Hearing loss and cognition: hearing well is thinking well

Julia Sarant, PhD, has been conducting clinical research with adults and children with hearing loss for 30 years. Julia’s research program is exploring co-morbidities of hearing loss, and the benefits of hearing instruments on cognition and well-being.

Here is our interview…

Your latest research is looking at hearing loss and its relationship to cognitive decline. Can you tell us more about why cognitive decline is such a hot topic?

Cognitive decline increases in prevalence with age. In older people, cognitive decline may reflect broader health issues such as depression, anxiety, poor cardiovascular health or poor nutrition, or signal the onset of dementia. After age 65 years, the risk of developing dementia is doubled every 5 years, with 3-12% of people aged 70-80 years affected, and 25-35% of people older than 85 years affected.

In 2010 there were 35.5 million people worldwide living with dementia. By 2030, that number is predicted to increase to 65.7 million, and by 2050 to 141 million. Dementia is now the 7th leading cause of death.

Wow, those numbers are staggering. But, how does that relate to hearing loss?

Cognitive decline is associated with decreases in sensory acuity, such as hearing loss. Hearing loss is very common in older adults, affecting about 32% of people aged 55 years, and more than 70% of people aged over 70 years.

Hearing loss has been found to be independently associated with the rate of cognitive decline in older adults in many recent studies, with affected people demonstrating an accelerated rate of cognitive decline. The rate of cognitive decline has also been found to increase with the severity of hearing loss.

Does this mean hearing loss causes cognitive decline?

Due to limitations in the ways in which various research studies have been conducted, there is still an ongoing debate about whether hearing loss is a causal factor for dementia. However, after conducting a meta-analysis of the results of three large studies in this field (the results of all studies were combined and analyzed together), the Lancet Commission concluded that hearing loss was a modifiable risk factor for dementia, contributing 9% of the risk for incident dementia (Livingston et al, 2017).

According to the Lancet analysis, compared with a person without hearing loss, whose risk of dementia could be said to be 1.0, a person with a mild hearing loss would have a relative risk of developing dementia of 1.89, one with a moderate loss would have a relative risk of 3.0, and a person with a severe loss would have a relative risk of 4.94.

If hearing loss contributes to 9% of the risk for incident dementia and someone with severe hearing loss is almost 5X more at risk than someone with a mild loss, what are the mechanisms underlying the association?

The mechanisms underlying the association between hearing loss and cognitive decline are currently unknown, but are likely to be multiple. It is possible that both poorer hearing and cognitive function could be the result of a degenerative/aging process. Increased demands of listening could also result in changes in the allocation of cognitive resources.

That makes sense. Are there other factors that play a part?

Communication difficulties, lack of stimulation, and subsequent isolation and loneliness caused by hearing loss could also lead to cognitive decline. This theory is supported by research that has shown that while having a limited social network increases the risk of dementia, having a high level of social engagement can significantly reduce this risk. Hearing loss is also associated with depression, and depression is thought to be a risk factor for dementia. There is also evidence of a decrease in brain volume in people with hearing loss, which may play a role in cognitive decline. In summary, there appear to be multiple potential mechanisms for the hearing-cognition relationship, including neuropathic degeneration, sensory degradation/deprivation, increased cognitive load, social isolation and depression.

Where does this leave us? How do we treat cognitive decline?

Although there are successful treatments for hearing loss, there is currently no successful treatment for cognitive decline or dementia. Given the potential for a causal relationship between hearing loss and cognition, it important to investigate whether the treatment of hearing loss with hearing instruments could delay the onset of cognitive decline/dementia.

It is known that the use of hearing instruments can significantly reduce communication difficulties, depression, anxiety, and loneliness and improve quality of life for people with hearing loss. However, the research to date that has examined the effects of hearing instruments on cognition in older adults has significant methodological limitations that makes their findings difficult to interpret, and has left many unanswered questions.

Can you tell us more. What have been the methodological limitations?

Some of these limitations include small numbers of participants, the use of self-report (rather than objective measurement) of hearing loss, a lack of continued measurement of hearing loss over time, and the use of cognitive assessment tools that have relied on verbal instructions and were therefore not suitable for use with people with hearing loss, who may not correctly hear the instructions.

There is also little or no information about how much participants wore their devices, and how much benefit they received from them, so that treatment effects are unknown. Therefore, the question of whether intervention with hearing instruments mitigates any effects of hearing loss on cognitive function is still unanswered.

We would love to hear about new research currently underway and how it overcomes the limitations in previous studies.

A new study, currently in its third year, is following cognitive function over time in a prospectively recruited cohort of people aged over 60 years at The University of Melbourne in Australia. The study addresses the limitations of previous studies in conducting objective audiological hearing assessments prior to hearing instrument fitting, with follow-up at 18-month intervals.

Other factors likely to influence cognition, including social isolation, loneliness, quality of life, mood, diet, exercise, and device use and benefit are assessed in order to be able to control for these factors in statistical analysis and in so doing to isolate the effects of use of hearing instruments on cognition.

That is very exciting. How are you able to measure cognitive ability?

Cognitive ability is measured using a computerized test battery that does not require the use of verbal instructions, and the amount of hearing instrument use, speech perception and other listening benefits are also being assessed in order to measure degree of success with treatment. Cognitive outcomes for this sample will be compared with those of a healthy aging comparison group of older Australians with typical hearing for their age.

Julia, I understand that these are very early data based on a small sample of participants who have been observed for only an 18 month period, but what could this mean for practicing clinicians?

Yes, we are following up with a larger sample and for a longer period of time to confirm our results but if these findings are confirmed, it will mean that clinicians can guide their clients with important evidence-based conversations. For example, if clients are told that their cognitive function can not only remain stable, but can improve over time with the use of hearing instruments, they might not take the ‘wait and see’ approach.

And if they know that more frequent use of hearing instruments is associated with greater improvements in cognitive function, I suspect they will use their hearing instruments more consistently. This could really change the way we talk about hearing loss in the future.

Wow, that would be wonderful. Your follow-up study will provide important and world-first rigorous evidence on whether treatment of hearing loss in older adults can delay onset of cognitive decline. We will definitely stay tuned for results soon!

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Three key takeaways for your clinical practice from Julia Sarant:

Initial results from a small sample* suggest that:

1. Clients who come into your clinic with greater hearing loss, greater age and less education are more likely to have poorer cognitive function.
2. Cognitive function in older adults with hearing loss who use hearing instruments can not only remain stable, but can improve significantly over time.
3. More frequent use of hearing instruments is associated with greater improvements in cognitive function.

*Based on a small sample of participants who have been observed for only an 18 month period. Follow up with a larger sample and for a longer period of time is needed to confirm these results. Results from this follow-up study are not yet available.

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For more information about the association between hearing loss and cognition you can read the following article in The Lancet.