Since the classic descriptions, depression has been conceived as an episodic and recurrent illness. Depressive episodes with clear onset and offset and sharp contrast with one’s usual mood and behaviors are perhaps the most conspicuous feature of severe mood disorders. However, systematic studies of unselected samples have been telling a different story: a large proportion of individuals suffer from low mood, lack of interest, and other symptoms of depression chronically, with some fluctuations but no clearly demarcated episodes.1 Chronic patterns of symptoms are often under-recognized and undertreated in the community.2
This article provides an update on the diagnosis, causation, and treatment of chronic depressive problems, with a focus on the recently introduced diagnostic category of persistent depressive disorder (PDD).
In DSM-III and DSM-IV, the protracted forms of depression have been conceptualized as dysthymia and by the chronic specifier of major depressive episodes. Dysthymia was characterized by milder symptoms not fully meeting criteria for MDD, but lasting 2 years or longer and meriting clinical attention because of the cumulative burden of long-standing symptoms. The symptomatic criteria for dysthymia differed in part from those for major depressive episode, with an emphasis on low self-esteem and hopelessness (Table 1).
In DSM-III and DSM-IV, dysthymia was trumped by MDD and was only diagnosed if the threshold for a major depressive episode was not met in the initial 2 years of symptoms. Major depressive episodes could be specified as chronic if the full criteria were continuously met for 2 years or longer.
The validity of dysthymia and its separation from MDD has been repeatedly discussed and questioned.3 When individuals with dysthymia were followed over long periods, it became clear that most of them also developed major depressive episodes, which suggests that dysthymia and major depressive episodes are phases of the same disorder rather than separate conditions.4 Dysthymia and MDD also run in the same families and respond to the same treatments. On the other hand, both dysthymia and chronic depression are associated with more impairment, comorbidity, and suicide risk than less persistent forms of depression.5
Chronic depression and dysthymia were merged into PDD in DSM-5. This new division of depressive disorders gives more weight to duration than to severity of symptoms. DSM-5 defines PDD on the basis of the set of symptoms for dysthymia, with the assumption that most individuals who meet the full symptoms for MDD also meet criteria for dysthymia. However, because of differences in symptomatic criteria, some individuals with chronic major depressive episodes will not meet the DSM-5 criteria for PDD.6
While the merger of dysthymia and chronic depression into PDD is well justified by their strong sequential comorbidity and similar implications for prognosis and treatment, several aspects of the new diagnosis are not well supported by evidence and may not be useful. Why do we need 2 different sets of symptomatic criteria for MDD and PDD? The reliability and validity of the dysthymia criteria has not been formally tested, prompting concerns about the value of the new diagnosis.3
The assumption that most individuals with chronic depression also fulfill the dysthymia criteria may not hold consistently enough—it creates a group of individuals who suffer from chronic depression but do not receive the PDD diagnosis. While it is undisputable that prolonged duration and nonepisodic character are relevant, there is no good justification for the 2-year cutoff. In fact, some of the work used to justify the validity of persistent depression is based on a duration of 1 year or longer.7 For clinical and prognostic purposes, it is important to emphasize that duration of depressive symptoms is important both below and above the 2-year mark regardless of whether the depression or dysthymia/PDD criteria are met.
Dr Uher is Associate Professor and Canada Research Chair in the department of psychiatry at Dalhousie University in Halifax, Nova Scotia. He reports no conflicts on interest concerning the subject matter of this article.
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