Until 1980, all recurrent severe mood episodes were called “manic-depression”. It didn’t matter if the mood episodes were manic OR depressive, the illness was called “manic-depressive illness.” This mean the presence of manic OR depressive episodes, not manic AND depressive episodes. Thus, one could have 20 depressive episodes, and the diagnosis would be manic-depressive illness (MDI).
In 1980, this approach was officially changed in DSM-III and the very large and broad concept of MDI was divided into two: the much smaller concept of bipolar disorder and the broad concept of “major depressive disorder” (MDD). Bipolar disorder meant manic AND depressive episodes, which is a much smaller group than MDI (which was manic OR depressive episodes). Those who had only depressive episodes were called “unipolar” depression, which later got relabeled MDD.
This distinction between bipolar and unipolar mood illness was based on research in the 1960s and 1970s which claimed that these conditions could be differentiated based not just on symptoms, but on other “diagnostic validators”, which means lines of evidence that are independent of symptoms but which indicate symptom differences represent different illnesses. For instance, pneumonia with cough is different in symptoms than pneumonia without cough; but those symptom differences are not sufficient to say they are different illnesses. Not all symptom differences represent different illnesses. The idea was that other diagnostic validators could tell us if different symptoms represent different illnesses.
Those other diagnostic validators were genetics and course, and as a smaller validator, treatment response.
Those studies found that bipolar illness was genetically specific: in families of bipolar patients, there was bipolar illness; in families without bipolar illness, there we no bipolar patients. If patients had unipolar depression, they did not have bipolar illness in their families.
Depression in persons with bipolar illness began around age 19, and episodes lasted months or less. Depression in persons with unipolar depression began around age 30, and episodes lasted 6-12 months or more.
Antidepressants can cause mania in bipolar illness, but do not do so in unipolar depression (25% occurrence in bipolar patients, <<<1% occurrence in unipolar depression). Antidepressants are ineffective in depression in bipolar illness, and somewhat more effective in unipolar depression.
SO, if you have depression, but no manic episodes, but you have a genetics of bipolar illness and/or a course of bipolar illness and/or mania with antidepressants or other medications, then you are a squared circle. You shouldn’t happen. The whole idea of having unipolar depression is that there would be no genetics of bipolar illness and the course of illness would be as described above.
The problem we have now in psychiatric diagnosis according to DSM and the standard textbooks is that the claim is made that we should diagnose solely based on symptoms: depression with or without mania. If there is no mania, there is no bipolar. But we ignore the genetics and course evidence which is the entire basis for making the claim that the symptom differences represent different illnesses. In other words, the whole reason we think presence or absence of mania represents a different illness (bipolar versus unipolar) is because presence or absence of mania correlated with a certain genetics and course. But if the genetics and course of a person is opposite the scientific evidence about the presence or absence of mania, then we have a paradox.
There are only two solutions to this paradox.
Manic-depressive illness is one disease: There is no such thing as "bipolar disorder" and no such thing as "major depressive disorder"
It could be that the old MDI concept is correct, and we should not obsess about presence or absence of mania, and simply say that all recurrent mood episodes represent the same illness. It has been shown that lithium is effective for prevention of mood episodes of any kind, not just bipolar illness. It has been shown that dopamine blockers (antipsychotics) are effective for acute treatment of mood episodes of any kind (depression or mania). Thus, the claim that depression versus mania makes a big difference in diagnosis or treatment (using “antidepressants” versus “antipsychotics” or “mood stabilizers” may be a false distinction.
There is a mood spectrum between classic bipolar disorder and classic unipolar depression ("Major depressive disorder" is still scientifically meaningless)
The other solution is to think of mood illness as a spectrum, with classic unipolar depression on one extreme, and classic bipolar illness on the other extreme, with a lot of people in the middle with mixtures of unipolar and bipolar features. We sometimes call the middle area the “bipolar spectrum” or we could just say the mood spectrum or the “manic-depressive spectrum.” Many people are in the middle of this spectrum, but DSM and our textbooks try to force people into the bipolar or MDD categories, with corresponding simplistic decisions of giving antidepressants over and over again for the MDD category (which is the broadest and thus where most people get shoved by our textbooks and DSM). If we accept a spectrum concept, we would be more flexible about using dopamine blockers or mood stabilizers for treating depressive conditions, and indeed those agents are effective for many depressive conditions that are not part of the narrow DSM defined bipolar disorder definition. Here, you can understand why there are more than one sort of depression, such as the important Mixed Type.
Another aspect of the manic-depressive spectrum is that there are mood temperaments, representing mild mood symptoms as part of one's personality: mild mania (hyperthymia), mild depression (dysthymia) and mild mood swings to both types (cyclothymia).
References and citations for the data in this article can be found here.