Sensitization: the sine qua non of the depressive disorders?

Available definitions of depressive disorders do not adequately distinguish depressive disorders from non-pathological mood states. When depression occurs as a reaction to life events (e.g., bereavement) a diagnosis is not usually warranted since these reactions represent normal and presumably adaptive reactions to life circumstances. Various approaches in DSM-IV such as the bereavement exclusion criterion and clinical significance criteria (addressing levels of distress and dysfunction) have not provided a fully satisfactory solution. The distinction between pathological and non-pathological expressions of depression would be facilitated were there to be a clearer understanding of the adaptive purpose of depression in its normal manifestations. Such an understanding would provide a basis for distinguishing adaptive manifestations of depression from those that represent a disorder. This paper presents the hypothesis that sensitization to life events is the core feature of depressive disorders. Sensitization may be an adaptive mechanism because it can fine-tune responses in a changing environment. Sensitization, however, may fail as an adaptive mechanism if it leads to an over-sensitized state in which a person cannot function or is intensely distressed even in a normal environment. This hypothesis predicts that a pattern of increasing reactivity to life events will be a better predictor of the need for, and response to, psychiatric treatment than current definitions.