4 ways hearing loss loss can lead to dementia
How might hearing loss contribute to cognitive problems and dementia? Lin suggests four possibilities. The most obvious is a common physiological pathway that contributes to both hearing loss and cognitive decline — something like high blood pressure, for instance. But he and other researchers used statistical methods to take into account the factors known to be associated with both conditions, so Lin doesn't give this explanation much credence.
Another possibility has to do with what researchers refer to as "cognitive load" — essentially, that the effort of constantly straining to understand stresses the brain. This one makes intuitive sense.
"If you put in a lot of effort just to comprehend what you're hearing, it takes resources that would otherwise be available for encoding [what you hear] in memory," says Arthur Wingfield, professor of neuroscience at Brandeis University. Research in Wingfield's lab has documented this effect on a short-term basis. The big question, he says, is whether years of drawing resources away from brain functions such as working memory will eventually reduce the brain's resilience.
M. Kathleen Pichora-Fuller, a psychologist from the University of Toronto, is conducting research to test the hypothesis that treating hearing loss in those with dementia will help to optimize communication, with positive effects on everyday well-being for the patient and caregivers. "I have no doubt that if a CI [cochlear implant] makes it easier for a person to listen, then they will be able to spend more of their power to do other cognitively demanding tasks."
A third factor, Wingfield and Lin suggest, is that hearing loss may affect brain structure in a way that contributes to cognitive problems. Brain imaging studies, Wingfield says, show that older adults with hearing loss have less gray matter in the part of their brain that receives and processes sounds from the ears. "It's not necessarily that you're losing brain cells," he adds. Certain structures of brain cells can shrink when they don't get enough stimulation. This raises the question, Wingfield says, whether getting clearer speech signals to the brain through use of a modern hearing aid might allow these brain structures to recover their previous size and function.
Finally, it seems very likely that social isolation plays a part. Being hard of hearing tends to isolate people from others: When you have to struggle to converse, you're less likely to want to socialize in groups or go out to restaurants. And being socially isolated has long been recognized as a risk factor for cognitive decline and dementia.
It will probably take much more research to tease out what factors might be at play. Most important, Lin says, is to find out whether providing state-of-the-art hearing loss treatment can prevent or delay cognitive decline and dementia. Lin and his colleagues have received the first phase of funding from the National Institutes of Health (NIH) to plan and develop a definitive clinical trial that will monitor a large group of older adults with hearing loss. Half will get best-practice hearing treatment, and the other half will get what Lin calls "watchful waiting." Over the following three to five years, researchers will track the participants' cognitive functions. The results won't be available until 2020 at the earliest.
In the meantime, Lin says, if you have hearing loss, it makes sense to get it treated as well as it possibly can be. There's lots of room for improvement — fewer than 15 to 20 percent of those with a clinically significant hearing loss even use hearing aids.
Katherine Griffin is a writer and editor in the San Francisco Bay Area. Updated by Katherine Bouton, an AARP hearing-health blogger and the author of Living Better With Hearing Loss: A Guide to Health, Happiness, Love, Sex, Work … and Hearing Aids.