A new review which analysed data from 50 cohort studies of more than 1.5 million people has provided “compelling evidence of adult-onset hearing loss being a robust and consistent independent risk factor for dementia”.
The systematic review and meta-analysis, the most comprehensive yet, suggest hearing loss significantly increases the risk of cognitive decline, dementia and Alzheimer’s disease. The researchers added there was “separate evidence that treating hearing loss with hearing aids may mitigate this association.”
They found hearing loss was associated with a 35% increased risk of incident dementia, a 29% increased risk of mild cognitive impairment and separately cognitive decline, and a 56% increased risk of Alzheimer’s disease dementia. The association with vascular dementia, however, was not statistically significant.
“Each 10-decibel worsening of hearing was associated with a 16% increase in dementia risk,” the researchers reported in the July edition of Ageing Research Reviews published online on 23 May 2024.
Furthermore, the impact of hearing loss on dementia did not vary significantly across dementia sub-types or other moderators such as baseline age or cardiovascular health.
“This meta-analysis of cohort studies provided compelling evidence across diverse study settings and designs of adult-onset hearing loss being a robust and consistent independent risk factor for dementia,” they concluded.
“(The) cohort studies consistently support that adult-onset hearing loss increases the risk of incident cognitive decline, dementia, mild cognitive impairment and Alzheimer’s disease dementia.
“Adult-onset hearing loss is also potentially treatable, most often with hearing aids. Our findings suggest that this treatment may also reduce dementia risk.”
Supports causal relationship
The study authors were from University College London including The Ear Institute, Johns Hopkins Medical Institutions, Baltimore USA and Edinburgh Napier University, Scotland.
They said theirs was the most comprehensive meta-analysis summarising the cohort evidence on adult-onset hearing loss as a risk factor for incident cognitive impairment and dementia, and included examining the evidence for dose-response, risk for various dementia subtypes, and other moderators.
“Our findings provide support to the possibility of a causal relationship between adult-onset hearing loss and dementia,” they added.
“First, our results are overall consistent in that despite heterogeneity between studies in population, methodology and type of incident cognitive outcome, most of the meta-analyses we conducted identified a statistically significant increase in risk across effect measures and cognitive outcomes, and even for those that were not significant, the magnitude of the effect consistently pointed towards risk increase.
“Second, we found evidence of a dose-response relationship. Both mild hearing loss and moderate-severe hearing loss were associated with increasing dementia risk, although the difference in risk increase by degree of hearing loss was not statistically significant.
“Third, our meta-analysis supported an appropriate temporal sequence between hearing loss and dementia by excluding studies with participants who already had dementia at baseline and excluding studies with less than two-year follow-up between hearing loss and subsequent dementia.”
They included cohort studies with participants who had no dementia and hearing assessments at baseline, minimum two years follow-up and incident cognitive outcomes. Twenty-four studies were conducted in the US, 10 in Europe, five in Australia and 11 in East Asia.
Both mild and moderate to severe hearing loss were associated with increased dementia risk, but the degree of hearing loss did not moderate the relationship between hearing loss and dementia risk.
The association with vascular dementia was not statistically significant but only three studies looked at this association, which limited statistical power, and two adjusted for cardiovascular factors which may have diluted the association.
Similar magnitude, more robust
The magnitude of the increased risk for dementia they reported for hearing loss as a yes-no variable was in line with previous meta-analyses and overlapping but on the lower end of the confidence interval of the effect reported by the 2017 Lancet commissions.
“These previous meta-analyses relied on a substantially smaller sample of studies, so the findings from our updated meta-analysis should be more robust,” they explained.
At the outset, they said that causal pathways had previously been suggested involving reduced social interaction and accelerated brain pathology. However, it was also possible that the observed association was not causal but due to residual confounding from inadequately controlled factors like age or cardiovascular health.
The large participant pool and settings, variations in hearing and cognitive assessment methods, follow-up durations, and analysis techniques introduced some residual heterogeneity, suggesting other unexamined factors might also play a role.
But researchers found no evidence that any of the potential moderating factors (the type of hearing assessments, the length of follow-up year, adjustment on cardiovascular risk factors, premorbid cognitive function, the severity of hearing loss, the age group of participants at baseline, and age and gender proportion at baseline) had a statistically significant effect on the magnitude of the association or explained a significant amount of the heterogeneity.
Their systematic review aimed to provide a comprehensive analysis of the existing epidemiological evidence to date, guided by the Bradford Hill criteria of nine principles to assess whether an observed association may be due to a causal relationship.
While their review suggested causality, experimental evidence from randomised, controlled trials was essential to establish causality, they added.
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