Major depressive disorder Health Article

Definition

Major depressive disorder (MDD) is a condition characterized by a long-lasting depressed mood or marked loss of interest or pleasure (anhedonia) in all or nearly all activities. Children and adolescents with MDD may be irritable instead of sad. These symptoms, along with others described below, must be sufficiently severe to interfere significantly with the patient's daily functioning in order for a person to be diagnosed with MDD.

Description

Major depressive disorder is a serious mental disorder that profoundly affects an individual's quality of life. Unlike normal bereavement or an occasional episode of "the blues," MDD causes a lengthy period of gloom and hopelessness, and may rob the sufferer of the ability to take pleasure in activities or relationships that were previously enjoyable. In some cases, depressive episodes seem to be triggered by an obviously painful event, but MDD may also develop without a specific stressor. Research indicates that an initial episode of depression is likely to be a response to a specific stimulus, but later episodes are progressively more likely to start without a triggering event. A person suffering major depression finds jobrelated responsibilities and such other tasks as parenting burdensome and carried out only with great effort. Mental efficiency and memory are affected, causing even simple tasks to be tiring and irritating. Sexual interest dwindles; many people with MDD become withdrawn and avoid any type of social activity. Even the ability to enjoy a good meal or a sound night's sleep is frequently lost; many depressed people report a chronic sense of malaise (general discomfort or unease). For some, the pain and suffering accompanying MDD becomes so unendurable that suicideis viewed as the only option; MDD has the highest mortality rate of any mental disorder.

Major depressive disorder may be limited to a single episode of depression; more commonly, it may become a chronic condition with many episodes of depressed mood. Other symptoms that may develop include psychotic symptoms (bizarre thoughts, including delusional beliefs and hallucinations); catatonia; postpartum onset (sometimes accompanied by psychotic symptoms); and seasonal affective disorder, or SAD.

Such conditions as postpartum depressionand seasonal affective disorder accompany MDD only under certain circumstances. Postpartum depression begins within four weeks of giving birth. Women with this disorder experience labile mood (frequent drastic mood changes). They may feel helpless and unable to care adequately for their infant, or they may be completely uninterested in the child. The symptoms of postpartum depression are much more severe than those of the relatively common "new baby blues," which affect up to 70% of new mothers. The presence of psychotic symptoms in the mother, too many ruminations (obsessive thoughts), or delusionsabout the infant are associated with a heightened risk of serious harm to the child. The symptoms of postpartum depression are usually attributed to fluctuations in the woman's hormone levels and the emotional impact of bearing a child. The condition is especially likely to occur in women who were highly anxious during pregnancy or had a previous history of mood disorder. Seasonal affective disorder (SAD) is also more common in women than in men; in this case, symptoms of MDD typically begin in fall and winter, especially in northern latitudes in the United States and Canada. Exposure to natural light is limited during the winter in these areas, but the symptoms of SAD typically improve during the spring and summer.

Causes

Because MDD is a relatively common mental disorder, researchers have performed a range of different studies to identify possible underlying causes. Three types of causes are commonly identified: intrapsychic, environmental, and biological.

INTRAPSYCHIC.Since Sigmund Freud attributed the development of mental disorders to intrapsychic (occurring inside the mind) conflicts occurring during early childhood, a sizeable number of theorists have suggested that MDD results from a tendency to internalize negative events. Cognitive behavioral treatment models assume that a person's interpretation of situations is responsible for the development of depression rather than the events themselves. Some people blame themselves for negative experiences while attributing positive outcomes to external sources; they may tend to feel guilty, undeserving, and eventually depressed. For example, they may think of their present job as something they obtained by a chance stroke of good luck; at the same time, they may regard being laid off as something they brought on themselves. When these patterns of thought become habitual, they lead to a style of coping characterized by a view of oneself as worthless, ineffectual, and inferior. In some cases, people pick up these patterns of thinking from their parents or other family members.

Another theory regarding intrapsychic causes attributes depression to so-called "learned helplessness." This theory grew out of research studies on animal learning, comparing dogs that were able to escape from mild electric shocks to dogs that could not escape. The researchers discovered that the dogs who could not escape the mild shocks became passive; later, when they were put in a situation in which they could escape the shocks, they made no attempt to do so but simply lay on their stomachs and whimpered. The animals had, in short, learned to be helpless; they had learned during the first part of the experiment that nothing they had done had any effect on the shocks. Applied to human beings, this theory holds that people tend to become depressed when they have had long-term experiences of helplessness— as would be the case for abused children. Later, when the children have become adults, they do not see themselves as grownups with some control over their lives; they continue to react to setbacks or losses with the same feelings of helplessness that they had as children, and they become depressed.

ENVIRONMENTAL.Environmental theories of the etiology (causation) of MDD emphasize the role of external events in triggering depression. According to this perspective, people become depressed primarily due to unfortunate circumstances that are difficult to change. In some cases, these misfortunes may include environmental disasters or personal losses; but such other factors as low socioeconomic status, oppression associated with one's sex or race, or unpleasant or frustrating relationships are also thought to contribute to depression.

BIOLOGICAL.Ancient medicine alleged that one's state of mind was related to the presence of specific "humors," or fluids, in the body, and various theories have emerged since the eighteenth century regarding possible constitutional factors in humans that affect mood. In recent years, researchers have found numerous abnormalities in the neuroendocrine systems, neurotransmitters, and neuroanatomy of the brains of both children and adults with MDD, as well as strong evidence for genetic factors in MDD.

Levels of cortisol, a hormone associated with the human "fight-or-flight" response, have long been studied as possible biological markers for depression. In many adults, cortisol levels rise when the person is acutely depressed and return to normal when the depression passes. Research findings have been inconsistent regarding cortisol levels in children and adolescents, although there is some evidence that higher levels of cortisol secretion are associated with more severe depressive symptoms and with a higher likelihood of recurrence. As of 2002, however, cortisol levels are not considered to be reliable enough to be useful in diagnosing MDD.

Another biological factor that has been studied in humans are changes in the levels of neurotransmitters, which are chemicals that conduct nerve impulses across the tiny gaps between nerve cells. Variations in the levels of certain neurotransmitters have been researched for many years due to their importance in the brain's limbic system, which is the center of emotions and has many important pathways to other parts of the brain. In depression, the system that regulates a neurotransmitter called serotonin does not function properly. A group of medications known as serotonin specific reuptake inhibitors, or SSRIs, are assumed to be effective in relieving depression because they prevent serotonin from being taken back up too quickly by receptors in the brain.

Differences in the anatomical structure of the brains of children and adults with MDD have suggested several possible explanations for its development. In particular, the prefrontal cortex has been thought to play a role, on the basis of findings in stroke patients with damage to the prefrontal area of the brain, and in children and adults with MDD. Researchers found that stroke patients experienced more severe depression if their stroke occurred closer to the frontal lobe of the brain; similarly, people with MDD have been found to have decreased frontal lobe volume. Studies of depressed children and adults included subjects who were currently depressed as well as those with a history of depression who were in remission, which suggests that abnormalities in the frontal lobe may be a structural marker of depression. Other neurological studies have reported lower levels of electrical activity in the left frontal cortex among depressed subjects (including the infants of depressed mothers) compared to persons who are not depressed.

Researchers have also been interested in the relationship of genetic factors to depression. It has been known for many years that depression tends to run in families. Convincing evidence of the heritability of depression has been obtained by comparing identical twins (who have identical genetic inheritances) with fraternal twins; these studies have consistently found a higher likelihood of depression between identical than between fraternal twins. Other data indicate that people with a higher genetic risk of depression are more likely to become depressed following a stressful event than people with fewer genetic risk factors.