"The general perception is that hearing loss is a relatively inconsequential part of aging," says Frank Lin, M.D., an otologist and epidemiologist at Johns Hopkins University. But recent findings, he says, suggest that it may play a much more important role in brain health than previously thought.
Several recent studies have suggested that by paying proper attention to and treating hearing loss, the rate of cognitive decline and dementia in seniors can be noticeably reduced.
In 2013 Dr. Lin and his colleagues published the results of a study that tracked the overall cognitive abilities (including concentration, memory and planning skills) of nearly 2,000 seniors with an average age of 77.
Initiated in 2009, those participants who suffered from hearing loss at the studies inception, to the degree that it interfered with conversation, were 24 percent more likely to acquire diminished cognitive abilities than those with little to no hearing loss. Essentially, the research points out that hearing loss seemed to speed up age-related cognitive decline.
In this study, focusing on dementia, Lin monitored the cognitive health of the participants who were mentally sharp when the study began. Over the next 18 years, these participants will be tested at regular intervals and the results were indeed notable: What has been determined to date is that the greater the hearing loss, the more likely the person will develop dementia. Compared with people with normal hearing, those with moderate hearing loss had triple the risk of development.
What's more, while the link between hearing loss and milder cognitive problems is becoming increasingly accepted, some researchers aren't convinced that hearing loss raises a person's chances of developing dementia.
"Everyone in the field agrees that hearing loss is a risk for cognitive problems," says P. Murali Doraiswamy, M.D., a professor of psychiatry at Duke University and author of The Alzheimer's Action Plan. "But I don't think the field takes mild hearing loss as a cause of Alzheimer's seriously yet." Nor, he says, do most researchers agree that hearing loss is related to other common types of dementia.
Still, he adds, "There are plausible reasons for why hearing loss might lead to dementia — the brain's hearing centers are very close to the regions where Alzheimer's first starts."
Dr. Lin suggests that there four avenues in whch hearing loss can contribute to cognitive problems and dementia. The most obvious is a common physiological pathway that contributes to both hearing loss and cognitive decline, such as high blood pressure. But in the studies, the 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.
Still another possibility has to do with what researchers refer to as "cognitive load," which can be understood as the extra effort involved in constantly straining to hear, which puts undue stress upon the brain.
"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, who heads the neuroscience program 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 like working memory eventually reduces the brain's resilience."
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 processes speech. "It's not necessarily that you're losing brain cells," he adds. Certain parts of the brain cells are known to shrink when they don't get enough stimulation. This suggests, he says, that getting clearer speech signals to the brain might allow these brain structures to grow back to 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 study to tease out what factors might be at play, Lin says.
He and other researchers 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 percent of those with a clinically significant hearing loss even use hearing aids.