A mental disorder characterized by sustained depression of mood, anhedonia, sleep and appetite disturbances, and feelings of worthlessness, guilt, and hopelessness. Diagnostic criteria for a major depressive episode (DSM-IV) include a depressed mood, a marked reduction of interest or pleasure in virtually all activities, or both, lasting for at least 2 weeks. In addition, 3 or more of the following must be present: gain or loss of weight, increased or decreased sleep, increased or decreased level of psychomotor activity, fatigue, feelings of guilt or worthlessness, diminished ability to concentrate, and recurring thoughts of death or suicide.
Major depression is the most common psychiatric disorder. According to the World Health Organization, it is the leading cause of disability worldwide among people aged 5 years and older. About 10% of men and 25% of women experience major depression at some time in their lives. Approximately 20 million people a year suffer depressive illness in the U.S., where the negative impact of this disease on the economy is estimated at $16 billion annually. Risk factors for depression are drug or alcohol abuse, chronic physical illness, stressful life events, social isolation, a history of physical or sexual abuse, and a family history of depressive illness. Depression can be masked by substance abuse. In old people, it may be mistaken for senile dementia, and vice versa; the two may coexist. The disorder is believed to result from an electrochemical malfunction of the limbic system involving disturbances in the metabolism of the neurotransmitters dopamine and serotonin. In people with familial depression, the number of glial cells in the subgenual prefrontal cortex is significantly smaller than in mentally healthy people. Treatment with psychopharmaceutical agents, including tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase (MAO) inhibitors, and others, effectively controls most cases of clinical depression. Cognitive-behavioral psychotherapy has demonstrated some success in reversing depression. Refined methods of electroconvulsive shock therapy (ECT) are used in cases that do not respond to other treatment. Even in severe depression the response rate with ECT is 80% or higher. This mode of therapy has a faster onset of action, causes fewer side effects than drug therapy, and is particularly useful in elderly patients.
Reference: Stedman's Medical Dictionary