Psychiatric Disease in Relation to Physical Illness

The science of epidemiology deals with the frequency of diseases in the community. The term incidence means "new cases" and prevalence means "all cases," new and old. So prevalence is incidence times duration. Psychiatric disorders have large prevalence rates mainly because they start in younger people, except for the so-called dementias like Alzheimer's disease. These psychiatric disorders are chronic or intermittent throughout life. A definitive study on these disorders was the Epidemiologic Catchment Area (E.C.A.) study, which was carried out in the United States around 1980 and published in detail in 1991 (Robins and Regier). This showed that 20 percent of all Americans had a mental illness at any one time and 32 percent at some point during their lifetimes. Depression and anxiety were common. Men had more mental illness than women, especially due to their having more substance abuse and antisocial personalities. Women tended to have more obsessive-compulsive disorder, major depressive disorder, and somatisation disorder (presenting psychological problems with physical symptoms). Perhaps the most interesting finding was that depressive illness declined with old age. The finding was counterintuitive since it is often thought that elderly persons have sad lives due to loss of status, income, marriage partners, and health. Other studies in different parts of the world have, however, confirmed the finding; yet it remains much argued. The arguments against the finding are that elderly persons tend to express psychiatric illnesses in physical terms, and so their true nature may be missed; and if all the different kinds of depression are added up the result is as high a rate for elderly persons as for the middle aged.

The E.C.A. study indicated that there was considerable comorbidity, that is, simultaneous occurrence of different psychiatric disorders and, separately, physical diseases. There was a tendency for those with psychiatric diseases to come from disadvantaged backgrounds. In other words, rates were higher among those who failed to complete high school, those on welfare, the unemployed, the unskilled, and the single. Thus, lower socioeconomic status is an important factor in mental illness. At the same time it should be remembered that the same applies to physical illness. So poverty, or relative poverty, plays a part in physical and mental illnesses, both helping to cause them and resulting from them. A more recent U.S. study, looking specifically at comorbidity, found even higher prevalence rates: almost half of the population studied (48 percent) had a psychiatric illness during their lifetimes and almost a third (29 percent) were ill at any one time.