More Evidence Ties Hearing Loss to Increased Dementia Risk

A large population-based study provides further evidence that hearing loss is a potentially modifiable risk factor for dementia.

The new study moves the field forward in that it used a measure of hearing impairment to assess an individual's ability to hear speech in the presence of background noise. Results of the study were similar to those of previous research linking hearing impairment that used more traditional methods to assess hearing.

The current study also included a larger population than previous studies, and it addressed the problem of reverse causation, showing that hearing loss leads to dementia, rather than the other way round.

Dr Thomas Littlejohns

"Our findings build upon existing evidence that hearing impairment is linked to the later development of dementia," senior investigator Thomas J. Littlejohns, PhD, senior epidemiologist at the Nuffield Department of Population Health, Oxford University, Oxford, United Kingdom, told Medscape Medical News.

"We still don't know for sure that this is a causal association, but I would say the evidence for this link is accumulating, and for patients who are concerned about their hearing, this is another reason to seek medical advice," he said.

"This is really a win-win situation, as there are interventions available to improve hearing which will make a difference to day-to-day quality of life and could also reduce risk of future dementia," Littlejohns added.

The study was published online in the journal Alzheimer's and Dementia on July 21.

"A Promising Target"

It is estimated that dementia affects 50 million people worldwide, and that number is expected to treble to 150 million by 2050.

"But rather than accepting dementia as an inevitable part of aging, there is growing evidence that dementia can be prevented or at least delayed in many cases," said Littlejohns.

To date, primary prevention of dementia has focused on vascular risk factors. There has been new interest in hearing impairment as a modifiable risk factor for dementia, he added.

Hearing impairment is common and increases with age. It has been estiamted that approximately two thirds of adults aged 75 years or older have a hearing problem that adversely affects communication, the investigators note.

"Because hearing impairment affects so many people, even if it just leads to a small increase in risk of dementia, that could make a big difference to the number of dementia cases that develop over time"' said Littlejohns.

The researchers cite a 2020 Lancet Commission report on dementia prevention, in which hearing loss was identified as one of 12 major, modifiable risk factors. In addition, it was estimated that if this was found to be a causal relationships, hearing impairment could be responsible for approximately 8% of dementia cases.

"Hearing impairment is a particularly promising target for dementia prevention due the widespread availability of cost-effective interventions. However, treatment varies depending on the cause of hearing impairment and it is important to understand which forms of impairment are associated with dementia risk," the investigators write.

Previous studies linking hearing impairment to dementia have used pure tone audiometry, which measures hearing sensitivity in a controlled, quiet environment. The current study used a different measure, known as "speech-in-noise," which assesses the ability to hear speech in a noisy environment.

"This is another measure of hearing impairment which may more accurately reflect experience in day-to-day life in that it assesses how someone follows a conversation, processes information, and distinguishes speech amidst background noise, such as hearing an announcement at a train station or having a conversation at a party," Littlejohns explained.

For the study, the researchers used data from the population-based United Kingdom Biobank cohort. Participants in the current study were aged 60 or older and were free from dementia at baseline. They underwent intensive investigations at baseline between 2009 and 2013 that included speech-in-noise hearing tests. Their hearing was categorized as normal, insufficient, or poor.

Participants were followed for a median of 10 years for the development of dementia, which was captured through linkage to electronic hospital inpatient and death registry records.

Results showed that after adjustment for sociodemographic, lifestyle, and health-related factors, both insufficient and poor speech-in-noise hearing were associated with a significantly increased risk for dementia.

Poor hearing was linked to a 61% increase in dementia (hazard ratio [HR] = 1.61; 95% CI, 1.41–1.84), and insufficient hearing was linked to a 91% increase (HR = 1.91; 95% CI, 1.55 – 2.36) relative to normal hearing.

Reverse Causation Bias?

A key concern in studies that investigate risk factors for dementia is reverse causation bias. Dementia pathology progresses several years before dementia is diagnosed, and this progression can affect other behavioral and physical measures. Preclinical dementia could adversely affect performance on a hearing test or sensory processing, which in turn would be associated with a future dementia diagnosis.

To address this, the researchers investigated whether associations differed by length of follow-up period or after exclusion of participants with poor or fair self-reported health at baseline. The investigators assumed that if reverse causation was a major source of bias, then any observed associations would become weaker over longer periods of follow-up or if the study was restricted to a sample of healthy participants.

The investigators found that associations were similar to the main findings over longer follow-up periods and when participants with poor or fair self-reported health were excluded.

"As dementia is a slow process that may start years before symptoms become apparent, it is difficult to know if hearing loss preceded the onset of dementia or vice versa. We didn't find any strong evidence for reverse causation in this study, but we still can't completely rule it out. Ideally, we would need an even longer follow-up of 15 to 20 years to really understand this issue," said Littlejohns.

The researchers also examined the possibility that the observed associations between hearing impairment and dementia may be mediated by other factors, such as social isolation and depression.

They found that that the percentage of excess risk mediated by depressive symptoms and social isolation combined was only 2.5% for insufficient hearing and 6.5% for poor hearing, suggesting these two factors do not substantially mediate the association between speech-in-noise–measured hearing impairment and dementia.

More research is needed to establish whether the association between hearing impairment and dementia is causal, the investigators note.

"If SiN [speech-in-noise] hearing impairment is a modifiable risk factor for dementia, then this may have important implications for the design and implementation of interventions to treat age-related hearing impairment and prevent dementia," they write.

"Further evaluation of the association between SiN hearing impairment and dementia is warranted in large prospective studies with concurrent measurement of SiN and pure tone hearing impairment," they add.

Experts Weigh In

Commenting on the study for the Science Media Center, Jason Warren, MD, professor of neurology at UCL Queen Square Institute of Neurology, London, United Kingdom, said the apparent link between hearing loss and dementia is both worrying and puzzling.

"This timely new study contributes an important piece to the puzzle. Evidence from one of the largest cohorts of older people yet assembled now suggests that reduced ability to understand speech in background noise carries a substantially increased risk of developing dementia.

"In focussing on a crucial aspect of everyday communication, this work raises far-reaching implications for how we assess and manage hearing impairment and dementia risk. We hear with our brains no less than our ears," Warren added.

Also commenting for the Science Media Center, David Curtis, MD, honorary professor at the Genetics Institute, University College London, London, United Kingdom, expressed caution over the idea that use of hearing aids may reduce future dementia risk.

"The authors themselves state that if hearing problems are a feature of early, preclinical dementia, then one might observe an increased risk of dementia within 3 years of hearing problems being present but less so after 9 years.

"In fact, this is exactly what we do see. Although the numbers involved are small, they are consistent with the notion that hearing problems can be a symptom of early dementia but do not cause dementia. If this is the case, then providing people with hearing aids or similar interventions would not be expected to reduce their dementia risk."

Curtis noted that it is very plausible that experiencing problems discerning speech in a noisy environment "could be an early symptom of dementia, because this task requires extraordinary computational abilities within the brain."

However, he noted that such problems are very common as people age and should not in themselves be taken as a worrying sign that dementia is imminent.

"On the other hand, it is quite difficult to see how hearing problems could impact on the mechanisms of neuronal degeneration and cell loss which can ultimately manifest in a diagnosis of dementia. Thus, while this study confirms that the conditions are associated with each other, I am not convinced that it establishes a causal relationship from hearing problems to dementia," Curtis added.

The research was funded by grants from the William Georgetti Scholarship (Universities New Zealand), the Otago Medical School (University of Otago), the Nuffield Department of Population Health (University of Oxford), and the Claus and Margrit Langbehn Foundation. The authors and commentators have disclosed no relevant financial relationships.

Alzheimers Dement. Published online July 21, 2021. Full text

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