What Is Dementia?

The term dementia does not describe a single, specific disease. It covers a wide range of conditions, including Alzheimer’s disease and vascular dementia. People with dementia may experience declines in memory, language, problem-solving, attention, reasoning, and other thinking skills to the extent that they have effects on normal daily activities. Behavior, feelings and relationships can also be affected. Although dementia mainly occurs in older adults (i.e., people aged over 65 years), it is not a part of normal ageing and is caused by abnormal changes in the brain. For example, Alzheimer’s disease is believed to be caused by two proteins, beta-amyloid and tau, forming plaques around brain cells that make it hard for them to stay healthy and communicate with each other. In contrast, vascular dementia develops when blood flow to parts of the brain is blocked or reduced, preventing them from getting all the oxygen and nutrients they need to function properly.

What Is Subjective Cognitive Decline (SCD)?

Most countries now have rising life expectancies, with the World Health Organization (WHO) estimating that 1 in 6 people in the world will be aged 60 years or older by 2030. An ageing global population and increased understanding of and information about dementia has led to increasing numbers of people reporting changes in cognition and seeking medical help. SCD is the name given when a person self-reports the experience of worsening or more frequent memory loss or confusion (“subjective” means “based on or influenced by personal feeling or opinions”) over the last 12 months. However, there is no objective evidence of cognitive decline, i.e., the results of standardized cognitive tests for mild cognitive impairment (MCI) and Alzheimer’s disease do not indicate that there is a problem. Dementia is a continuum, progressing from MCI to mild, moderate, and eventually severe dementia, and the boundary between SCD and MCI has not been defined clearly. Some individuals report SCD as early as 5 years before MCI is detected by objective test results. It is thought that improved understanding and management may reduce the future effects of SCD.

What Is Cognition and How Is It Assessed?

Cognition is an umbrella term that describes a combination of processes that take place in the brain, such as the ability to learn, remember, and make judgements based on experience, thinking, and information from the senses. These processes affect every aspect of life and our overall health. For example, how we form impressions about things, fill in gaps in knowledge, and interact with the world. A variety of tests have been developed to assess cognitive skills. These include the Rey Complex Figure Test, in which participants are asked to reproduce a complicated line drawing, and the Stroop Color Word Test, in which participants are asked to view a list of words printed in colors that differ from the colors that the words describe (for example, the word “blue” might be printed in yellow ink) and then name the color the word is printed in.

How Is Hearing Loss Thought to Be Linked to Cognitive Decline?

Research studies have identified a link between hearing loss and dementia, and some suggest that hearing loss may be a major risk factor for its development. This may be partly due to something called “cognitive reserve”, which is the idea that people build up a reserve of cognitive abilities during their lives, and that this reserve can protect them against some of the cognitive decline that can happen as the result of ageing or disease. In other words, some brains keep working more efficiently than others despite them experiencing similar amounts of cognitive decline and/or damage. It has been suggested that cognitive reserve may be affected by hearing loss because the cognitive resources (the capacity that a person has to carry out tasks and process information) of people with hearing loss are under greater demand than people unaffected by hearing loss. This is due to the increased effort that it takes for people with hearing loss to process auditory (relating to the sense of hearing) information.

What Did the Study Show?

The authors investigated whether hearing loss (as assessed by audiometry, which measures the range and sensitivity of a person’s hearing) affects cognitive function in people with SCD. Participants in the study were aged 60 years or older and were grouped according to whether they had normal hearing or bilateral (affecting both sides) hearing loss. They were then assessed using series of cognitive tests that evaluate attention, language, visuospatial functioning (the visual perception of the spatial relationships of objects), memory and executive functions (responsible for processes like planning, focused attention, self-control, and juggling multiple tasks). Participants also gave blood samples so that particular biomarkers could be measured and had magnetic resonance imaging (MRI) scans to look for differences in areas of the brain.

Although there were no differences between the two groups regarding biomarkers and other tests of cognition, the group with hearing loss performed worse in the Stroop Color Word Test. It is not clear why, but the authors suggest that it may be linked to the robustness of the Stroop test and its ability to measure executive function, particularly aspects to do with control of attention. When a person takes the Stroop test, they need to be able to selectively control their attention, so that they can suppress the automatic response of reading the word presented and instead focus on naming the color that the word is written in. If the idea that cognitive reserve is affected by hearing loss is correct, it might explain why the group with hearing loss did worse in the Stroop test. Another possibility is that people with SCD and hearing loss may participate in cognitive and social activities less often than people with unaffected hearing, reducing their cognitive reserve. High-level engagement in social activity and having large social networks is known to be linked to better cognitive functioning in later life.

The authors also found that people in the hearing loss group had smaller volumes of grey matter, one of the main components of the brain, in four brain regions. One of these is a major component involved in memory. However, it is unclear whether there is a causal link between hearing loss and reduced volumes of grey matter, and it may be more likely that they both result from a common cause, such as accelerated aging in some individuals.

How Can You Increase Your Cognitive Reserve?

Keeping your brain and body healthy and active is the best way to increase your cognitive reserve. Activities that engage your brain, such as learning a language or new skill or solving puzzles, as well as high levels of social interaction, are known to reduce your risk of developing dementia. However, doing the same type of puzzle every day isn’t enough. Novelty and variety are needed to stimulate the brain most effectively, even to the extent that deliberately taking routes to places that differ from the ones that you normally take can help. Regular physical activity, not smoking, and a healthy diet are also important.

Take-Home Message

There may be a link between hearing loss and cognition in people with SCD. People with SCD may be at increased risk of developing dementia in the future. As a result, it is important that people with SCD report any signs of hearing loss to healthcare practitioners promptly so that it can be managed effectively to reduce the risk of further cognitive decline. Importantly, everyone can take steps to increase their cognitive reserve and investing in making small, positive lifestyle changes now may pay dividends in the future.