DSM-IV criteria for major depressive episode and major depressive disorder are listed in Table 8.

An episode of major depressive disorder may be classified as mild, moderate, or severe. Mild episodes are characterized by little in the way of symptoms beyond the minimum required to make the diagnosis and by minor functional impairment. Moderate episodes are characterized by the presence of symptoms in excess of the bare diagnostic requirements and by greater degrees of functional impairment. Severe episodes are characterized by the presence of several symptoms in excess of the minimum requirements and by the symptoms' marked interference with social and/or occupational functioning. In the extreme, afflicted individuals may be totally unable to function socially or occupationally or even to feed or clothe themselves or to maintain minimal personal hygiene. The nature of the symptoms, such as suicidal ideation and behavior, should also be considered in assessing severity.

The melancholic subtype is a severe form of major depressive disorder with characteristic somatic symptoms, and it is believed to be particularly responsive to pharmacotherapy and ECT.

Major depressive disorder may be accompanied by hallucinations or delusions; these may be congruent or noncongruent with the depressive mood.

The differential diagnosis of dysthymia and major depressive disorder is particularly difficult, since the two disorders share similar symptoms and differ primarily in duration and severity. Usually major depressive disorder consists of one or more discrete major depressive episodes that can be distinguished from the person's usual functioning, whereas dysthymia is characterized by a chronic mild depressive syndrome that has been present for at least 2 years. If the initial onset of what appears to be dysthymia directly follows a major depressive episode, the appropriate diagnosis is major depressive disorder in partial remission. The diagnosis of dysthymia can be made following major depressive disorder only if there has been a full remission of the major depressive episode that has lasted at least 6 months before the development of dysthymia.

People with dysthymia frequently have a superimposed major depressive disorder, and this condition is often referred to as double major depressive disorder. Patients with double major depressive disorder are less likely to have a complete recovery than are patients with major depressive disorder without dysthymia.

The Epidemiologic Catchment Area study indicates that major depressive disorder has a 1-month prevalence of 2.2% and a lifetime prevalence of 5.8% in Americans 18 years and older (84). Other studies estimate the lifetime prevalence to be as high as 26% for women and 12% for men. The illness is 1.5 to 3 times as common among those with a first-degree biological relative affected with the disorder as among the general population. Major depressive disorder is frequently accompanied by comorbid conditions. For example, in one study of patients with major depressive disorder under the care of psychiatrists in the United States, 84% had at least one comorbid condition: 61% had a co-occurring axis I condition, 30% a comorbid axis II condition, and 58% a comorbid axis III condition (85). Frequently a major depressive episode follows a psychosocial stressor, particularly death of a loved one, marital separation, or the ending of an important relationship. Childbirth sometimes precipitates a major depressive episode. Patients with major depressive disorder identified in psychiatric settings tend to have episodes of greater severity and to have recurrent forms of major depressive disorder and also are more likely to have other mental disorders than are subjects from the community and primary care settings.

The average age at onset is the late 20s, but the disorder may begin at any age. The symptoms of major depressive disorder typically develop over days to weeks. Prodromal symptoms, including generalized anxiety, panic attacks, phobias, or depressive symptoms that do not meet the diagnostic threshold, may occur over the preceding several months. In some cases, however, a major depressive disorder may develop suddenly (e.g., when associated with severe psychosocial stress). The duration of a major depressive episode is also variable. Untreated, the episode typically lasts 6 months or longer. Some patients with major depressive disorder will eventually have a manic or hypomanic episode and will then be diagnosed as having bipolar disorder.

Although some people have only a single episode of major depressive disorder, with full return to premorbid functioning, it is estimated that from 50% to 85% of the people who have such an episode will eventually have another episode, at which time the illness will meet the criteria for recurrent major depressive disorder (86). People with major depressive disorder superimposed on dysthymia are at greater risk for having recurrent episodes of major depressive disorder than those without dysthymia.

The course of recurrent major depressive disorder is variable. Some people have episodes separated by many years of normal functioning, others have clusters of episodes, and still others have increasingly frequent episodes as they grow older.

Functioning usually returns to the premorbid level between episodes. In 20%–35% of the cases, however, there are persistent residual symptoms and social or occupational impairment. Patients who continue to meet the criteria for a major depressive episode throughout the course of the disturbance are considered to have the chronic type, whereas those who remain symptomatic are considered to be in partial remission.

A seasonal pattern of major depressive disorder is characterized by a regular temporal relationship between the onset and remission of symptoms and particular periods of the year (e.g., in the northern hemisphere, regular appearance of symptoms between the beginning of October and the end of November and regular remission from mid-February to mid-April). Patients should not receive this diagnosis if there is an obvious effect of seasonally related psychosocial stressors, e.g., seasonal unemployment.

The most serious complications of a major depressive episode are suicide and other violent acts. Other complications include marital, parental, social, and vocational difficulties (87). The illness, especially in its recurrent and chronic forms, may cause distress for other individuals in the patient's social network, e.g., children, spouse, and significant others. If the patient is a parent, the disorder may affect his or her ability to fulfill parental role expectations (88). Major depressive disorder episodes are associated with occupational dysfunction, including unemployment, absenteeism, and decreased work productivity (89). Major depressive disorder may also complicate recovery from other medical illnesses. Major depressive disorder has been demonstrated to be a major risk factor in the post-myocardial-infarction period.