Epidemiology Of Mood Disorders

Epidemiology is the study of the distribution of a disease or condition in a population and of the factors that influence this distribution. Due to the high prevalence and morbidity of mood disorders for which effective treatment is available, there has been growing interest in recent years in assessing the epidemiological distribution of these disorders. Results from the National Comorbidity Survey (NCS) show that mood disorders, along with simple phobia and alcohol dependence, are the most frequent psychiatric disorders. Since World War II there have been major changes in the epidemiology of mood disorders, such as an increase in the overall cumulative lifetime rates of depression and the occurrence of these disorders at earlier ages in both men and women.

In the early 1980s, approximately 20,000 community and institutionalized (e.g., in mental hospitals, nursing homes, or prison) subjects were selected according to age group proportion by the 1980 U.S. census of adults ages 18 and older as part of the Epidemiological Catchment Area (ECA) study. The overall rate for mood disorders was 5.1%, with significantly higher rates in women (6.6%) compared to men (3.5%). The highest rates were found in the age group 25-44 years (6.4%), whereas the lowest rates were found in the are group X65 years old. Of the sociodemographic variables, marital status accounted for the only significant difference, with a 4.1% rate of mood disorders in married subjects compared to an 11.1% rate in separated and divorced subjects. A major depressive episode occurred in 2.2% of the population during the previous year, whereas a manic episode (necessary for the bipolar I disorder diagnostic criteria) occurred in the previous year in 0.4% of the population. Dysthymia was identified on a lifetime basis in 3.3%.

The NCS assessed a probability-based sample population of subjects between 15 and 54 years of age in the United States. In order to increase the reporting sensitivity of lifetime psychiatric disorders and substance use, the interviewers encouraged respondents to recall accurately past episodes of psychiatric illnesses using a structured interview that incorporated DSM-III-R criteria. This survey, conducted from 1990 to 1992, surprisingly showed much higher prevalence rates than the ECA. The NCS found that the lifetime prevalence of major depressive episode was 17.1% overall (12.7% in males and 21.3% in females). For major depressive disorder, the lifetime prevalence was 14.9% overall (11.0% among males and 18.6% among females), whereas the overall prevalence for dysthymia in this age group was 6.4% (4.8% in males and 8.0% in females). The NCS also found that in those respondents who had a substance use disorder on a lifetime basis, there was a prevalence of 51% of lifetime mental disorder. The variation in the assessment instruments used in the ECA and the more recent NCS could account for the substantial difference observed in the rates of prevalence between the two surveys.

A review by Rouchell, Pounds, and Tierney of recent epidemiological studies showed the prevalence of major depression in specific medical conditions: dementia, 11%; stroke, 27%; Parkinson's disease, 28.6%; epilepsy, 55%; diabetes mellitus, 24%; coronary artery disease, 16-19%; and cancer, was 20-38%.

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