ARTICLE IN BRIEF
Investigators reported that compared with individuals who have normal hearing, people with hearing loss at baseline had a 24 percent increased risk for incident cognitive impairment, and between a 30- and 40-percent accelerated rate of cognitive decline.
Hearing loss is independently associated with accelerated cognitive decline and incident cognitive impairment in older adults, according to a large new study by researchers at Johns Hopkins University, which was published in JAMA Internal Medicine online ahead of print on Jan. 21.
Compared with individuals who have normal hearing, people with hearing loss at baseline had a 24 percent increased risk for incident cognitive impairment, and between a 30- and 40-percent accelerated rate of cognitive decline. Put another way, according to the study, it would take only 7.7 years for people with hearing loss to lose five points on the Modified Mini-Mental State (3MS) examination, compared with 10.9 years for those with normal hearing.
Several prior studies have also suggested a link between hearing loss and cognitive decline or dementia. Two years ago, in the Archives of Neurology, the same Johns Hopkins group published research that found that degree of hearing loss was associated with risk for dementia: individuals with mild hearing loss had a twofold greater rate of dementia; moderate hearing loss was associated with a threefold increased risk of dementia; and severe hearing loss brought with it a fivefold elevation in dementia risk.
“In this study, we treated hearing loss as a binary variable — either you have hearing loss or you don't,” said lead author Frank Lin, MD, PhD, an assistant professor in Hopkins' Division of Otology, Neurotology and Skull Base Surgery and a faculty member in the Center on Aging and Health. “But in our analysis, we did find the same dose-dependent effect: the greater your amount of hearing loss, the faster your rate of cognitive decline.”
The study initially involved some 1984 participants in the Health ABC (Health, Aging, and Body Composition) Study, a prospective observational investigation of the health of community-dwelling older adults. The study subjects, ages 74-83, all had no evidence of cognitive impairment at baseline in 2001 and 2002. Participants were followed for six years, and the researchers analyzed the association of baseline hearing loss with incident cognitive impairment in the 1626 participants with at least one follow-up visit.
Over the course of the entire follow-up period, on average, individuals with hearing loss had adjusted 3MS scores that declined from 90.3 (95% CI, 89.8-90.8) at baseline to 86.4 (95% CI, 85.7-87.1) at the end of the follow-up period, compared with 91.0 (95% CI, 90.5-91.6) at baseline and 88.3 (95% CI, 87.5-89.1) at the end of the follow-up period for individuals with normal hearing.
MIXED RESPONSE TO THE RESULTS
The study results and methodology drew mixed responses from experts not involved with the research. The study has a number of important strengths, said Sandra Gordon-Salant, PhD, professor and director of the doctoral program in clinical audiology in the University of Maryland's department of hearing and speech sciences.
“Even though they're doing a retrospective analysis of pre-existing data, they're looking longitudinally at the same individuals, which is an improvement over the cross-sectional studies that have been done in the past,” she said. “They also used standard audiometric evaluations, conducted in a sound-attenuating booth, and calculated a 4-frequency pure-tone average, which is the gold standard set by the World Health Organization for determining hearing loss. Some other studies have relied on self-reporting or booths at health fairs.”
But Elliott D. Ross, MD, professor of neurology at Oklahoma University Health Science Center and director of the Center for Alzheimer's and Neurodegenerative Disorders at the Oklahoma City Veterans Affairs Medical Center, was less impressed with the data, calling the effect size modest at best.
“The subjects in the study with hearing loss already start the study at a lower level on the 3MS and the DSS [Dementia Signs and Symptoms Scale] than did subjects with normal hearing,” says Dr. Ross. “You can convert the 95 percent confidence interval to a standard error of the mean and then convert that to a standard deviation to calculate the actual effect size. And when you do that, their results explain less than 1 percent of the variance. Clinically speaking, what does a difference in fall between hearing impaired and non-hearing impaired subjects of 1.2 points, over six years, mean on the 3MS when the range goes from 0 to 100? It's a teensy change that is not clinically relevant.”
Such a small effect, explaining less than 1 percent of the data variance, said Dr. Ross, could be due to a wide range of variables, including the fact that, overall, the subjects with hearing loss were on average slightly more than one year older than those without hearing loss (77.9 vs. 76.8 years).
The data are perfectly plausible; “they're just small,” said David Poeppel, PhD, professor of psychology and neural science at New York University.
“It's not surprising that hearing poorly is associated with getting worse on cognitive tests,” Dr. Poeppel said. “I believe the data. When you have hearing loss, there is a lower signal-to-noise ratio; what typically came to you as a very clear signal now has very low quality. This means that higher-order cognitive systems have to compensate for the low-quality input. It's like trying to listen to Mahler on a very bad stereo. You're putting a burden on the sweet spot of the operating system, and as the hearing system declines, the downstream systems are affected.”
That additional cognitive load on the brain has been documented with functional MRI imaging showing that people with hearing loss “recruit” other parts of the brain to help with hearing and speech processing. “That may come at the expense of cognition and memory,” he said.
Another factor that may contribute to an association between hearing loss and cognitive decline is social isolation. “We know that older individuals tend to become more socially isolated as their hearing declines, and studies have documented for years that social isolation is linked with cognitive decline,” Dr. Lin said.
Could treating hearing loss reduce an older adult's risk of cognitive decline or dementia? The answer to that isn't clear. Dr. Lin's study found no significant modification of the risk of cognitive decline among people who used hearing aids. “Does that mean it's too little, too late? We don't know,” he said. “The fact is that treating hearing loss isn't just about getting someone a hearing aid. The hearing aid needs to be properly fitted, and there needs to be good follow-up along with counseling, rehabilitation, and the use of other assistive devices.”
Ideally, says Dr. Lin, neurologists should establish working partnerships with audiologists to ensure that their patients are receiving the best hearing care possible. “You want your patient to be going to an audiologist who isn't just focused on fitting a hearing aid, but also on ensuring that the patient can communicate effectively in all settings,” he said.