I have neglected so far to discuss a very important diagnostic category: bipolar disorder.
In bipolar disorder, there can be episodes of severe depression, in fact this may be the presenting or initial problem. Bipolar disorder is strongly heritable (it runs in families), even more strongly than other types of mental illness. Yet there are cases in which bipolar disorder can arise in an individual without an obvious family history.
The other "pole" in bipolar disorder is mania: this is a state in which mood is abnormally elevated or irritable, with a variety of other accompanying symptoms-
-increased energy (in its extreme form, severe uncontrollable physical agitation)
-decreased need for sleep (in its extreme form, no sleep at all despite high energy)
-racing thoughts or speech (in its extreme form, leading to incomprehensible speech)
-elevated self-esteem (in its extreme form, delusions of grandeur such as believing oneself to have supernatural powers)
-reckless and uncharacteristic behaviour (such as driving dangerously, taking other unusual risks such as substance use or gambling)
-uncharacteristic increase or change in social behaviour (e.g. promiscuity, socializing freely with strangers)
-increased spending (sometimes this leads to financial catastrophe, giving away one's savings, buying new cars, etc.)
-there may be psychotic symptoms such as hallucinations, paranoia, or severely disorganized thinking
-increased "goal-directed activity": many new plans, ideas, and actions, but often these are disorganized and chaotic
-usually these symptoms last for weeks or months at a time. For some people their symptoms fluctuate much more rapidly, sometimes between depressed symptoms and manic symptoms, or some combination simultaneously. This is so-called "rapid cycling".
A manic state can be very severe, leading to the police needing to bring the afflicted person to the hospital. There can be catastrophic life consequences, affecting relationships, finances, or physical health.
In other cases, though, a manic state can be quite mild (a so-called "hypomanic" state), and may even be quite a pleasant and productive period of time.
For any person seeking treatment for depression, it is extremely important to examine closely whether there have been any manic symptoms in the past--even mild ones--or if there is a family history of bipolar disorder. One important reason for this is that antidepressants can provoke manic episodes in persons with bipolar disorder. Treating depression in bipolar disorder requires extra care to prevent a manic episode from arising. This can involve a so-called "mood-stabilizer" drug such as lithium carbonate. Or, it can involve choosing a different type of treatment for the depression, such as a newer drug called lamotrigine, which can help with bipolar depression without causing mania.