Diabetes, physical inactivity, and hypertension are well-known, often-studied risk factors for cognitive decline. Now researchers are homing in on another possible risk factor—hearing loss.
"In the past, hearing loss was considered no more than a benign nuisance of aging," says Justin S. Golub, MD, MS, assistant professor of otolaryngology-head and neck surgery at New York-Presbyterian/Columbia University Irving Medical Center in New York City. "A growing body of evidence shows that age-related hearing loss is associated with a variety of health problems common in aging, including depression, frailty, falls, and, perhaps most significantly, cognitive decline."
That hearing loss could play a more important role in brain health than previously thought could have big consequences, since as many as 26 million Americans over age 50, including two-thirds of adults over age 70, are hard of hearing, according to a 2011 study published in the Archives of Internal Medicine.
Hearing Loss and Dementia
Researchers first uncovered a link between dementia and hearing impairment in the elderly nearly three decades ago. In a 1989 study published in the Journal of the American Medical Association (JAMA), investigators reported that a hearing loss of 30 decibels or greater—the equivalent of a whisper—was significantly higher among people with dementia compared to healthy controls. They also observed that greater hearing loss was associated with higher odds of having dementia.
More recent research has confirmed these findings, including a 2011 study published in Archives of Neurology led by Frank Lin, MD, PhD, associate professor in the division of otology, neurology, and skull base surgery at Johns Hopkins University in Baltimore. (Dr. Lin has consulted for hearing aid manufacturers and received research grants from the National Institutes of Health. Our other experts have no disclosures.) Among the 639 participants, whose ages ranged from 36 to 90 years at the start of the study, those with mild, moderate, and severe hearing impairment had, respectively, a twofold, threefold, and fivefold increased risk of developing dementia over a median follow-up period of 11.9 years.
To build on his research, Dr. Lin and colleagues published another study in 2013 in JAMA Internal Medicine that tracked the overall cognitive abilities (including concentration, memory, and planning skills) of nearly 2,000 older adults.
After six years, those who began the study with hearing loss severe enough to interfere with their ability to have conversations were 24 percent more likely than those with normal hearing to experience cognitive impairment. The researchers also tested cognitive ability after five, eight, 10, and 11 years to measure changes in cognitive function and the rate of decline. Those with severe hearing loss were also more likely to experience cognitive deficits within a shorter period of time than participants whose hearing loss was more moderate.
A 2017 meta-analysis on hearing impairment and the risk of Alzheimer's published in Neurological Sciences noted that "hearing impairment significantly increases the risk of cognitive disorders."
Hearing Aids May Help
Scientists are still puzzling out whether recouping hearing with hearing aids and other devices will make a difference in cognition. "Studies show that people who use a hearing aid or cochlear implant do better cognitively, but it's tough to trust that data," says Dr. Lin. "People who choose to get hearing aids are more likely to be affluent and socially engaged, so it's hard to attribute the improvement to better hearing."
But a June 2018 study from the University of Maryland published in Neuropsychologia suggests that it may be more than that. Researchers fit a group of older adults with impaired hearing but normal cognitive function with hearing aids for the first time. Over the course of six months, they tested their brain function and compared it to an age-matched group of adults who did not receive hearing aids.
At the end of the trial, "brain processing and working memory improved significantly in the hearing aid group, but we did not see that in the control group," says co-author Samira Anderson, AuD, PhD, assistant professor in the department of hearing and speech sciences at the University of Maryland. "This is a small study, but the findings do suggest that better hearing through hearing aids can free up available cognitive resources for remembering spoken conversation."
Other experts argue that hearing loss and dementia are both conditions of aging and may not be causally linked. "Hearing loss may make it more difficult for people to participate in interviews performed to assess dementia. So hearing-impaired people might only appear to have cognitive problems," says Robert Dobie, MD, clinical professor of otology and neurotology at the University of Texas Health Science Center at San Antonio. "It's also possible that impaired cognition makes it more difficult for people to adequately perform hearing tests, most of which demand full attention and best cognitive effort. People with dementia might only appear to be hearing impaired. Finally, it is possible that there is no real association at all."
Regardless of whether or not there's a definitive link, experts agree that protecting your hearing is key. While most hearing enhancement devices are risk-free and widely available, they are underused, according to the National Institute on Deafness and Other Communication Disorders. The Institute says fewer than 30 percent of people who need hearing aids actually get them, perhaps because hearing aids are expensive (the average cost is $2,500) and are not covered by Medicare. The Institute says treating hearing loss is one of the few proactive steps people can take to lower their risk for cognitive decline later in life.
Dr. Lin agrees. Other factors such as controlling high blood pressure and vascular disease can help reduce the risk of dementia, but they need to be modified decades in advance. Treating hearing loss, he says, even when patients are in their seventies and eighties may positively impact the rate of cognitive decline and dementia.
How Hearing Loss May Cause Dementia
Researchers have several theories about the association between hearing loss and cognitive decline.
- Common risks. "Hearing loss and dementia could share risk factors, perhaps genetic, that lead to both more impaired hearing and more cognitive problems," explains Robert Dobie, MD, clinical professor of otology and neurotology at the University of Texas Health Science Center at San Antonio. While age is the biggest risk factor for both hearing loss and dementia, conditions such as high blood pressure, heart disease, and diabetes may also play a role, Dr. Dobie says, as they can cause blood vessels in the body to narrow. The cochlea of the inner ear, which receives sound in the form of vibrations, is fed by a network of small blood vessels. When they narrow, hearing can worsen.
- Mental overload. Age changes the central auditory pathways that analyze and process sound. To compensate for these changes, other parts of the brain, especially those involved in thinking and memory, pitch in to help with hearing and speech processing, says Sandra Gordon-Salant, PhD, professor and director of the doctoral program in clinical audiology in the department of hearing and speech sciences at the University of Maryland. "If you're listening to a distorted signal coming through your auditory pathway, you may have to tap into more cognitive resources to help you resolve that signal." As the brain is forced to redirect thinking and memory resources to help with hearing, those resources may diminish, says Dr. Gordon-Salant.
- Structural changes. "Hearing loss may lead to faster rates of shrinkage in areas responsible for processing sound, including the superior, middle, and inferior temporal gyri," explains Frank Lin, MD, PhD, associate professor in the division of otology, neurology, and skull base surgery at Johns Hopkins University in Baltimore. (These regions sit on the part of the brain directly above the ears.) "But these areas don't work in isolation, and their responsibilities don't end with sorting out sounds and language." In fact, he adds, hearing loss affects the same areas of the brain that are implicated in mild cognitive impairment and Alzheimer's disease.
- Social isolation. When people struggle to converse, they're less likely to socialize in groups or visit restaurants, movies, and places of worship. In a 2016 study published in Ear and Hearing, researchers at the University of British Columbia found that for every 10-decibel drop in hearing, the odds of social isolation increased by 52 percent. Furthermore, among the sample of seniors (aged 60 to 69) included in the study, a 10-decibel reduction in hearing was associated with cognitive declines equal to almost four years of cognitive aging.
"People with age-related hearing loss often report that they aren't socializing as much, so they don't have as many stimulating conversations and aren't going out and learning new things from friends," says Justin S. Golub, MD, MS, assistant professor of otolaryngology-head and neck surgery at New York-Presbyterian/Columbia University Irving Medical Center in New York City. "They may withdraw from society to some degree. Over time, you could see how individuals who don't have these meaningful intellectual interactions could develop impaired cognition."
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