Physical Functioning Decline and Mortality in Older Adults With Multimorbidity: Joint Modeling of Longitudinal and Survival Data

Abstract

Background: Multimorbidity is common among older adults and strongly associated with physical functioning decline and increased mortality. However, the full spectrum of direct and indirect effects of multimorbidity on physical functioning and survival has not been quantified. We aimed to determine the longitudinal relationship of multimorbidity on physical functioning and quantify the impact of multimorbidity and multimorbidity-attributed changes in physical functioning on mortality risk.

Methods: The Health and Retirement Study (HRS) is a nationally representative population-based prospective cohort of adults aged 51 or older. In 2000, participants were interviewed about physician-diagnosed chronic conditions, from which their multimorbidity-weighted index (MWI) was computed. Between 2000 and 2011, participants reported their current physical functioning using a modified Short Form-36. With MWI as a time-varying exposure, we jointly modeled its associations with physical functioning and survival.

Results: The final sample included 74,037 observations from 18,174 participants. At baseline, participants had a weighted mean MWI of 4.6 ± 4.2 (range 0-36.8). During follow-up, physical functioning declined: -1.72 (95% confidence interval [CI] -1.77, -1.67, p < .001) HRS physical functioning units per point MWI in adjusted models. Over follow-up, 6,362 (34%) participants died. Mortality risk increased 8% (hazard ratio 1.08, 95% CI 1.07-1.08, p < .001) per point MWI in adjusted models. Across all population subgroups, MWI was associated with greater physical functioning decline and mortality risk.

Conclusions: Multimorbidity and its associated decline in physical functioning were significantly associated with increased mortality. These associations can be predicted with an easily interpreted and applied multimorbidity index that can better identify and target adults at increased risk for disability and death.

Figure 1.

Causal diagram for joint modeling of multimorbidity on long-term physical functioning and mortality. α, direct effect of multimorbidity (adjusted for covariates) on survival. β, direct effect of multimorbidity (adjusted for covariates) on long-term physical functioning, also referred to as the ‘trajectory function of physical functioning’. The trajectory function of physical functioning may be considered as a mediator of the association between multimorbidity and survival. γ, indirect effect of association between predictors (multimorbidity and covariates) and long-term physical functioning trajectory model on survival adjusted for predictors (multimorbidity and covariates). A value of γ = 0 implies the trajectory function of physical functioning has no significant effect on survival and thus there is no improvement of using joint modeling over separate survival analysis. A value of γ <0 implies that the hazard ratio decreases and that better physical functioning is protective for survival. The overall (direct and indirect) effect of multimorbidity on survival is denoted as βγ + α.

Figure 2.

Kaplan–Meier survival curves for mortality by multimorbidity-weighted index (MWI) quartiles at baseline in 2000. Participants were followed from the 2000 to 2010 study waves. Due to various times, participants were entered within each 2-year study wave, participants were followed up to 12 years.