Hearing loss and cognitive decline are two of the most common bugaboos of aging. Up to 360 million people worldwide have impaired hearing, according to the World Health Organization. Nearly 45 million people have Alzheimer's or another form of dementia, a number that is predicted to increase to nearly 136 million in the next 40 years, according to Alzheimer's Disease International.
Many scientists have seen these age-related maladies as unrelated. Recent research, though, has raised the possibility that untreated hearing loss may lead to or exacerbate cognitive impairment.
"This area of research is wide open and the potential impact is huge," says Frank Lin, an associate professor of otolaryngology-head & neck surgery, geriatric medicine, mental health, and epidemiology at the Johns Hopkins University School of Medicine. "If hearing loss affects brain function, treating it could turn out to be a powerful way to address age-related cognitive issues."
In 2013, Lin published a study in the Journal of the American Medical Association, Internal Medicine showing that older adults with hearing loss were at greater risk for problems with thinking and memory than were people of the same age who had normal hearing. Analyzing data from the Health, Aging and Body Composition study, which from 2001 to 2007 tested the hearing and cognitive abilities of nearly 2,000 adults between the ages of 75 and 84, Lin found that the cognitive abilities of participants with hearing loss declined up to 40% faster than other participants. Over the seven years of testing, hard-of-hearing subjects also tended to develop memory and thinking problems three years earlier, on average, than did participants with normal hearing.
A handful of studies from the early 1980s and mid-'90s explored links between hearing and brain function; Lin's study has revived questions about precisely how and why hearing loss might diminish brainpower.
Taxing the brain
A few theories as to why have gained ground in recent years. First is the idea that hearing loss imposes an extra, detrimental workload on the brain.
"We take for granted that processing sound is simple, but for the brain it's very energy intensive," Lin says. "The most powerful computers in the world are no match for the sound-processing capabilities of the brain of a three-year old-child." When we consider that sound is comprised of countless vibrations of varying frequencies and intensities, Lin says, "we can better appreciate the amount of neural energy required to precisely segment auditory signals and make sense of them by, for example, recognizing speech and separating it out from background noise."
The more garbled the auditory information is when it's sent to the brain, as in the case of people with hearing loss, the harder the brain has to work to parse it, commandeering resources usually involved with other functions, perhaps including memory and thinking.
Many studies provide strong evidence that the brains of rats, at least, do have to work harder to untangle poor sound signals, says researcher Jonathan Peelle, a cognitive neuroscientist at Washington University in St. Louis. Peelle's work focuses on what happens in human brains when hearing is compromised.
"It's early days yet, but we have seen that if you take an adult with normal hearing and put her in an MRI scanner while listening to garbled speech, the scans reveal that the brain has to spend extra energy to decode it," says Peelle, who received a Dana Foundation research grant in 2013.
Hearing loss, social isolation, and the shrinking brain
Another theory is that hearing loss may cause parts of the brain to atrophy. In a study published in 2014 in the journal NeuroImage, Lin and his associates found that people who have been diagnosed with hearing loss for at least seven years were more likely to have brains with smaller lateral temporal lobes, which are involved with retaining visual memories, processing and deriving meaning from sensory input, and storing new memories.
But is it the hearing loss that causes loss of brain volume and consequent cognitive decline-or might some of the consequences of hearing loss, most notably social isolation, be to blame? Hearing troubles often lead people to become more solitary; dozens of studies have found strong correlations between social isolation and cognitive decline and between robust social activity and stronger mental function. In a 2009 study in the journal Experimental Aging Research, for example, Kristin R. Krueger of Rush University Medical Center found that "[a] higher level of social engagement in old age is associated with better cognitive function."
This effect is also seen in studies of other animals. Rats raised in isolation tend to have smaller brains than rats raised is social settings, notes Indiana University speech and hearing researcher Charles Watson. However, Watson-who co-founded and is president of The National Hearing Test-and other researchers also emphasize that links between hearing loss, lack of social contact, and cognitive decline are so far only correlational. While it may be the case that social isolation leads to a loss of cognitive ability, it's also possible that people with stronger cognitive function are more apt to be socially engaged. Plus, even if social engagement does help maintain and possibly improve brain function, it's not yet clear how and why.
To try to answer these and related questions, Lin has received funding from the National Institute on Aging to do a five-year clinical trial testing around 1,000 people. "One of our central assumptions heading into the trial is that hearing loss does in fact cause cognitive decline and that the more socially engaged you are, the greater your chance for avoiding a decline in mental abilities," says Lin, whose study will be the first on this scale to test the hypothesis.
More questions than answers
While links between compromised hearing and cognitive decline seem compelling, the research is still relatively new, and many questions remain. If further studies did show that degraded hearing led to a slowdown in mental processes and that improving hearing eased or reversed the decline, the benefits would be profound.
"Hearing loss is one of the few late-life risk factors for cognitive decline and dementia, meaning that we may be able to intervene even in old age," Lin says. "People are living longer than ever on average, which means that in the next several decades millions more people will be living with dementia. If we can slow cognitive decline by addressing hearing loss, the public health benefit would be significant."