Hearing loss has long been considered a normal, and thus acceptable, part of aging. It is common: Estimates suggest that it affects two out of three adults age 70 and older. It is also rarely treated. In the U.S., only about 14 percent of adults who have hearing loss wear hearing aids. An emerging body of research, however, suggests that diminished hearing may be a significant risk factor for Alzheimer’s disease and other forms of dementia — and that the association between hearing loss and cognitive decline potentially begins at very low levels of impairment.

In November, a study published in the journal JAMA Otolaryngology — Head and Neck Surgery examined data on hearing and cognitive performance from more than 6,400 people 50 and older. Traditionally, doctors diagnose impairment when someone experiences a loss in hearing of at least 25 decibels, a somewhat arbitrary threshold. But for the JAMA study, researchers included hearing loss down to around zero decibels in their analysis and found that they still predicted correspondingly lower scores on cognitive tests. “It seemed like the relationship starts the moment you have imperfect hearing,” says Justin Golub, the study’s lead author and an ear, nose and throat doctor at the Columbia University Medical Center and NewYork-Presbyterian. Now, he says, the question is: Does hearing loss actually cause the cognitive problems it has been associated with and if so, how?

Preliminary evidence linking dementia and hearing loss was published in 1989 by doctors at the University of Washington, Seattle, who compared 100 patients with Alzheimer’s-like dementia with 100 demographically similar people without it and found that those who had dementia were more likely to have hearing loss, and that the extent of that loss seemed to correspond with the degree of cognitive impairment. But that possible connection wasn’t rigorously investigated until 2011, when Frank Lin, an ear, nose and throat doctor at Johns Hopkins School of Medicine, and colleagues published the results of a longitudinal study that tested the hearing of 639 older adults who were dementia-free and then tracked them for an average of nearly 12 years, during which time 58 had developed Alzheimer’s or another cognitive impairment. They discovered that a subject’s likelihood of developing dementia increased in direct proportion to the severity of his or her hearing loss at the time of the initial test. The relationship seems to be “very, very linear,” Lin says, meaning that the greater the hearing deficit, the greater the risk a person will develop the condition.

In 2017, the medical journal The Lancet convened a commission to review all published research on risk factors for dementia that might be modified to prevent or delay the onset of symptoms. The surprising conclusion was that hearing loss is the largest — accounting, statistically speaking, for approximately 9 percent of all current diagnoses. That, Lin says, was “a big wake-up call.” Historically, hearing loss has received relatively little attention from clinicians, scientists and the general public simply because, Lin believes, “it’s something everyone gets as they get older. Everyone gets white hair and wrinkles, it can’t really matter, right? It’s an invisible disability.”