By Emma Young
In 2011, a US-based study published in the Journal of the American Medical Association found that people with hearing loss were more likely to develop dementia. This alarming result prompted a number of follow-up studies, which have substantiated the link and further explored the risk. But the mechanism of how hearing loss raises this risk has not been clear.
Now a new study, by a team at Ruhr University Bochum in Germany, offers an explanation. The researchers found that gradual hearing loss (the sort commonly experienced into older age) “profoundly” alters normal processes in the brain’s cortex and hippocampus, and that this impairs memory. This work was conducted on mice, not humans. But it provides useful new insights into what might happen in people.
Sudden sensory loss is known to trigger widespread reorganisation of key brain areas. This “cortical plasticity” is an adaptive way of dealing with a challenge. If someone suddenly loses their vision, for example, areas of the cortex that were dedicated to vision can switch to processing touch and hearing data, enhancing these senses.
Information from our senses plays a big part in the formation of our memories, so we have strong links between the sensory cortices and the hippocampus, our most important memory structure. The sudden loss of a sense can therefore also produce disruptive changes in the hippocampus — and memory impairments. Eventually, though, this settles, limiting the disruption to memory.
A gradual deterioration is different, though. Typical age-related hearing loss starts with impairments in hearing higher frequencies, then mid-range frequencies, then low frequencies. “During cumulative hearing loss, a steady state in terms of sensory input is not achieved,” comments senior author Denise Manahan-Vaughan. “Rather the brain has to constantly adapt to an ever-changing sensory input.”
For the study, led by Daniela Beckmann, the team used a strain of mice that serve as a model for typical gradual hearing loss in older people. To look at how this hearing loss might reorganise the brain, the researchers examined changes in the animals’ neurotransmitter receptors, a key part of the chemical messaging system that allows for healthy communication between brain cells.
The team found that progressive hearing loss caused a reorganisation of brain areas, with neurotransmitter receptor changes taking place in both sensory-processing regions in the cortex and the hippocampus. This process also impaired cellular processes related to memory that take place in the hippocampus, and the mice developed clear deficits in spatial memory (their memory for where they’ve been and the layout of their environment).
This constant change in the mice’s neurotransmitter receptor expression during cumulative hearing loss uses up tremendous resources, says Manahan-Vaughan. Not only do memory structures receive less sensory information — because hearing is becoming impaired — but also the information that these structures receive from the cortex constantly changes (with the progression from high-frequency through to low-frequency loss). “This is very likely why memory becomes impaired,” she says.
If this process is mirrored in the human brain, hearing loss may, then, amplify or accelerate deficits in cognition caused by the degeneration of brain cells with older age. As the researchers write, “our results suggest that age-related hearing loss may be a contributor to loss of function in the hippocampus that occurs with age.”
This process does not itself cause Alzheimer’s disease (or vascular dementia, which is also relatively common). But in acting as such a drain on the brain’s resources, and interfering with memory, hearing loss could make it much harder for the brain to cope with another challenge — such as the protein plaques and tangles that are associated with Alzheimer’s disease. And this has practical implications, the researchers argue.
“I think my study shows that it is extremely important to start wearing a hearing aid when hearing loss becomes apparent,” says Manahan-Vaughan. “I wouldn’t go so far as to say this will prevent dementia — this is another physiological process entirely — but wearing a hearing aid is likely to slow down the progress of memory impairments that occur in healthy ageing, simply because one reduces the demands on the brain to adapt to the progressive loss of a sensory modality.”