More than one-third of dementia cases can potentially be prevented by controlling nine lifestyle factors. That’s the startling conclusion of a report presented at the Alzheimer’s Association International Conference in London this summer. And the top modifiable risk factor might surprise you.
Some of them, such as smoking, high blood pressure and midlife hearing loss, were not news. What was striking was that midlife hearing loss accounted for 9% of the risk of dementia across global populations — the greatest single factor. (The other modifiable risk factors were low education, midlife obesity, late-life depression, diabetes, physical inactivity and social isolation.)
The good news is that treating the hearing loss may help reverse or slow cognitive decline.
“We’re optimistic it will make a difference,” said Dr. Gill Livingston, professor of psychiatry at University College London and lead author of the report, which was published in The Lancet in July. “We’re beginning to do studies to check, here and at Johns Hopkins University.”
Said Jennifer Deal, an epidemiologist and gerontologist at Johns Hopkins on the research team headed by Dr. Frank Lin that’s looking into the connection between hearing loss and accelerated cognitive decline: “What we don’t know yet definitely is whether treating hearing loss will prevent cognitive decline. “We’re excited about asking older adults about this.”
The Lin Research Group at Johns Hopkins has published a pilot study and will start recruitment for a clinical trial in November that will answer that question. Researchers will follow subjects for three years and will have results in about five years, said Deal.
Why the connection between dementia and hearing loss?
The reasons for the connection between hearing loss and cognitive decline are not clear-cut, but researchers have some theories. An important one is that hearing loss tends to lead to social isolation, another risk factor for dementia.
“What keeps your brain healthy is a cognitively rich environment,” said Livingston. “One of the easiest ways to get that is to talk. [But] people who can’t hear often avoid complex social situations.”
When researchers at Johns Hopkins did the pilot study, they saw improvements in social measures among people who had their hearing loss treated with hearing aids, such as the diversity of participants’ social network. Among people in the control group who did not receive hearing interventions, the number of people in their social network decreased in just six months.
The Johns Hopkins study will target a group aged 70 to 84 at risk for cognitive decline who are not yet cognitively impaired. Researchers will not measure subjects’ hearing objectively, but by the older adults’ own perception.
“There’s a lot of information that suggests it could be true that treating people with hearing aids would help [prevent or slow dementia], but there are a lot of complications,” said Deal.
Because hearing aids can cost thousands of dollars, “people who get hearing aids can be quite a select group. People who use hearing aids can be better cognitively, but it could be because of higher socioeconomic status, higher education or other reasons,” Deal said. Failure to complete secondary education is second on the list of modifiable risk factors for dementia in The Lancet report.
Hearing and the brain
Another possible reason for the connection between hearing loss and dementia has to do with the two-part mechanism necessary for hearing, said Deal.
The sensory input from sound is encoded by the inner ear through a signal sent to the brain and the brain interprets the sound, a higher-level cognitive process. If the individual has a hearing loss, the brain has to work harder to decode the degraded sound. That is what happens when someone says, “I hear you, but you’re mumbling.”
Because the brain is working harder to decode the sound, it is harder for the person to remember what he or she heard. Experiments have been done with young people with healthy hearing where the researcher plays a sound and asks the subject to remember it. If the sound is garbled, even the young, healthy subjects have trouble remembering it, said Deal.
Researchers speculate that over time, the repeated effort to decode sound may start to draw down a hearing-impaired individual’s neural resources, said Arthur Wingfield, professor emeritus of neuroscience at the Volen National Center for Complex Systems at Brandeis University.
A third theory for the connection between hearing loss and dementia is that there may be changes within the brain long-term resulting from hearing loss, Deal said.
Dementia not predictable at the individual level
The research comes with two important caveats: All the investigation done on this subject is for age-related hearing loss and would not apply to people who have lost hearing for other reasons. And the connection between hearing loss and dementia holds true for a small portion of large populations. It is impossible to say that an individual with age-related hearing loss is or is not at greater risk for dementia, the experts said.
Dr. Debara L. Tucci, professor of surgery at the Duke University division of Neck and Head Surgery and Communication Sciences, emphasizes that although there is a strong correlation between hearing loss and dementia, this does not mean that hearing loss or social isolation causes dementia. We just don’t know.
But the research presents “a good argument, once you reach middle age, to see an audiologist and have your hearing tested,” said Wingfield. “And if it’s warranted, get a hearing aid. The overwhelming majority of people who need a hearing aid do not use one.”
At a minimum, your quality of life could improve. If it turns out to help prevent or delay dementia, all the better.