MDE) (APA, 2013). The diagnostic criteria for a depressive episode are similar in both systems. Both DSM-V and ICD-10 define recurrent depression as the occurrence of two or more episodes that are separated by at least two months during which the criteria for a depressive episode are not met (APA, 2013; WHO, 1993). In DSM-V the term 'major depressive disorder' (MDD) is used to denote the occurrence of one or more major depressive episodes.

Major depression is a clinically heterogeneous disorder (Rush, 2007). The diagnostic criteria are designed to account for such heterogeneity, which means that depressed individuals with markedly divergent symptoms are assigned to the same diagnostic category (APA, 2013; Krueger, Watson, & Barlow, 2005). For example, two individuals diagnosed with a major depressive episode may both experience depressed mood and concentration difficulties. However, one of them may have the accompanying symptoms of significant weight loss and insomnia, while the other may experience significant weight gain and hypersomnia. These differences may be important for the selection of appropriate treatment, and prognosis (APA, 2013; WHO, 1992; Rush, 2007), and therefore DSM-V enables the specification of depressive subtypes and of episode severity (APA, 2013).

Diagnostic Criteria for Major Depressive Disorder

The diagnosis of a major depressive episode requires that at least five of the symptoms listed in Table 1.1 are met for a period of at least two weeks. Importantly, one of the symptoms must be either a depressed mood or a loss of pleasure/interest in everyday activities. It is also necessary that the symptoms reach clinically significant levels, which typically compromise occupational and social functioning.

Table 1.1 Summary of DSM-V criteria for an episode of major depression.

A closer inspection of the nine main symptoms of depression in Table 1.1 shows that individuals meeting diagnostic criteria for a depressive episode may have minimal overlapping symptoms. Nevertheless, researchers and clinicians have observed what appears to be relatively consistent constellations of depressive symptoms that may respond differently to treatment (Rush, 2007). Consequently, successive revisions of the DSM since version III have included specifiers that enable potentially important clinical characteristics of episodes to be recorded (APA, 2013). These episode specifiers relate to symptom severity, remission status, chronicity, and symptomatic features that may denote depressive subtypes.

The Epidemiology of Major Depression

Surveys of the prevalence of psychiatric disorders have been undertaken since the Second World War. However, estimates of prevalence varied widely, due to differences in methodology. Early estimates of the prevalence of MDD were derived from screening instruments that were not fit for purpose (Kessler et al., 2007). There were two main problems; (1) the screening instruments were prone to poor specificity or sensitivity (or both), which undermined confidence in the resultant prevalence estimates; and (2) the use of different instruments between surveys hindered the interpretation of results. This has become less of an issue since the World Health Organization commissioned the Composite International Diagnostic Interview (CIDI) in the 1980s (Kessler & Ustun, 2004) in order to compare psychiatric prevalence rates between countries according to standardized criteria (Kessler et al., 2007). The CIDI was based on the Diagnostic Interview Schedule (Robins, Helzer, Croughan, & Ratcliff, 1981) and was designed to be administered by lay interviewers. It was also designed to support psychiatric diagnoses according to both ICD and DSM criteria. However, the original version of the CIDI was not designed to capture detailed demographic and clinical data. This meant that countries could only be broadly compared, in terms of overall prevalence rates (Kessler & Ustun, 2004).

The CIDI (version 3) was designed for the World Mental Health Survey Initiative (WMHS) (Kessler, 1999) for the purpose of facilitating the acquisition and comparison of psychiatric epidemiological data within the participating countries (Kessler & Ustun, 2004). In addition to enabling the quantification of lifetime and 12-month diagnoses according to both DSM-IV and ICD-10 criteria, the CIDI-3 also includes items that assess severity, demographic, quality-of-life, and disability data (Kessler & Ustun, 2004). Unlike previous versions, the CIDI-3 included interview probe questions that increase the reliability of autobiographical recall. The methodological rigour used to produce different translations of the CIDI-3 has led to its being described as 'state of the art' for comparing epidemiological findings across participating WMHS countries (Alonso & Lepine, 2007). Two large-scale surveys within the WMHS framework have specifically examined the epidemiology of MDD. These are the European Study of the Epidemiology of Mental Disorders (ESEMeD) (Alonso et al., 2002) and the American National Comorbidity Survey Replication Study (NCS-R) (Kessler et al., 2003).

Overall prevalence rates

The NCS-R and ESEMeD surveys estimated that the 12-month prevalence of MDD according to DSM-IV criteria is 6.6 per cent in American adults and 4.1 per cent in European adults (Alonso et al., 2004; Kessler et al., 2003). In absolute terms, these results indicate that at least 13.1 million US adults experienced a major depressive episode in the preceding year (Kessler et al., 2003). In terms of lifetime rates, 16.2 per cent of Americans and 13.4 per cent of Europeans will experience at least one depressive episode.

In terms of DSM-IV symptomatology, the NCS-R results estimated that 10 per cent of the people identified within the 12-month prevalence time frame were mild, 39 per cent moderate, 38 per cent severe, and 13 per cent very severe according to the Quick Inventory of Depressive Symptomatology Self-Report (QIDS-SR) (Rush...