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The term depression has been used to refer both to an emotional state and a group of psychiatric disorders. As an emotional state, it is also known by various comparable terms: dejection, despair, sadness, despondency, lowering of spirits. Cognitions (perceptions and judgments) of a negative nature often accompany depressed mood.

Most people experience brief periods of depressed or despondent mood, often in response to a disappointing life event. Each individual utilizes different COPING skills and relies on available social supports to deal with such episodes, which generally pass within hours to days.

When a dysphoric mood becomes more severe, is persistent, and impairs functioning, a major depression as a clinical syndrome has developed. Concurrent clinical features include a loss of interest or pleasure in usual activities, a sense of hopelessness, poor or alternatively increased sleep, loss of appetite or overeating with resultant changes in weight, fatigue, anxiety, restlessness, obsessive thinking, difficulty concentrating, irritability, feelings of worthlessness, recurring thoughts of death, and suicidal ideation or an actual attempt to end one's life. Suicidal disturbances are of serious concern; approximately 66 percent of depressed patients contemplate suicide, and it is estimated that 10 to 15 percent succeed. In some cases, psychotic features such as hallucinations and delusions may develop.

Depression is one of the most common psychiatric disorders seen in adults. The lifetime prevalence of major depressive disorder (using DSM-III-R criteria) in the United States is estimated to be 12.7 percent in men and 21.3 percent in women. Some individuals suffer from chronically depressed mood of a less intense nature than that experienced in a major depressive episode; this is referred to as dysthymia. A depressive syndrome may occur as part of manic-depressive illness, and depression as a symptom (i.e., a depressed mood) can be found in many other psychiatric disorders.

Depression should be distinguished from the normal despair of bereavement and from the various medical disorders (e.g., Parkinson's disease) and chemical agents (e.g., alcohol or drugs for heart conditions) that can produce symptoms of depressed mood. The cause of depression is unknown. Biological factors (e.g., dysregulation of neurotransmitter systems), genetic factors, and psychosocial factors (e.g., life events, learned behaviors, and cognitions) have been proposed, and it is likely that all interact to varying extents. Depression is a treatable (but not really curable) illness in the vast majority of people. Treatment consists of a number of modalities, depending on the type and severity of the depression. PSYCHOTHERAPY, anti-depressant medications, and electroconvulsive therapy are the main interventions used.