What exactly is "bipolar disorder" and how does it differ from depression?
Bipolar disorder, as currently defined in the Diagnostic and Statistical Manual of Mental Disorders, consists of three subsets:
First, there is "bipolar disorder one." This is a disorder that has been recognized since the turn of the last century and is also known as manic depression. It's characterized by episodes of major depression, for example; depressed mood, difficulty enjoying oneself, difficulty in sleeping and eating, low energy, problems with concentration, suicidal ideation, et cetera. But in addition to having episodes of depression, people with manic depression, or bipolar disorder one, also have episodes of mania or up-swings in mood. In mania, the presentation is one of either euphoria or extreme irritability.
Then, there is "bipolar disorder two." This is a more subtle form of the illness, in that the person still has episodes of major depression, but on the other side of the coin, there is hypomania. The up-swings never reach the same magnitude as they do in full mania, so that patients with bipolar disorder two have episodes of what we refer to as hypo-manic episodes, where they have some of the symptoms for mania, but in a much more attenuated fashion. One of the difficulties of this disorder is that the subtlety of those symptoms can often make it hard to diagnose.
The third type of bipolar disorder, which is what's being included in so-called "bipolar spectrum disorder," is "bipolar disorder NOS" ("Not Otherwise Specified"). This is a diagnostic criterion that's available when the clinician sees a patient who he or she believes has a bipolar condition, but doesn't strictly meet the criteria of either bipolar disorder one or bipolar disorder two. So that's where the word "spectrum" is utilized, to refer to this type of problem.
So it is the episodes of either manic or hypo-manic episodes that separate bipolar from depression.