Community-dwelling older adults with hearing loss experience greater decline in cognitive function over time than those with normal hearing
- Correspondence to Dr Aimee M Surprenant, Department of Psychology, Memorial University of Newfoundland, Science Building, Prince Philip Drive, Science Building, St John's, Newfoundland, Canada A1B 3X9;
Implications for practice and research
Age-related hearing loss (ARHL) is associated with cognitive decline.
Early identification and remediation may offset or delay the onset of incident dementia.
Randomised controlled trials (RCTs) of interventions including counselling, hearing aids, cognitive training and social support will elucidate best practice.
Even mild ARHL should not be ignored.
As the world's ageing population increases, rates of incident vascular and Alzheimer-type dementias in the USA are projected to nearly triple from their 2010 levels by 2050.1 ARHL is also expected to increase sharply from the 48 million Americans who currently suffer from hearing loss.2 Demonstrating an independent association of ARHL with cognitive impairment and resultant dementia, as Lin and colleagues do in the current article, is an important step towards determining causal mechanisms and developing efficacious interventions. Using a longitudinal design they show that hearing loss is associated with increased rates of cognitive decline and incident dementia, even when other factors are accounted for.
A total of 1984 participants aged 70–79 (mean=77.4) with no cognitive impairments at baseline were enrolled in a 6-year longitudinal study (Health ABC) whose original goal was assessing ethnicity differences in body type. Participants underwent standardised cognitive assessments each year. In year 5 hearing was assessed. The participants were divided into normal (n=822), and hearing loss groups (n=1162).
Repeated cognitive measures included the Modified Mini Mental-State (3MS) for global functioning and the Digit Symbol Substitution (DSS) for executive functioning. The 3MS has items that assess orientation, language, praxis, concentration and verbal memory. The DSS, a subtest of the Wechsler Adult Intelligence Scale is a non-verbal task involving translation of symbols to digits. Covariates including risk factors for cardiovascular disease, depression, hearing aid use and demographic information such as age, sex, ethnicity and education were obtained.
To determine the association of hearing loss with repeated cognitive measures, multivariate mixed-effects models were used.
Compared with the normal hearing group, those with hearing loss demonstrated a 41% accelerated rate of cognitive decline for the 3MS global cognitive test and a 32% faster rate of decline on the DSS non-verbal cognitive test. Greater hearing loss was associated with a faster rate of incident cognitive impairment. Adjustment for depression did not change the results.
Hearing aid use was not associated with a lower risk of cognitive impairment. However, since this was by self-report, factors that may impact listening ability, such as duration of hearing aid use and type of hearing aids were unknown.
Lin and colleagues demonstrated an independent association between ARHL and incident cognitive impairment. The strengths of this study include its large sample and precise measurement of baseline cognitive and hearing status. Controlling for confounding factors including cardiovascular risk and depression further strengthen the findings.
As with any longitudinal design, it is subject to several limitations. Coincident changes in auditory and cognitive processes could contribute to the strong relationship between hearing and cognitive functioning via a third factor. One such variable, vascular disease was controlled for. However, there could be other mechanisms such as sensory-perceptual abilities that are sensitive to a third factor, which similarly acts on cognitive abilities (common-cause explanation).3
Another critical factor is that the hearing loss group was significantly older and started with a lower baseline measure of cognition. Over the age of 70, the trajectory of cognitive decline is quite steep, thus, only a few years could make a substantial difference even if the groups are on the same curve of cognitive decline.4
Finally, retest effects are an issue. Even though the groups took the same tests, it is possible that those with better hearing and/or cognitive abilities benefitted more from practice. In particular, as shown in the data, older adults with hearing loss may be more affected on those cognitive tasks that are reliant on auditory-verbal skills (3MS) than tasks that are less reliant on auditory-verbal skills (DSS).
Despite its limitations, the study reported here strongly supports other findings in the literature.5 Determining mechanisms underlying the relationship between hearing loss and cognitive impairment will inform researchers about the interaction between hearing and cognition. Understanding the causal mechanism(s) will aid the practitioner in developing and testing potential efficacious interventions.