Depression—or Bipolar Disorder?
In the 1970s, The National Institutes of Health began a study on 559 patients diagnosed with a depressive disorder. One of the study aims was to see how a serious depressive illness evolved in patients over their lifetimes. The researchers followed some of these volunteers for up to 31 years.
In 1995, researchers published a series of reports with results from the first ten or so years of the study. One of those papers* reported that nearly 9 percent of these patients with "depression" developed a hypomanic episode during that time; that is, they turned out to have bipolar II disorder. The first hypomanic episode usually occurred within several months of the onset of depression, but sometimes it took up to nine years for the correct diagnosis to become apparent. Occasionally, these "depression" patients developed a manic episode—that is, they turned out to have bipolar I—but this was far less common (only 3.9 percent).
Depressed patients who later developed hypomanic periods (the bipolar II patients) were about five years younger when they first developed depression. Their first episodes of depression were significantly longer: nearly three times as long.
In 2011, another report on these same patients was published. It was now 31 years since the bout of depression that brought them into the study initially. With the passage of more time, nearly one in five of the patients with "depression" developed some form of bipolar disorder (19.6 percent), 12.2 percent bipolar II, and 7.5 percent bipolar I.
This report also noted that subjects who reported even a few hypomanic symptoms at the beginning of the study were at greater risk of developing bipolar disorder. Also, the more symptoms they reported, the higher their risk.
These studies found that persons with "depression" who go on to develop a bipolar disorder (either bipolar I or II) were:
younger when they first developed a major depression,
more likely to have a relative with a diagnosis of bipolar disorder,
more likely to have a few mild hypomanic symptoms.
We also recognize now that some people with mostly depression symptoms have a “soft” form of bipolar disorder and may benefit from drugs used to treat that illness. This observation has led to the idea that there is a spectrum of mood disorders.
Dr. Emil Kraepelin, who coined the term "manic depressive disorder," argued that all mood disorder patients should be diagnosed with "manic depression." That would also include patients who only have symptoms of depression. This idea fell out of favor in the first half of the twentieth century. When the term unipolar depression" was coined, psychiatrists started classifying purely depressive illnesses separately from bipolar disorders.
Today, many mood disorder researchers argue that Kraepelin was right and that mood disorders vary along a spectrum, with pure depressive disorders at one end, bipolar I disorder at the other, and many patients in-between.
A review paper published in 2007 found more than eleven hundred articles that argue either for and against this idea.
I would argue that most psychiatrists who treat patients with mood disorders eventually come to agree with Dr. Kraepelin—they see patients every day who don't fit neatly into a DSM category. These “soft” bipolar disorders are not only difficult to diagnose, but they are also difficult to treat. Many of these patients end up with complicated combinations of medications.
Fortunately, more and more clinical trials of medications for bipolar disorder include these patients, so hopefully, clinicians will have more guidance on treating them.
*F. Benazzi, "Is There a Continuity between Bipolar and Depressive Disorders?" Psychotherapy and Psychosomatics 76, no. 2 (2007): 70–76.
*Hagop Akiskal, Jack Maser, Pamela Zeller, Jean Endicott, William Coryell, Martin Keller, Meredith Warshaw, Paula Clayton, and Frederick Goodwin, "Switching from 'Unipolar' to Bipolar II: An Eleven Year Prospective Study of Clinical and Temperamental Predictors in 559 Patients," Archives of General Psychiatry 52 (1995): 114–23.
†J. G. Fiedorowicz, J. Endicott, A. C. Leon, D. A. Solomon, M. B. Keller, W. H. Coryell, "Subthreshold Hypomanic Symptoms in Progression from Unipolar Major Depression to Bipolar Disorder," American Journal of Psychiatry 168, no. 1 (2011): 40–48.