Bipolar I and Bipolar II: It’s Time for Something New for a Better Understanding and Classification of Bipolar Disorders

As Prof Malhi and his colleagues have appropriately pointed out, bipolar II disorder has an unstable status in the classificatory system. From its introduction in the Diagnostic and Statistical Manual of Mental Disorders, Fourth edition, it has been separated as a distinct diagnostic entity and simultaneously regarded as a disorder with a milder form of mania.

Instead of dividing the disorders into bipolar I and bipolar II, Dr Malhi and his colleagues propose “bipolar disorder” as a single disorder entity and suggest a multidimensional approach comprising duration, severity, and impairment. Their proposal is based on the observation that (1) bipolar II disorder has little clinical and biological research evidence as a valid, distinct syndrome; (2) bipolar II disorder lacks specific treatment; and (3) overdiagnosis of bipolar II disorder can lead to the unnecessary risk of improper and excessive management.

Certainly, Dr Malhi’s proposal makes sense in some aspects, especially considering the instances of diagnostic change in some cases from bipolar II to bipolar I disorder as time goes on and the disease evolves. Although we agree with Prof Malhi’s proposal, problems still remain and are as follows:

The issue related to misdiagnosis originating from ambiguous diagnostic boundaries will still exist even if we adopt a single bipolar disorder or multidimensional approach. If we lower the threshold from 4 days to 2 days of hypomania, the issue of overdiagnosis could even be intensified. ¹ In fact, underdiagnosis of bipolar disorder is a more problematic mental health issue than overdiagnosis in most regions of the world including Asia, and the consequences of underdiagnosis are not smaller than the risk of unnecessary management due to overdiagnosis. ²

Prof Malhi proposes to consider duration, severity, and impairment independently in the multidimensional threshold for mania. We agree with Prof Malhi’s assertion to include impairment as one of the criteria. We propose that “frequency” should also be taken into consideration. The issue still unsolved is that these variables are very much interrelated. Current descriptive psychiatry has not yet developed valid methodologies for measuring impairment as a function of the duration, frequency, and severity of bipolar disorder.

Another important point is that the course or prognosis of bipolar disorder is determined by the duration, severity, frequency, or impairment not only of manic episodes but also of depressive episodes. ^3,4 Although the points of differentiation between bipolar I and II in the Diagnostic and Statistical Manual of Mental Disorders, fifth edition, come from the severity and duration of manic episodes, it is depressive episodes that draw attention of the clinicians who diagnose or manage bipolar II disorder, in which the depressive phase is longer, more severe, more frequent, and more debilitating than the hypomanic phase. In the past, before the introduction of the concept of bipolar II disorder, bipolar disorder was mainly regarded as “manic” disorder in most Asian countries. It was only after the introduction of bipolar II disorder that most clinicians in Asia started to pay attention to the diverse nature and manifestations of depressive episodes and hypomania of bipolar disorders. Increased attention to the depressive phase in turn increased awareness of subclinical mood fluctuation, rapid cycling, mixed state, and various comorbid conditions such as anxiety, obsessive compulsive symptoms, and addictive behaviors. All these changes have led clinicians to understand bipolar disorder from a longitudinal perspective more than before. The introduction of bipolar II disorder also brought changes in the view of mood disorders as a spectrum from unipolar depression to unipolar depression with possible bipolarity, bipolar II disorder, and to bipolar I disorder.

It is true that clinicians have been motivated to explore hypomanic episodes among depressive patients when they found clinical characteristics suggesting bipolar II disorder, that is, earlier age of onset, highly cyclic nature, variable duration of depressive episodes, lifetime trait of seasonality and premenstrual syndrome, circadian preference, and higher comorbid conditions including anxiety and personality traits, which is different from major depressive disorder and also from bipolar I disorder. ^{5 –8} Without such thorough exploration, many patients with hypomania were diagnosed as having major depressive disorder with various comorbid conditions, which resulted in poor treatment response. A proportion of patients with hypomania were diagnosed as borderline or other cluster B personality disorders in Asia, where resources for appropriate psychotherapeutic intervention were extremely limited. It is unknown whether clinicians will continue to vigorously explore the possible hypomanic episodes without the separate existence of bipolar II disorder.

Most psychiatric research has been aimed at exploring interdisease commonalities or intradisease heterogeneities. Actually, there are many biological studies reporting overlapping or similar results between bipolar disorder and major depressive disorder, between schizophrenia and bipolar disorder, between depressive disorder and obsessive-compulsive disorder, and so on. It is also true that a single medication has effectiveness for several mental disorders. Like many of the other interdisease commonalities stated earlier, bipolar I and bipolar II disorders share clinical and biological characteristics, which does not necessarily mean or indicate that the two conditions belong to a single disease entity and are unclassifiable. Biological studies suggesting quantitative or qualitative differences between bipolar I and II disorders are also growing. ^{9 –12} Neglecting intradisease heterogeneity could be more problematic than neglecting interdisease commonality. If we combine two heterogeneous groups into a single bipolar disorder, we might miss important biological signals or characteristics.

We fully agree with Prof Malhi’s premise and opinion that we need to research and modify the concept, definition, and diagnostic criteria for bipolar disorders. We fully agree that it is time for something new. However, the breakthrough should not be limited to the discussion related to the manic–hypomanic distinction, which would inevitably be ambiguous. Prof Malhi’s proposal and current discussion tells us that it is time to promote studies focusing on bipolar II disorder per se, to explore its intrinsic nature of a longitudinal course, and other characterizing features including family history, personality, seasonality, comorbidities, and others that were suggested by previous studies. Clinical trials specifically targeting bipolar II disorder are also warranted. These approaches would provide a better solution to what Prof Malhi raised in his paper than discarding the concept of bipolar II disorder.