Mood disorders are among the most prevalent forms of mental illness. Serious depression is especially common; based on a face-to-face survey conducted from December 2001 to December 2002, the past-year prevalence rate of clinically significant major depressive disorder (MDD) was estimated to be 6.6%, affecting at least 13.1 to 14.2 million Americans.1 Although many patients with recurrent episodes of illness have good symptom remission between episodes, with few residual symptoms, approximately a quarter of patients with major depression have chronic residual depressive symptoms of varying severity with only incomplete remission for many years.2 There is evidence that chronic depression is more familial, more refractory to treatment, and more impairing than episodic major depression.Diagnostic criteria
Current nosologies of depressive illnesses do not, however, do a very good job of categorizing chronic depression. In DSM-IV, there are 2 major categories for depressive illness: MDD, for which there are a number of subcategories and qualifiers; and dysthymic disorder, conceptualized as a more chronic but less severe depressive illness (Table). (DSM-IV also lists minor depressive disorder and recurrent brief depressive disorder among the "criteria sets . . . for further study.")
DSM-IV relies heavily on lists of symptoms to define the categories. For MDD, 5 symptoms are required to make a diagnosis of a major depressive episode: low mood; anhedonia; changes in appetite, weight, sleep, or psychomotor activity; feelings of guilt or worthlessness; cognitive problems (such as poor concentration); and recurrent thoughts of death or suicide. For dysthymic disorder, depressed mood, along with a similar list of symptoms is specified: changes in appetite, weight, or sleep; low energy; low self-esteem; cognitive problems; and hopelessness. Minor depressive disorder requires fewer of the same symptoms as MDD.
When DSM-IV addresses the course of illness, the situation becomes much more confusing and complicated. For MDD, symptoms must be present continuously for 2 weeks and may be characterized by a single episode or be recurrent. Either can be chronic if symptoms present continuously for 2 years. The qualifier, without full interepisode recovery, can be added as well. For dysthymic disorder, symptoms must present for 2 years (1 year in children and adolescents) with no absence of symptoms lasting more than 2 months. Also, there can be no major depressive episode during the first 2 years of the disturbance (1 year for children and adolescents).
When the validity of these distinctions is examined, it becomes apparent that this multitude of diagnoses does not reflect the clinical reality of chronic depressive illnesses. The term "double depression" was introduced by Keller and colleagues3 in 1982 to describe patients with MDD and a preexisting chronic minor depression (now called dysthymic disorder). Although this term appears commonly in the clinical literature and comes closest to reflecting the clinical reality of chronic depression, it is not a DSM diagnosis and must be captured in DSM-IV by assigning 2 diagnoses (MDD and dysthymia).
The natural history of chronic depression was well described in the work of the NIH Collaborative Study on the Psychobiology of Depression. In one report from that project, 431 patients with a major depressive episode were monitored for 12 years, assessed every 6 months, and assigned to 1 of 4 symptom levels of depressive illness: major depression, dysthymia, subsyndromal symptoms, or no symptoms.2 The authors reported that 23% of patients were never symptom-free and that 88% of patients spent some follow-up weeks at 3 or 4 different symptom levels with level changes 2 to 3 times per year. They concluded that "the traditional . . . focus on MDD level of symptoms represents only the tip of the iceberg of this common, chronic and disabling disease." These data further support the idea that rather than a collection of diagnoses, chronic depressive illness is best understood as a fluctuating condition with different levels of severity over time.
McCullough and colleagues4 compared 681 outpatients with chronic depression for a broad range of demographic, clinical, psychosocial, family history, and treatment response variables. Using DSM-IV criteria, they assigned participants to 4 categories of chronic depression:
- Those who had a chronic major depressive episode lasting longer than 2 years.
- Those who had suffered at least 2 major depressive episodes but without full interepisode recovery.
- Those who had dysthymia and a major depressive episode (double depression).
- Those who had both dysthymia and a chronicmajor depressive episode.