Depressive disorders Diagnostic issues

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A key issue for the epidemiology of depressive disorders is defining the boundaries of major depression and dysthymia. Depressive symptoms in the community are common, and defining both the symptom count and the duration at which depressive symptoms count as part of a clinical disorder is arbitrary. Recently, Kendler and Gardner*7) examined the boundaries of major depression as defined by DSM-IV in a population-based twin sample of women. They found that, if a twin had four or fewer depressive symptoms, syndromes composed of symptoms involving no or minimal impairment, and episodes lasting less than 14 days, then the individual's co-twin was still at an increased risk of major depression. Kendler and Gardner concluded that they could find no empirical support for the DSM-IV requirement of duration for 2 weeks, five symptoms, or clinically significant impairment. These authors suggested that major depression, as articulated by DSM-IV, may be a diagnostic convention imposed on a continuum of depressive symptoms of varying severity and duration. Wainwright et al.,(8) using data from the National Psychiatric Morbidity Survey of Great Britain, have also suggested that research should move beyond a binary decision of case versus non-case, and utilize a probablistic measure of psychiatric case status, replacing the arbitrary threshold with a smooth transition. This type of approach allows the benefits of syndrome diagnosis to be retained, while not falling into the dilemma of an arbitrary threshold that may lack validity.

Provided that one accepts the arbitrary definition of major depression, then determining the rates of current depressive disorders is not especially problematic. However, there are major methodological issues involved in determining whether an individual has ever had a lifetime episode of major depression. Lifetime prevalence rates vary from 4.4 per cent in the United States ECA study, to 17.1 per cent in the NCS, and to over 30 per cent in Kendler's Virginia twin sample of women. In part, subjects in the community may forget or fail to report past episodes of major depression, and the manner in which the questions are asked may importantly influence lifetime rates of depression. In the Diagnostic Interview Schedule, which was used in the ECA, respondents were asked about lifetime symptoms, a lifetime diagnosis was made, and then recency of the lifetime diagnosis was determined. More recent diagnostic interview schedules, such as the Composite International Diagnostic Interview, first ask about current depressive symptoms and then, having 'primed' individuals about depressive symptoms, go on to enquire about past depressive episodes. Interviews that follow the schedule of 'priming' before asking about past episodes appear to obtain considerably higher rates of lifetime major depression. Determining lifetime rates of depression with greater precision is an important task, as the vulnerability to depression conferred by risk factors such as genetic factors and childhood experiences may be wrongly estimated if lifetime rates of major depression are imprecise. For instance, when Kendler et al.(9) examined the heritability of major depression and corrected for the moderate reliability of a lifetime diagnosis of major depression, the heritability estimate increased from 40 per cent to over 70 per cent. As concluded by Kendler, major depression is not a disorder of high reliability and moderate heritability, but is a diagnosis of moderate reliability and high heritability.

DSM-IV allows major depression to be further subclassified into subtypes, such as melancholia and atypical and psychotic depression. Most of the traditional epidemiology studies have tended to ignore the issue of subtyping major depression. Recently, however, the issue of the atypical depression subtype has received particular attention in the study of the Virginia twins and in the NCS. In both these studies, latent class analysis suggests that atypical depression is a distinct subtype with several distinctive features, such as higher rates of parental alcohol- and drug-use disorders, higher interpersonal dependency, and higher rates of conduct disorder. If risk factors for atypical depression are, in part, distinct from risk factors for other subtypes of major depression, then for epidemiology to contribute to an understanding of aetiology it will be important to undertake further work on depressive subtypes. (1 i1,)

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