In order to diagnose someone with Major Depression, they must have had at least one Major Depressive Episode (in which they suffer from depressed mood, or the loss of interest or pleasure in nearly all activities) for at least two weeks. In addition, depressive symptoms must persist for most of the day, nearly every day, and cause problems in social interactions, work, or someone's ability to function in daily life. For some individuals with milder depressive episodes, social and occupational functioning may appear to be normal, but the affected person is making a serious, concerted effort to appear unimpaired.
When a person has experienced only one episode of depression, it is classified as Major Depression, Single Episode. When multiple Major Depressive Episodes occur in a row, and no manic or mixed episodes are observed, the diagnoses changes to Major Depression, Recurrent.
Untreated, a Major Depressive Episode may last, on average, about 4 months. Some people only experience a single depressive episode and thereafter are symptom free. However, many people who experience one major depressive episode will go on to experience multiple major depressive episodes. The more major depressive episodes an individual experiences, the more likely they are to develop future episodes. Approximately 60% of people with MDD who experience a single depressive episode go on to have a second episode, and about 70% are likely to experience a third episode. Similarly, having had three episodes increases the chances to about 90% that a fourth episode will occur.
The course of Major Depression, Recurrent varies across individuals. Some people have isolated depressive episodes that are separated by many years during which mood is normal, whereas other individuals experience clusters of major depressive episodes that occur closely together in time. Still other people with Major Depression experience increasingly frequent episodes as they grow older.
Episodes of Major Depression can be triggered by highly stressful events. Stressors capable of triggering major depression may include the death of a loved one and other significant losses such as a job layoff; or relationship difficulties such as divorce or separation. Other more typical sorts of life changes may trigger depression as well. Even normal developmental milestones such as puberty, marriage, or retirement may serve to trigger depression when a particular event is personally distressing to a given individual. Stressors that trigger depression may be fresh, or they may be past events that are intensely remembered for some reason or another. For example, thinking deeply about stressful past experiences such as emotional, physical, or sexual abuse can be sufficient to trigger an onset of depression. Individuals with Post-Traumatic Stress Disorder (PTSD), who have experienced a traumatic event such as a military battle, rape, severe automobile accident or natural disaster, are more likely to suffer from depression than people who have not experienced such trauma.
A wide range of medical conditions and medications (see below) can also cause or worsen MDD. Major depression frequently co-occurs with other mental illnesses such as: substance-related disorders, panic disorder, obsessive-compulsive disorder, anorexia nervosa, bulimia nervosa, and borderline personality disorder.
At the present time, there is no diagnostic laboratory test (e.g., no blood test or brain scan) that can confirm whether you have Major Depression. However, some laboratory tests can appear abnormal during an active depressive episode. For example, sleep electroencephalograph (measurements of electrical activity in the brain during sleep) abnormalities have been found in 90% of people who are hospitalized for MDD. Even depressed people who do not have symptoms severe enough to require hospitalization often show EEG abnormalities. Other abnormalities in brain chemicals and hormones also occur with depression, but none of the tests examining these factors are stable, reliable, or specific enough at present to develop a foolproof way of diagnosing Major Depression.
Even though lab tests aren't used to diagnose depression, don't be surprised if your doctor still sends you to the lab. Medical tests can be helpful in uncovering other conditions such as thyroid trouble, cancer, arthritis, and other diseases discussed later in this article that might be causing depressive symptoms. We will discuss more about how clinicians diagnose depression using lab tests, psychiatric interviews and self-report questionnaires in a later section of this article as well.
Symptoms of Major Depression usually develop over a few days or weeks. Many people feel anxious or mildly depressed for a while before a full depressive episode becomes apparent. Often, the symptoms eventually disappear and functioning returns to normal. The DSM provides labels that describe the course of a person's Major Depression: "full remission" means no current depressive symptoms; "partial remission" means that the person currently has fewer than five depressive symptoms or has had no symptoms at all for less than two months; and "chronic" means that a person has met all of the diagnostic criteria for Major Depression for two or more years. Approximately 20-30% of people with Major Depression experience partial remission of symptoms, while 5-10% of people have chronic MDD.