One of the most important distinctions in all of psychiatry is often also the most difficult to make: Is the patient's depression part of a bipolar or a unipolar course of mood disorder? This is so consequential a decision because treating the depression of bipolar disorder with antidepressants can trigger problematic irritability, mood swings, and rapid cycling. To reduce this risk, patients receiving antidepressants for bipolar depression usually also receive either a mood stabilizer or an antipsychotic (or too often both). But the reduced risk of mood swings conferred by the covering medication comes at a potentially heavy cost in side effects and complications (especially dangerous weight gain and diabetes). The tough question is where to draw the diagnostic line between bipolar and unipolar mood disorder in a way that best balances the risks of taking vs. not taking the covering medication.
The trend in diagnostic habits over the past 25 years has been clear. The boundaries of bipolar disorder have expanded widely at the expense of unipolar; the prevalence ratio between them is now about 1:3 as opposed to the previous 1:6. This has caused a markedly increased use of mood stabilizing and antipsychotic drugs; they have protective effects in those who need them, but harmful side effects in those who may not.
Two factors account for this move toward bipolar (and away from unipolar) disorder. The first was the introduction by DSM-IV of a new official category (Bipolar II) that expanded the bipolar category into unipolar territory. Bipolar II describes patients who have depressions that alternate with hypomanic episodes (rather than with the manic episodes of classic Bipolar Disorder). Hypomanic episodes are milder and often briefer versions of classic mania and are therefore very much more difficult to diagnose. This is complicated further by the fact that brief periods of elevated mood can be provoked by drugs or medications. And it is often hard to distinguish the elevated mood of hypomania from a temporary return to normal mood in someone who has become accustomed to always feeling depressed.
Patients with depression and hypomania are at the boundary separating bipolar and unipolar disorder. They could have been classified in either camp. We made the decision to describe them as Bipolar II in DSM-IV because the weight of the course, family history, and treatment evidence suggested that they sorted better with Bipolar Disorder. This was not an easy decision. We had to balance: 1) concerns that patients with bipolar tendencies would be harmed iatrogenically if they received antidepressants without coverage, against 2) concerns that some patients identified as Bipolar II were really unipolar and would receive the added burden of unnecessary and potentially harmful medication. This was a close call, but on balance it seemed safer to include Bipolar II as a new category. I still think this made sense, although the extent of the bipolar fad that followed was certainly surprising and remains a serious concern.
We perhaps should have, but did not, anticipate how intense and effective the marketing and "education" campaign mounted by the pharmaceutical industry would be, alerting psychiatrists, primary care doctors, other mental health workers, and patients about the perils of previously "missed" bipolar disorder. This led to more accurate diagnosis and safer treatment for many patients, but like all fads, it overshot and has led to unnecessary medication for others who are now misdiagnosed as bipolar on very flimsy grounds.
This brings us to two problematic changes being considered by the DSM-5 mood disorders work group: 1) allowing hypomania to be diagnosed just on the basis of increased energy/activity (no longer requiring the presence of elevated mood or irritability), and 2) reducing the duration requirement for a hypomanic episode (now set at four days by DSM-IV). As is usual with all the changes being considered for DSM-5, the lowered thresholds are meant to avoid missed diagnoses, but this seems far outweighed by the risks of overdiagnosis of Bipolar II Disorder and the consequent overtreatment with harmful medications.
We must accept that there is no currently available method that infallibly distinguishes bipolar from unipolar disorder at their boundary. Until we have a much deeper understanding of the different pathogeneses of unipolar vs. bipolar disorder, this crucial distinction must be based exclusively on clinical judgment using the admittedly arbitrary DSM rules of thumb defining the symptoms and duration of a hypomanic episode. Any decisions lowering the symptom and duration requirements for a hypomanic episode will feed what is already a bipolar fad.
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