Persistent Depressive Disorder: Commentary on Parker and Malhi

In their recent Perspective article, Parker and Malhi¹ provide an impassioned critique of the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) diagnosis of persistent depressive disorder (PDD). They argue that the diagnosis should be abolished, as it lacks a rationale and supporting evidence for combining chronic major depressive disorder and dysthymic disorder in a separate category. They also contend that PDD includes many cases that do not have a clinically significant disorder and that the diagnosis may be better conceptualized as a reflection of personality disturbance than as a mood disorder. They conclude by advocating that the diagnosis be subsumed as a chronic course modifier within a unitary depressive disorder category. However, Parker and Malhi overlook most of the evidence that led to establishing the PDD diagnosis. In this Commentary, I will summarize the rationale and evidence for PDD, beginning with some historical context, and will then briefly address some of Parker and Malhi’s other points.

The first DSM category for chronic depression, Dysthymia, was introduced in DSM-III, following pioneering work by Weissman and Klerman² and Akiskal et al³ demonstrating that, contrary to the prevailing view of depression as an episodic/remitting condition, many depressed patients experience a chronic course of illness. Indeed, approximately half of depressed patients in clinical settings exhibit chronic forms of depression.^4–6 At the time it was introduced, Dysthymia was highly controversial as many believed it should be classified as a personality, rather than a mood, disorder.⁷ However, this controversy faded in light of evidence that dysthymia responded to antidepressant medication, was a powerful risk factor for developing major depression in the future, and was associated with an increased risk of major depression in first-degree relatives.^7,8 DSM-III-R furthered the recognition of chronic forms of depression by introducing a specifier for Chronic Major Depressive Episode. DSM-IV continued this trend by introducing a series of course specifiers that recognized a variety of chronic presentations of depression, including the pattern sometimes referred to as “double depression,”⁹ in which a major depressive episode develops in the context of a preexisting dysthymia.

However, subsequent studies found few differences between dysthymia, double depression, and chronic major depression on comorbidity, personality, depressive cognitions, coping style, childhood adversity, familial psychopathology, response to pharmacotherapy and psychotherapy, and naturalistic course and outcome.^8,10,11 Moreover, follow-up data indicated that the various forms of chronic depression often shifted to other types of chronic depression over the course of the illness.¹² Together, these studies suggested that the various types of chronic depression in DSM-IV represented distinctions without a difference and that simplification was warranted. Hence, the multiform presentations of chronic depression were brought together under the rubric of PDD. However, DSM-5 conservatively preserved much of the information on course patterning in the form of specifiers reflecting dysthymia, chronic major depression, and dysthymia with current or past major depressive episodes, for, as Parker and Malhi note, current severity of symptoms has important treatment implications.

Although there are few differences between the various forms of chronic depression, there are substantial and well-replicated differences between chronic depressions and episodic (or nonchronic) major depression.^8,13 As Parker and Malhi note, chronic depression is characterized by higher rates of personality disorders and greater childhood adversity and maltreatment compared to episodic major depression. Chronic depressions are also characterized by higher rates of mood disorders in first-degree relatives than episodic major depression.^14,15 More strikingly, family studies have demonstrated specificity of familial aggregation, with increased rates of chronic depression in the relatives of probands with chronic depression compared to relatives of probands with episodic major depression and healthy controls.^14–16 In addition, follow-up data indicate that the chronic-episodic distinction is stable over time: recurrences of episodic major depression are almost always episodic, whereas recurrences of chronic depression are almost always chronic (although, as noted earlier, the precise form of chronic depression may vary).¹² Finally, we recently reported evidence for a qualitative, rather than quantitative, relationship between duration of depression and long-term outcomes.¹⁷ Hence, there is a great deal of evidence supporting the distinction between episodic major depression and the chronic forms of depression that comprise PDD in DSM-5.

Parker and Malhi are also concerned that milder forms of DSM-5 PDD (i.e., dysthymia without a concurrent major depressive episode) are not clinically significant. However, these cases are rare, and almost all patients with dysthymia eventually experience exacerbations that meet criteria for major depression.¹² Moreover, even “pure” dysthymia (without major depression) is associated with as much impairment in social functioning as episodic major depression, and both groups of patients exhibit greater impairment than healthy controls (although less than patients with “double depression”).^18–20 On the other hand, Parker and Malhi are correct that PDD is associated with a high rate of personality disorders and a high level of neuroticism (as well as lower extraversion), making it difficult to disentangle traits from depression symptoms. Their position seems to be similar to the argument at the time DSM-III was introduced that dysthymia should be classified as a personality, rather than mood, disorder, and harkens back to the older distinction between neurotic and endogenous depression. The relationship between personality and chronic depression is unquestionably complex and worthy of sustained consideration.⁸ However, it is difficult to reconcile Parker and Malhi’s position with their larger brief against separating episodic from chronic forms of depression. Indeed, this complex mixture of state and trait is one of the critical distinctions between these two forms of depression.

Parker and Malhi conclude by arguing that PDD should be abolished and that a chronic course should be incorporated as a specifier in a unitary depressive disorder category. While there is virtue in considering a system with separate axes for symptom severity and course,²¹ reducing chronicity to the status of a modifier would revert back to the period in which the default view of depression was that of an episodic and remitting condition and imply that chronic presentations are an exception, and a suspicious one at that, with questionable legitimacy as a psychiatric disorder or a reflection of personality disturbance.

In conclusion, the DSM-5 PDD diagnosis is consistent with a number of studies that indicate that splitting chronic depression into multiple diagnoses had little validity, but that there are many clinically and etiologically significant distinctions between chronic depression and nonchronic major depression.^8,13 While the classification of depressive disorders can certainly be improved, downgrading the significance of the longitudinal course of depression is not the way forward.