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Bipolar II Disorder Is NOT a Myth

The challenge to define and refine psychiatric diagnostic categories has led researchers to a healthy debate about the usefulness or lack thereof of particular diagnoses, especially bipolar II disorder. The National Institute of Mental Health (NIMH) and others have challenged the overall validity of the Diagnostic and Statistical Manual of Mental Disorders rubric, at times dismissing it as an arbitrary set of symptoms, observations, severity, duration, and impairment, which fail to “carve nature at its joints.” As an alternative, NIMH proposed and implemented the multidimensional Research Domain Criteria (RDoC), a translational project to better understand psychopathology from molecular to cellular to animal model to clinical phenomena. Yet, after considerable investments, RDoC has yet to deliver on its promise as a top-down deterministic paradigm to uncover previously unknown pathophysiology to supplant the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Ultimately, the overarching project of understanding the physiologic basis of complex psychiatric disorders and developing better treatments, including bipolar disorder, will most likely be more difficult than anyone had anticipated and will need to use all of the tools available, including categorical and dimensional approaches when appropriate to the task at hand. Which brings us back to the controversy around the diagnosis of bipolar II disorder. Ultimately, while reversing the causal chain of research from molecules to diagnoses implied by the RDoC, will the diagnosis of bipolar II disorder prove pathophysiologically or clinically distinct from bipolar I? The history of how the diagnosis of bipolar II was established and defined is well covered elsewhere. Hypomanic episodes, rather than manic episodes, distinguish bipolar II from bipolar I. Malhi and colleagues are correct in pointing out the vagaries of how the DSM-5 and International Classification of Diseases, 11th Revision (ICD-11) define hypomania—and one could argue that hypomania differs from mania as a matter of degree (severity, duration, and impairment, along with the presence or absence of psychosis). More important, episodes of major depression and subthreshold depressive symptoms appear to be worse in bipolar II versus bipolar I. In the now-classic naturalistic longitudinal Collaborative Study of Depression, Judd and colleagues found that people with bipolar II spent over 50% of their time with depression compared to those with bipolar I who spent about 30% of time. Malhi et al. argue that this distinction should be folded into a spectrum of bipolar disorder rather than a categorical difference, letting the data speak for themselves (i.e., just like light, bipolar disorder should be considered a “wave” rather than distinct “particles”). I argue that, like light, one should consider bipolar disorder as a spectrum and as distinct categories depending on the purpose, be it exploring pathophysiology, longitudinal course, or response to treatment.