Research has shown a strong link between hearing loss and developing Alzheimer type dementia and other dementias. People with severe hearing loss are five times more likely to develop a dementia. Even those with mild hearing loss have double the chance of acquiring the condition than people with normal hearing.
One study led by Dr Frank Lin, Professor of Geriatric Medicine at Johns Hopkins University in the US, shows auditory impairment is associated with a 30-40 per cent rate of accelerated cognitive decline. His team is now trying to identify if better treatment of hearing loss could reduce dementia rates.
It is still not yet clear why people with hearing problems are more likely to develop dementia. Being able to hear well relies on our hearing apparatus - the eardrum, cochlea, ear bones, cochlear nerve and neural circuits, including the brain’s auditory cortex - functioning efficiently.
We know hearing loss is said to affect mental efficiency by making it harder for the brain to think and concentrate. Also, if certain neural pathways are not regularly used, their efficiency diminishes. It’s a case of ‘use it, or lose it’.
In many dementias, neural cells die off faster than they normally would and parts of the brain shrink. In people with Alzheimer’s, shrinkage tends to start in the hippocampus – the brain’s memory zone, making forgetfulness one of the first signs of the illness. Deposits of two proteins tau and amyloid and the neurotransmitter acetylcholine, the chemical messenger needed to trigger memories, also affect normal connections between brain cells.
Brain shrinkage, particularly in the auditory cortex and right temporal lobe, also happens with hearing deterioration, as noted in a 2014 study by Dr Frank Lin’s team. This tends to happen in similar areas involved in the early stages of Alzheimer type dementia. These brain regions are associated with spoken language processing, conceptual knowledge and sensory integration.
As we age, hearing hair cells die off too and communication between the ear and the brain falters as biological systems function less slickly and our neural wiring degrades. Added to this is the burden of listening fatigue. A combination of these factors can lead to cognitive decline.
How to identify patients with hearing loss and dementia
As there is no definitive test for dementia, diagnosis is usually made by documenting a case history, observing behaviours and undergoing ongoing memory tests and brain scans. Doctors usually use the Mini-Mental State Examination, which asks questions such as what year, month and time it is and requires patients to repeat sentences and remember three spoken words.
Greg Savage, Professor of Clinical Neuropsychology at Macquarie University, says there is a strong case for early screening. “We are starting to develop disease-modifying drugs that can arrest the process or even revert it but we are going to have to give them much earlier when there is only a whiff of something wrong – possibly even before then. We need screening for people in high-risk categories,” Professor Savage says.
In research, lumbar puncture and blood tests are being developed, aiming to identify proteins that could predict if someone with memory problems might develop dementia. Emerging technology could be harnessed to produce apps that help measure cognitive decline.
Six symptoms that could trigger misdiagnosis
Because hearing loss and dementia share many similar symptoms, auditory issues can be overlooked when a dementia diagnosis is made. Also, people with hearing problems who become withdrawn or misunderstand conversations could be viewed as experiencing mental decline.
Professor Savage is currently working on the Australian Imaging, Biomarkers & Lifestyle (AIBL) Flagship Study of Ageing – Australia’s largest study of brain features and health and lifestyle factors that determine who develops Alzheimer type dementia.
He explains how misdiagnosis can happen. “Clinicians need to be made aware of the fact that the two can often happen together. I do worry that someone who comes across as withdrawn, seemingly forgetful or not themselves – some of it might be due to hearing loss,” he says.
“You give them a spoken memory test and the person might fail because of their hearing not because of neurodegeneration. I think this is being recognised now and screening tests are being modified,” he explains. Here are six symptoms to look out for that can occur in both hearing loss and dementia.
- Difficulty communicating
- Struggling with everyday tasks
- Difficulty concentrating
- Emotional changes
- Repetitive questions
- Depression and anxiety
Scanlan believes it’s important for GPs to always refer people who have cognitive impairment for hearing tests because that is the way to help determine how much of their behaviour is due to cognitive problems and how much to hearing.
“Audiologists should undertake a thorough case history and actively check for behavioural evidence of dementia. A range of hearing tests should be used that do not always rely on verbal and physical responses,” says Scanlan.
“What you tend to see with people who have cognitive difficulties is that they may respond at random intervals, lose attention very quickly or stop responding altogether,” she says.