What is new?
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This article tests the assumption derived from the theory of a dynamic equilibrium that severe disability is related to proximity to death, whereas mild disability is not.
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By means of analysis of disability in relationship to time to death, we demonstrated that moderate and severe disabilities were indeed related to proximity to death, whereas mild disability was not.
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Based on these associations, it was calculated that a 10-year increase of life span would result in substantial expansion of mild disability (4.6 years) but small expansion of moderate (0.7 years) and severe disability (0.9 years).
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These findings support the theory of a dynamic equilibrium and imply that projections of the future burden of disability could be improved by connecting to this theory and incorporating information on proximity to death.
In the coming decades, human life expectancy is likely to further increase [1], [2]. The issue whether this increase of life expectancy will correlate with either compression of disability [3] or expansion of disability [4], [5] has been the subject of debate ever since the publication of Fries' [3] seminal article on compression of morbidity. A compression, that is, a decrease in the number of years that people may expect to live with disability, can only occur when the increase in life expectancy is surpassed by a stronger increase in the mean age of the onset of disability. When the increase in the age of the onset of disability will not keep pace with rising life expectancy, expansion will occur, that is, an increase in the number of years that people may expect to live with disability. Explorative analyses of future compression or expansion in disability are needed to prepare future health care systems to cope with the health effects of ageing of the population.
Projections of future trends in the total burden of disability have been applied that were based on past trends of disability [6], [7]. Unfortunately, application of these methods involved some difficulties. Past trends of disability were often not unambiguously pointing toward one direction and therefore constituted a weak basis for projections [6], [8]. Furthermore, studies do not present univocal methods to extrapolate trends, as it is not clear to what extent past trends will keep pace in the future [6], [7]. The complex mechanisms that simultaneously drive the life expectancy and the expectancy of life with disability are still not fully understood. This complicates the modeling of the effect of increasing life expectancy on the future burden of disability [9].
The extent to which gained life years will be spent in disability will greatly be determined by the relative part of disability in the population that is specifically related to end-of-life processes and will therefore shift to older ages as life expectancy increases. Assuming that most severe disability is reserved to the end of life [10], [11], [12], but that mild disability mostly occurs independent to the end-of-life processes, it is to be expected that with increase of life expectancy most severe disability will shift to older ages, but that mild disability will mostly expand. This scenario, which is expressed in the theory of a dynamic equilibrium, is graphically represented in Fig. 1 [13]. Severe disability is fully dependent on proximity to death and, consequently, with increase of life expectancy, the onset of severe disability equivalently shifts to older ages. Mild disability is fully dependent on age and, therefore, the onset of mild disability does not change as life expectancy increases. The years spent in severe disability did not change, whereas the years in mild disability expanded.
The present article aims to assess whether the theory of a dynamic equilibrium [13] provides a valid framework for projections of the future burden of disability. We will test the key assumption of this theory that the occurrence of severe disability is associated with proximity to death, whereas the occurrence of mild disability is not. Based on estimates of associations of disability with age (time since birth) and proximity to death, we will calculate to what extent the lifetime burden of mild, moderate, and severe disability will expand in the hypothetical case of a 10-year increase of life span.