The cause of depression is unknown, but there are many theories based on research.
Most likely, depression is caused by a combination of genetic, biological, environmental, and psychological factors.
Depressive illnesses are disorders of the brain.
Longstanding theories about depression suggest that important neurotransmitters—chemicals that brain cells use to communicate—are out of balance in depression. But it has been difficult to prove this.
Brain-imaging technologies, such as magnetic resonance imaging (MRI), have shown that the brains of people who have depression look different than those of people without depression.
The parts of the brain involved in mood, thinking, sleep, appetite, and behavior appear different.
These images do not reveal why the depression has occurred. They also cannot be used to diagnose depression.
Some types of depression tend to run in families.
However, depression can occur in people without family histories of depression too.
Scientists are studying certain genes that may make some people more prone to depression. Some genetics research indicates that risk for depression results from the influence of several genes acting together with environmental or other factors.
In addition, trauma, loss of a loved one, a difficult relationship, or any stressful situation may trigger a depressive episode. Other depressive episodes may occur with or without an obvious trigger.
One in five people will suffer from depression during their lifetime, and it is the leading cause of disability worldwide. The good news is that depression is treatable.
There are several forms of depressive disorders.
Major depression—severe symptoms that interfere with your ability to work, sleep, study, eat, and enjoy life. An episode can occur only once in a person’s lifetime, but more often, a person has several episodes.
Persistent depressive disorder—depressed mood that lasts for at least 2 years. A person diagnosed with persistent depressive disorder may have episodes of major depression, along with periods of less severe symptoms, but symptoms must last for 2 years.
Some forms of depression are slightly different, or they may develop under unique circumstances. They include:
• Psychotic depression, which occurs when a person has severe depression plus some form of psychosis, such as having disturbing false beliefs or a break with reality (delusions), or hearing or seeing upsetting things that others cannot hear or see (hallucinations).
• Postpartum depression, which is much more serious than the "baby blues" that many women experience after giving birth, when hormonal and physical changes and the new responsibility of caring for a newborn can be overwhelming. It is estimated that 10 to 15 percent of women experience postpartum depression after giving birth.
• Seasonal affective disorder (SAD), which is characterized by the onset of depression during the winter months, when there is less natural sunlight. The depression generally lifts during spring and summer. SAD may be effectively treated with light therapy, but nearly half of those with SAD do not get better with light therapy alone. Antidepressant medication and psychotherapy can reduce SAD symptoms, either alone or in combination with light therapy.
Bipolar disorder¬— also called manic-depressive illness, is not as common as major depression or persistent depressive disorder. Bipolar disorder is characterized by cycling mood changes—from extreme highs (e.g., mania) to extreme lows (e.g., depression).
Dysthymia—less severe type of depression, dysthymia (or dysthymic disorder), involves long-lasting symptoms that do not seriously disable, but keep one from functioning well or feeling good.
(National Institute for Mental Health)
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