Previous research has demonstrated a strong association between hearing impairment and future dementia. However, few studies have used a long follow-up period to demonstrate this association.
Addressing this gap in research, a recent eClinicalMedicine study has considered a follow-up period of more than two decades to account for reverse causality and confounding.
Study: Hearing impairment and risk of dementia in The HUNT Study (HUNT4 70+): a Norwegian cohort study. Image Credit: Peakstock/Shutterstock.com
Background
Around 40% of dementia cases worldwide can be accounted for by a dozen risk factors that can be delayed or prevented. Among these, the most important is hearing impairment. There are some concerns that this association may be spurious because of the presence of reverse causality and the possibility of misdiagnosis.
Neurodegenerative illnesses often take a long time to develop, and hearing impairment could just be an early symptom. Some illnesses may take up to 20 years to develop before clinical symptoms are seen.
Few studies have explored the risk of dementia and objective hearing impairment while allowing 10 or 20 years of follow-up.
The primary aim of this study was to make the best use of available evidence and evaluate whether hearing impairment could be deemed an independent risk factor for dementia.
About the study
This largest longitudinal study uses the gold-standard dementia diagnostic assessment and audiometric testing. A huge strength of this analysis is the inclusion of more than two decades of follow-up and accounting for potential confounding factors.
Besides the primary, a secondary objective of the current study was to explore the association with Alzheimer’s dementia (AD) and non-ADs.
Individual data from The Trøndelag Health Study (HUNT), Norway, was used for this analysis. Individuals aged at least 20 years were asked to take part in four decennial surveys. These were HUNT1 (1984–1986), HUNT2 (1995–1997), HUNT3 (2006–2008), and HUNT4 (2017–2019).
Additionally, a sub-study (HUNT4 70+) was conducted that included individuals aged 70 years and over. The present study reports findings from the HUNT4 70+ study.
7,135 individuals were included who were assessed for dementia and had audiometry between 1996 and 1998. The Diagnostic and Statistical Manual of Mental Disorders was used for dementia assessment.
All associations between key variables of interest were studied using Poisson regressions after appropriately accounting for confounders.
Key findings
Post the adjustment for confounders, the association between dementia and hearing impairment was 1.04 per 10 dB loss in hearing threshold, i.e., the relative risk (RR). This was based on the entire sample.
When the results were stratified by age, for the individuals below 85 years of age, the risk increased by 12%. In other words, the RR for this group was 1.12. With hearing impairment as a bivariate exposure, for the whole sample, the RR was 1.09.
This increased by 36% when only the individuals under 85 years were evaluated. Additionally, there were moderate associations with AD for men and with non-ADs for women. For individuals above 85 years of age, there was no association.
The association between hearing impairment and dementia weakened as more confounders were accounted for. The study participants, relative to those who dropped out, had fewer mid-life comorbidities, better hearing, and higher education levels.
These factors could have attenuated the association, which is proved by the absence of association among individuals above 85 years and the presence in those below 85 years of age. An additional implication is that death could act as a competing risk to dementia.
Conclusions
In sum, the present study documented a moderate association between hearing impairment and dementia in individuals less than 85 years of age. However, this was not true in individuals aged 85 years and over, where death could be a competing risk of death.
Future research should investigate questions such as the risk of all-cause dementia or dementia subtypes among both sexes and the types of non-AD subtypes associated with hearing difficulties.
The key strengths of the study include its gold-standard identification of cognitive ability and hearing impairment. The cognitive diagnoses conducted here were far superior to other studies that mainly rely on death records and hospital admissions.
Additionally, allowing for more than two decades of follow-up and a large sample size minimized the risk of confounding factors and reverse causality.
The main limitation of this study centered around the fact that the associations could be underestimated owing to considerable comorbidities within a geriatric population.
Further dementia subtypes could have been misclassified as brain imaging was not performed, and AD biomarkers were not collected.
Owing to the observational nature of the study, it was difficult to know when the conditions began, which made it difficult to disentangle mediating effects and confounders.
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