The old classification of depression into Reactive and Endogneous that are still observed in the clinical practice cannot all be accommodated under the current rubric of Major Depression. This is because psychiatric nosology under DSM and its latest 5th edition is still descriptive, and not etiologic. In this article both reactive and endogenous categories of depression are revisited from the perspective of today’s understanding of etiological pathways. From an epigenetic perspective, the old dichotomy of Reactive vs. Endogenous are inter-related through the impact of the environment (e.g. stress). This includes familial or prenatal depression, where the environmental impact is before birth, or childhood depression where the early life stress is the precipitating factor to the genetic susceptibility. In conclusion, searching for both environmental impact (e.g. stressors) and genetic predispositions in depression, even at a clinical level could help clinicians with better therapeutic decisions.
The differentiation of major depression into ‘reactive (stress-induced)’ vs. ‘endogenous (e.g. genetic)’ dates back to the German psychiatrist, Kurt Schneider (Schneider, 1920) who borrowed the term ‘endogenous’ from Emil Kraepelin. The differentiation was an early attempt at an etiological classification of depression (Mendels & Cochrane C, 1968). Despite the extensive use of these terms and despite the popularity of the catecholamine deficiency hypothesis of depression (Schildkraut, 1965) and the effectiveness of tricyclic antidepressants that began with the introduction of imipramine in the 1950’s, psychiatric nosology then gave up on the attempt of classifying depressions according to etiology.
Although the aim of DSM-III in 1980 was for psychiatry to do what the rest of medicine does, to classify disease according to cause, this proved impossible and a non-etiological, purely descriptive system was devised that relied on categories based on symptoms and their severity. DSM-III divided the depressions into major and minor (DSM-III, 1980). Almost four decades later, DSM5 continues to be descriptive and non-etiological (DSM5, 2013). This has continued despite research that points to distinguishable pathways leading to the symptoms of major depression (Ghaemi & Vohringer, 2011; Malki et al. 2014; Mizushima et al. 2013; Parker 2000).
In this article an attempt depression is reviewed on a pathophysiological basis through 1) the impact of stressful events and their timing 2) gene-environment interactions and 3) biological circuits affected by different kinds of depression. The generic term of “depression” that has been used in this paper, refers mostly to major or unipolar depression, though it can at times also applies to minor depression and dysthymia. This article also excludes the normal reaction of mood to stress below clinical level of severity and dysfunction.
The timing of the stress onset:
In reference to stress leading to depression, there is a major differentiation between an early childhood adversary or later in life (adulthood) stress. While these two types of depression, one with an early onset in childhood or adolescence, and the other one with a later onset in adulthood, could be referred to as “Reactive Depression”, they are fundamentally different. (Hazel NA., et al., 2008) This differentiation between reactive depression in the past decade has been recognized in the literature as “Juvenile” and “adult” onset with different pathophysiological pathways that perhaps demand different treatment pathways as well. (Jaffee SR, et al., 2002; Weissman MM, 2002)
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