Senile deafness is considered more of a nuisance than a serious health problem. It’s time to abandon that idea. Because, among other things, it is the single most important preventable risk factor for Alzheimer’s disease.
As is well known, the consequences of this disease are devastating. There is a progressive loss of memory and the ability to think, which ultimately affects behavior and personality. Life expectancy is shortened and independence disappears. The individual, family, social and economic losses are very serious.
As the population ages and life expectancy increases, the impact of Alzheimer’s disease is growing. Around 10 million cases are diagnosed worldwide each year, and the number of cases is estimated to increase from 1,000,000 in 2025 to 1,700,000 in 2050 in Spain, for example.
Therefore, every advance in understanding its causes is a step towards a higher quality of life for people suffering from it. And, ultimately, towards its cure.
Accumulation of waste
In Alzheimer’s disease, key mechanisms fail to stabilize and recycle certain proteins in brain cells that are essential building blocks for their survival. When these mechanisms fail, remnants of the defective blocks—literally “protein junk”—build up in the brain. Because they were observed by German scientist Alois Alzheimer in the early 20th century, they helped identify the disease.
The debris (balls of hyperphosphorylated tau protein inside cells and plaques of beta-bamyloid protein around them) damages neural circuits, preventing signal transmission. This leads to memory loss, inability to solve problems, depression, behavioral changes, and personality disintegration.
Sum of risks
It is not known why these basic cellular mechanisms fail. The key appears to be the sum of multiple environmental factors acting on genetic predisposition conditions. But it is not yet possible to join the links.
in the magazine Lancet(a committee of experts periodically analyzes risk factors for dementia). According to their latest estimates, genetic factors are responsible for 55% of the risk. The remaining 45% are 14 environmental factors that operate throughout life. And among them, adult deafness occupies a special place.
The diagram was prepared by the authors.
This is how deafness progresses
Senile deafness begins after the age of 50. It affects up to 40% of people over 65 and 80% of people who have passed the eighth decade of life.
The highest pitches are the first to go, and this is the early stage at which the hearing loss usually goes unnoticed. As it progresses, those affected lose the ability to perceive deeper sounds, which include a significant portion of the sounds of words. At this point, the problem begins to show.
At this stage, using hearing aids slows the process because the receptor cells of the inner ear are especially damaged. They are like microscopic antennas that receive, amplify, and convert sound into electrical signals that are transmitted to the brain. Hearing aids amplify sound where damaged cells cannot.
But eventually the brain circuits responsible for hearing are also damaged. So, amplification with hearing aids helps less: auditory signals, even amplified, are not transmitted correctly through damaged cables. Serious communication problems arise, leading to isolation, withdrawal, anxiety and even depression.
We must realize
Despite this, the idea remains that the impact of deafness is relatively small when consistent messages are given in Lancet show that its elimination would reduce the incidence of new cases of dementia by 7–9%. These are high values that were unknown before the publication of these studies.
It is important to raise awareness among health professionals and the general public. In the field of dementia prevention and treatment, deafness continues to be underestimated compared to other risks. For example, recent work shows that amplifying sound with hearing aids reduces cognitive decline in high-risk adults.
Actions to prevent age-related hearing loss must therefore be prioritised. For example, the link between adult deafness and early life noise exposure cannot be ignored. Targeted educational measures are needed.
Vicious circle
But we also need to investigate the biological mechanisms that influence the link between hearing loss and Alzheimer’s disease. In this context, our research group examined whether the disease itself accelerates deafness, creating a vicious circle.
To find out, we use animal models that simultaneously reproduce the features of deafness associated with aging and neurodegenerative diseases. In this way, we have verified that by experimentally inducing the manifestations of Alzheimer’s disease, hearing problems appear earlier. This indicates the existence of this vicious circle.
What could be causing this? There are cells in the inner ear that function like an electrical battery: they supply the energy needed by the receptor cells, the “antennae,” to generate and send auditory signals to the brain.
The area of the inner ear where these “battery cells” are located contains a large number of small blood vessels. As we age, cells of the “fast” system of the body’s defense against attacks appear there: inflammation. According to our observations, inflammatory cells will be more numerous and active in mice with signs of Alzheimer’s disease.
Original photographs by the authors.
As we get older, these defense mechanisms get out of control and end up damaging other cells, and to make matters worse, Alzheimer’s disease can increase inflammation and accelerate the wear and tear of the inner ear’s “batteries.” This will accelerate the development of deafness, which in turn will worsen the neurodegenerative condition.
Research into the link between hearing loss and Alzheimer’s disease could open up new avenues for improved prevention and treatment.