In this prospective cohort study of community-living older persons, we found that illnesses and injuries leading to hospitalization were associated with worsening functional ability for nearly all transitions between states of no disability, mild disability, severe disability and death over the course of more than ten years. Furthermore, illnesses and injuries leading to restricted activity but not hospitalization increased the likelihood of transitioning from no disability to both mild and severe disability and from mild disability to severe disability, but were not associated with recovery from mild or severe disability. Finally, these associations of hospitalization and restricted activity with functional transitions were accentuated by the presence of physical frailty. These results provide strong evidence that intervening events play an important role in precipitating and, subsequently, perpetuating the disabling process.
Unlike conditions that invariably progress, such as Alzheimer Disease, disability is a recurrent disorder, characterized by high rates of recovery (7
). The dynamic nature of disabilty has only recently been elucidated (8
), and relatively little is known about the factors that are associated with clinically relevant transitions in functional status. We have previously shown that illnesses and injuries leading to either hospitalization or restricted activity are strongly associated with the initial onset of disability (6
). The current study extends this earlier work by demonstrating that exposure to these intervening illnesses and injuries is also associated with the subsequent course of disability.
Hospitalization was associated with a particularly pronounced risk, with relative risks for developing new and worsening disability much greater than those for physical frailty, which is the single strongest risk factor for disability and functional decline (9
). For the transition from no disability to severe disability, the high hazard ratio associated with hospitalization is likely attributable not only to the potent disabling effects of serious illness, and hospitalization itself (26
), but also to the low incidence of severe disability in the comparison group of nondisabled participants without an acute hospital admission or restricted activity. Our results support the hypothesis that illnesses and injuries leading to hospitalization act not only to precipitate and worsen disability, but also to hasten death and to impede recovery from disability, thereby prolonging the disabling process. Hospitalization was not associated with the transition from severe disability to no disability; this was likely due to the short duration of severe disability among participants who regained independence (median [interquartile range], 1 [1-1] month) and, for participants with severe disability, the strong association of hospitalization with the 2 competing outcomes of mild disability and death.
Illnesses and injuries leading to restricted activity were also associated with developing new and worsening disability, although these associations were not as strong as those for acute hospital admissions. Because restricted activity was much more common than hospitalization, the overall magnitude of its effects could be heightened. We were unable to calculate population attributable fractions, but in our earlier report (6
), which focused only on the initial onset of disability, the population attributable fractions were considerably greater for hospitalization than restricted activity for three distinct disability outcomes. In the current study, we found that episodes of restricted activity were not associated with transitions from mild disability to no disability or from severe disability to mild disability, suggesting that intervening events less potent than those leading to hospitalization do not impede recovery from disability.
Physical frailty was independently associated with each of the six transitions between no disability, mild disability and severe disability and accentuated the associations of the intervening events in absolute terms for all nine of the possible transitions. Because physical frailty was assessed every 18 months but could have changed over shorter periods of time, it is possible that its effects were underestimated. Nonetheless, in the setting of an intervening event, the change in absolute risks associated with physical frailty on transitions to new or worsening disability was greater than that of sex and age.
We determined absolute risks for subgroups defined on the basis of physical frailty, sex, and age, which greatly enhances the clinical relevance of our findings. For example, in the setting of an acute illness or injury leading to hospitalization, the absolute risk of transitioning from no disability to severe disability within one month ranged from only 3.3% in men younger than 85 years without physical frailty to 16.6% in physically frail women aged 85 years or older. Although the multivariate nature (i.e. nine different outcomes) of our analytic strategy did not permit us to evaluate the risks associated with specific reasons for hospitalization or restricted activity, we found that falls and fall-related injuries almost invariably conferred the highest likelihood for developing new or worsening disability.
The results of the current study, coupled with those of our earlier report (6
), provide strong evidence that disability among older persons is driven largely by illnesses and injuries leading to hospitalization or restricted activity. Both types of intervening events greatly increased the likelihood of developing new or worsening disability, while only the most potent events, i.e. those leading to hospitalization, reduced the likelihood of recovery from disability. Given the central role of intervening illnesses and injuries on the disabling process, more aggressive efforts are warranted to prevent their occurrence (27
), to manage them more effectively and reduce subsequent complications, especially in the hospital setting (31
), and, post event, to enhance restorative interventions in the subacute, home care, and outpatient settings (35
Although causality cannot be established by an observational study, the frequency of our assessments increases the likelihood that the intervening events at least preceded the functional transitions. However our data do not allow us to determine how often the intervening events resulted immediately in new or worsening disability, as may occur with a sudden acute process such as a stroke or hip fracture. The validity of our results is strengthened by the nearly complete ascertainment of intervening events and disability, by the high reliability and accuracy of these assessments, by the low rate of attrition, and by adjustment for several relevant covariates at 18-month intervals with few missing data.
Our study has at least two additional limitations. First, information was not available on the duration of the intervening events. It is possible that the likelihood of recovery may be reduced by long episodes of restricted activity, but not by short episodes. Second, because our study participants were members of a single health plan in a small urban area and were oversampled for slow gait speed, our results may not be generalizable to older persons in other settings. However, the demographic characteristics of our cohort did reflect those of older persons in New Haven County, Connecticut, which are similar to the characteristics of the U.S. population as a whole, with the exception of race or ethnic group (38
). The generalizability of our results is enhanced by our high participation rate, which was greater than 75%.
Despite the reductions observed in the prevalence of disability over the past two decades (39
), the absolute number of disabled older Americans could increase substantially in the coming years with the aging of the baby boom generation (40
). To obviate this increase, more aggressive efforts will be needed to prevent and manage intervening illnesses and injuries, given their apparent role in precipitating and perpetuating the disabling process.