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When I speak to audiences about mood disorder- when I summarize Ken Kendler's model or the stress-and-failed-resilience hypothesis-they may concede that depression has the form of a disease. But inevitably, someone will express doubts about depression's gravity. Looking at the range of illnesses that afflict human beings, just how bad is depression?
This question has been answered with some decisiveness in the past fifteen years. Because public health dollars are scarce, statisticians have worked to quantify the harm diseases cause. Their findings have surprised even the researchers who devised the major studies: Depression is the most devastating disease known to humankind. This result holds across the board-in the developing world and the industrialized nations, in current surveys and in projections of future health care needs.
You might imagine that fatal diseases do the most damage. But researchers tend to rank-order conditions by asking questions like: How many good days does a given disease steal? Although there are obvious exceptions, like AIDS, most terminal illnesses arrive in old age. They may account for only a year or two of lost life. In contrast, a chronic disease that begins in childhood or adolescence will blight many decades.
Since the late 1980s, researchers have relied on a standard measure of burden of disease, called "disability-adjusted life years." This figure takes into account a variety of calculations. It begins with problems with mobility, self-care, and daily activities, such as work and study. It rates cognitive dysfunction, pain, and discomfort, including severe mood alteration. These factors contribute to a summary estimate of the severity of a typical bout of the illness under study-in this case, the severity of an episode of depression.
Researchers then look at the data on prevalence-how often does a disease occur, and how long does the illness last? Combining estimates of severity with estimates of prevalence, it is possible to calculate the "burden" attributable to a disease, in terms of time lost to disability. That figure is then added to years of life lost due to premature death. Various adjustments are factored in. (A year in young adulthood is valued slightly more than a year in late adulthood.) The intention is to create a number that represents the difference between the health status resulting from illness and an ideal situation in which a person lives free of disease to a ripe old age.
Assume that, under
benign circumstances, women live to age eighty. Then, consider a young woman who
at twenty succumbs to a severe, early-onset arthritis and suffers a one-third
disability for the next sixty years. Her burden of disease is twenty good years
lost-roughly the same as the loss suffered by a previously healthy woman who dies
suddenly at sixty. Since the actuarial discounting values early years more, the
final calculation will rate the crippling illness as slightly more disabling,
over the course of a lifetime, than the premature death. Researchers check results
of this sort against people's actual preferences:
Groups around the world have undertaken the same effort using different assumptions and weightings. The results of these analyses are similar. Varied assumptions lead to a single conclusion: Not AIDS, not breast cancer, but depression is the major scourge of humankind.
The most extensive global-burden-of-disease study is one conducted by the World Health Organization, the World Bank, and the Harvard School of Public Health. The study was massive. Its results began appearing in 1996 in volumes of almost a thousand pages each; six of a projected ten have been published. The findings are often quoted to the effect that by the year 2020, depression will be second only to ischemic heart disease-narrowing of blood vessels and related cardiac problems-in terms of disability caused.
Astonishing though it is, the estimate for 2020 serves to mask the current reality. As of 1990 (the year for which data was analyzed), the afflictions that stood ahead of depression were ones that steal years by killing children young-respiratory infections, diarrhea, and the illnesses of early infancy. These conditions are grouped-they represent not one disease but many-while major depression stands alone, independent of bipolar disorder (manic depression), minor depressions, and alcoholism.
Among the chronic diseases of midlife, depression was (by 1990) already the most burdensome, and not by a small margin. Major depression accounted for almost 20 percent of all disability-adjusted life years lost for women in developed countries-more than three times the burden imposed by the next most impairing illness. The story was similar in developing regions: depression was still the fourth most burdensome disease (after conditions that affect the very young) and the most disabling disease for both men and women age fifteen to forty-four. In the 2020 projections, depression becomes the single most disabling disease in developing regions.
These findings may be counterintuitive, but they hold up solidly in the face of a variety of approaches to the evidence. In fact, the globalburden data on mental fitness were calculated conservatively; they probably underestimate both the loss of life due to suicide in depressives and the indirect harm depression causes when it complicates other diseases, such as diabetes or stroke. Looking at depression, the study considered only full-blown acute episodes; periods of residual symptoms were excluded. The undercounting probably amounts to tens of millions of life years lost annually to disability caused by depression.
There is room for concern in the other direction as well. The disability-adjusted life years measure is necessarily inexact. Still, it is impressive when internists, surgeons, and public health officials conclude that depression is the disease that injures patients most. And the numbers are overwhelming. Cut the estimates for depression in half, and it would still cause much more disability than asthma and diabetes combined. Make what adjustments you will, the result is the same. Depression has no rival as a disabling disease.
impact of depression in these studies is not only a matter of prevalence-how
common depression is. The data create an estimate of the disability caused by
a single instance of illness, in this case, an episode of depression. There are
seven categories of severity. The depressive episode is in the second group, next
to the top. The highest ranking, most severe group includes conditions like dementia
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