Depression encompasses a wide range of clinical entities, from a mild mood disturbance commonly found in both normal and anxious persons to a serious, potentially lethal medical illness. Moreover, depression can be either a central or an associated feature of many types of mental disorders. Clinical depression can be defined as a condition characterized by a persistent and abnormal lowering of mood (feeling sad, blue, unhappy) and/or a loss of interest in usual activities, accompanied by a variety of characteristic signs and symptoms. The signs and symptoms of clinical depression are observed in five distinct, but related areas:

Since first described by Hippocrates as a medical illness, depression has undergone many transformations through the centuries. In the 1930s, Manfred Blueler classified depressive disorders under the term “affective” disorders. More recently, the revised fourth edition of the Diagnostic and Statistical Manual (DSM-IV-TR), published by the American Psychiatric Association (2000), replaced affective disorders by the term “mood” disorders in order to emphasize the gross deviation in mood. Although the DSM-IV-TR classification of mood disorders is not entirely satisfactory, it is the most widely used classification. Since no perfect system of classification exists, any classification of mood disorders is likely to be controversial. Because this book is confined to major depressive disorder (MDD), the terms depression, affective disorder, mood disorder, or unipolar depression are used interchangeably to denote nonbipolar depressions.

DSM-IV-TR divides mood disorders into bipolar and depressive disorders. Bipolar disorders are characterized by the presence of depressive symptoms and one or more episodes of mania (elevated mood, increased activity, and expansive and self-important ideas) or hypomania (mild mania). Depressive
disorders are subdivided into major depressive disorder (MDD), dysthymic disorder (DD), and depressive disorder not otherwise specified (DDNOS). These disorders exclude a history of manic, mixed, or hypomanic episodes, and they should not be due to the physiological effects of substances of abuse, other medications, or toxins (First & Tasman, 2004).

Depression is among one of the most common psychiatric disorders treated by psychiatrists and psychotherapists, and approximately one half of all psychiatric admissions are depressed patients. Both American and British statistics suggest that the lifetime expectancy of developing unipolar depression is approximately 20% in women and 10% in men. The prevalence rate of depression is higher among people from the lower socioeconomic group; divorced and separated persons (Smith & Weissman, 1992); primary care outpatients with chronic disease (ranging from 9% to 20%; Barry, et al., 1998); and medical inpatients (15% to 36%; Feldman, et al., 1987). However, the majority of depressed people do not seek or receive treatment. The estimated percentages of severely depressed patients receiving treatment range from 20% to 33% worldwide (Kaplan & Sardock, 1981).

Apart from being one of the most commonly diagnosed psychological disorders, major depressive disorder is reported to be on the increase (World Health Organization, 1998). It is estimated that out of every 100 people, approximately 13 men and 21 women are likely to develop the disorder at some point in life (Kessler, et al., 1994), and approximately one third of the population may suffer from mild depression at some point in their lives (Paykel & Priest, 1992). In fact, the rate of major depression is so high that the World Health Organization (WHO) Global Burden of Disease Study ranked depression as the single most burdensome disease in the world, in terms of total disability adjusted life years
among people during the middle years of life (Murray & Lopez, 1996). Major depression is also a very costly disorder in terms of lost productivity at work, industrial accidents, beds occupancy in hospitals, treatment, state benefits, and personal suffering. The disorder also adversely affects interpersonal relationships with spouses and children (Gotlib & Hammen, 2002), and the rate of divorce is higher among depressives than among nondepressed individuals (e.g., Wade & Cairney, 2000). The children of depressed parents are found to be at elevated risk of psychopathology (Gotlib & Goodman, 1999). According to WHO (1998), by 2020, clinical depression is likely to be second only to chronic heart disease as an international health burden, as measured by cause of death, disability, incapacity to work, and medical resources used.

Moreover, depression has significant impact on mortality. The link between depression and mortality is most noticeable in patients with cardiovascular disease. A twofold elevated risk of mortality is noted in patients with depression recovering from myocardial infarction (Frasure, Smith, Lesperance, & Talajic, 1995). Similarly, the mortality rate for depressive older adults residing in nursing homes is twice of those without depression (Rovner, 1993). Furthermore, about 15% of patients suffering from primary mood disorder ultimately take their own lives (Stolberg, Clark, & Bongar, 2002).

A major depressive episode is the most common form of depression diagnosed in clinical practice. If two or more major depressive episodes occur, separated by a period of at least 2 months, during which the individual was not depressed, major depressive disorder, recurrence, is diagnosed. Approximately 60% of people who have a major depressive episode will have a second episode. Among those who have experienced two episodes, 70% will have a third, and among those who have had three episodes, 90% will have a fourth (American Psychiatric Association, 2000). Recurrence is very important in predicting the future course of the disorder, as well as in choosing appropriate treatments. As many as 85% of single-episode cases later experience a second episode. The median lifetime number of major depressive episodes is four, and 25% of depressed patients experience six or more episodes (Angst & Preizig, 1996). Depression therefore is considered to be a chronic condition that waxes and wanes over time but seldom disappears (Solomon, et al., 2000). The median duration of recurrent depression is 5 months. Episodes of MDD are often triggered by severe psychosocial stressors, such as the death of a loved one or divorce. Studies indicate, however, that psychosocial stressors play a more significant role in the precipitation of the first or second episodes than in the onset of subsequent episodes (American Psychiatric Association, 2000).

Over 50% of depressed patients have their first episode of depression before the age of 40. An untreated episode of depression may last 6 to 13 months, whereas most treated episodes last about 3 months. Approximately 50% to 85% of patients have a second depressive episode within the next 4 to 6 months. The risk of recurrence is usually increased by coexisting dysthymia, alcohol or drug abuse, anxiety symptoms, older age at onset, and a history of more than one previous depressive episode. Approximately 50% of all depressed patients recover completely, but in 20% to 35% of
cases, a chronic course ensues, with considerable residual symptomatic and social impairment (DSM-IV-TR, 2000).

In trying to define the course of major depressive disorder, researchers have come to realize that depression is a heterogeneous disorder, with many possible courses. Moreover, people with the same diagnosis may vary greatly from one another. For example, some depressed patients may be diagnosed as having psychotic features, such as delusions and hallucinations. Such distinction among unipolar depressives is important because it has implications both for treating and recognizing the severity of the illness. It has been proved that depressed patients with delusions do not generally respond well to antidepressants, but show better response when an antipsychotic drug is combined with the antidepressant. It is also known that depression with psychotic features tends to be more severe than depression without delusions, and it involves more social impairment and less time between episodes (Coryell, et al., 1996).

Moreover, depression co-occurs with other disorders, both medical and psychiatric. Kessler (2002), from his review of the epidemiology of depression, concludes that comorbidity is the norm among people with depression. For example, the Epidemiologic Catchment Area Study (Robins & Regier, 1991) found that 75% of respondents with lifetime depressive disorder also met criteria for at least one of the other DSM-III disorders assessed in that survey. Anxiety is the most frequent comorbid condition with depression. Approximately 50% to 76% of depressed patients experience anxiety (Dozois & Westra, 2004). In fact, considerable symptom overlap occurs between these two conditions. The presence of poor concentration, irritability, hypervigilance, fatigue, guilt, memory loss, sleep difficulties, and worry may suggest a diagnosis of either disorder. The symptom overlap between the two conditions may be indicative of similar neurobiologic correlates. At a psychologic level, it seems reasonable to assume that depression can result from the demoralization caused by anxiety, for example in a case of an agoraphobic who becomes withdrawn because of the fear of going out. Conversely, a person with depression may become anxious due to worry about being unable to hold gainful employment. Although there is an apparent overlap between anxiety and depression, it is common clinical practice to focus on treating one disorder at a time. Lack of an integrated approach to treatment may mean that a patient is treated only for depression while still suffering from anxiety. One of the rationales for combining hypnosis with cognitive behavior therapy, as described in this book, is to address symptoms of anxiety.

Despite the well-known fact that many types of depression exist, each involving multiple factors in the genesis of the disorders, researchers have expended a great deal of energy finding a single cause of depression. Some of the well-known biological and psychological theories of depression are briefly and selectively reviewed here to underline the limitation of searching for a single cause of depression. The cognitive theory of depression is described in greater details in the next chapter, because it forms
part of the integrated model of depression (Cognitive-Dissociative Model of Depression) described in Chapter 4.