Jenna Griffiths and Arun V. Ravindran of the Royal Ottawa Hospital in Ontario, Canada, offers some helpful details that complement the earlier responses to this question:
"Mild chronic depression has long been known to exist. Although it has been referred to by different names such as neurotic depression, minor depression, intermittent depression and depressive personality, it has, since its appearance in the DSM-III (the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders,1980), been standardly labeled 'dysthymia.' The term literally means 'ill-humored.' According to DSM-IV (1994) criteria, the core features of the disorder include at least two years of depressed mood for more days than not, with at least two of the following: poor appetite or overeating; insomnia or oversleeping; fatigue or low energy; low self-esteem, poor concentration or problems in decision making; and hopelessness. Dysthymic individuals tend to be self-deprecating, brooding about the past, socially withdrawn; they may feel irritable and unproductive. Dysthymia is also characterized by anhedonia (an inability to derive pleasure from events or stimuli previously found pleasurable).
"It is now generally accepted that dysthymia belongs to the classification of mood/affective disorders, rather than representing a depressive temperament. Yet a condition known as double depression also exists, in which dysthymia may be superimposed on a major depressive episode. Dysthymia can be further broken down based on age at onset and vegetative symptoms (for instance, increased or decreased appetite, weight, sleep). In addition, dysthymia has been subclassified, according to clinical symptoms and presence of family history, as subaffective dysthymia and character-spectrum dysthymia. Subaffective dysthymics tend to respond to antidepressant medication and often have a family history of mood disorder, whereas character-spectrum dysthymics respond less well to medications and more often report a major loss as well as family history of substance abuse.
"It had long been thought that dysthymia was best classified as a character disorder, in which the individual's core problems stemmed from a depressive personality or temperament. For this reason, and because there had been few systematic studies on the efficacy of antidepressants in the treatment of dysthymia, the treatment of choice had long been of a psychotherapeutic nature. The negative attitude toward pharmacotherapy was the result in part of the types of medication that were available (for example, tricyclic antidepressants characterized by unpleasant side effects) and of the clinicians' hesitation to prescribe these medications in adequate doses and duration. Because dysthymia was conceptualized as a mild form of mood disorder, it was routinely treated with subthreshold doses of antidepressant, over inadequate durations of time.
"The past decade has provided evidence for dysthymia's positive response to antidepressant medication, especially to the newer generation of drugs such as Prozac, Zoloft, Paxil, Effexor and Serzone. It is now understood, however, that although the symptoms may be less severe than those of major depression, dysthymia requires just as aggressive, often longer-term antidepressant treatment.
"The success in treating dysthymia with antidepressant medication supports the contention that, like major depression, dysthymia may have biological underpinnings. Research has been under way into the immunologic, hormonal and neurotransmitter correlates of dysthymia, as well as its genetic transmission.
"Although dysthymia is not a new disorder, the possibility exists that its diagnosis is becoming more talked about because of the success of the newer generation of antidepressants in treating its symptoms. When the treatment of choice is that of prescribing medication, the disorder enters the realm of the medical world, and the stigma associated with a mental disorder once thought to be a personality disturbance is lessened."