Out of sight, out of mind : population estimates and dynamic interplay between age-related decline in hearing and cognitive functioning in late life

Abstract

Age-related sensory loss and major neurocognitive impairment are two of the leading drivers of non-fatal disability burden among the oldest-old, and are often reported to co-occur. Both biological and social explanations have been given to account for links between these two functional domains. This thesis explores inter-associations between age-related hearing-loss with cognitive function. The broad substantive aims of this dissertation are: 1) to document the levels of hearing impairment, dual sensory loss, and co-morbid hearing-loss with cognitive impairment in an older adult population; 2) to identify predictors of decline in hearing acuity and its association with all-cause mortality risk; 3) to investigate longitudinal pathways between hearing thresholds, hearing aid use and processing speed. The Dynamic Analyses to Optimise Ageing (DYNOPTA) project is a collaborative inter-disciplinary project that has pooled nine Australian longitudinal studies of ageing. The DYNOPTA project constitutes an important methodological backdrop to this thesis. Data pooling is advantageous because it can enhance representativeness of a population, increased statistical power and allows for direct replication of effects. However, variability in study protocols and the need to orientate functionally equivalent measures onto a common scale can create analytic challenges. A subsidiary aim of this thesis will be to illustrate and evaluate the use of harmonised longitudinal data pooled from independently designed epidemiological surveys. This research presented in this thesis primarily draws upon data from two contributing DYNOPTA studies that began in the early 1990s and are ongoing. These two studies were selected because they collected functionally equivalent clinical measures of hearing, vision and cognition, as well as a range of comparable contextual variables including data on socio-demographics, health, noise exposure, and hearing aid use. Multistate Markov Chain models estimated transition rates and expected years lived with sensory impairment. Joint Survival-Growth Curve models demonstrated that hearing loss was associated with increased mortality risk in women but not in men. Linear Mixed Models were used to identify predictors of hearing trajectories. Bi-variate Dual Change Score models demonstrated that low levels of hearing were leading indicators of subsequent rates of decline in processing speed. Finally, hearing-aid use was shown to be associated with improved levels of processing speed after adjusting for the effects of hearing thresholds, but did not attenuate rates of decline in processing speed. Hearing loss and cognitive impairment are highly prevalent and contribute to a significant number of years lived with functional impairment in late life. Links between hearing and cognition may be due to common biological processes. Alternatively, hearing loss could limit opportunities to engage in activities that promote and maintain cognitive reserves. Reductions in cognitive resources may also mean that older adults are less well equipped to deal with sensory ageing. In the context of this thesis, the main benefits of pooling and harmonization were the capacity to derive coarse population level estimates and the fostering of inter-disciplinary collaboration. However, it was necessary to return to the use of single study data to facilitate investigations into more fine grained causal pathways between hearing and cognition. Show more