What is the impact of hearing loss in older adults? Would screening for the condition and treating it make a difference in how people function as they age? These very basic questions are at the heart of research that is yielding some provocative findings about the relationship between hearing loss and healthy aging. The results suggest that hearing loss has an important role to play in how people fare as they grow older. There may even be pathways through which hearing loss contributes to cognitive decline.
“By the time people are 70, two-thirds of adults have a clinically significant hearing loss,” said Frank R. Lin, MD, PhD, whose research group at Johns Hopkins University School of Medicine in Baltimore is examining these broader questions (see FastLinks). “For almost everyone, it develops over time, yet we don't necessarily think about it as possibly contributing to other, more important outcomes in later life.”
In some cases, hearing loss may be a component of a more complex syndrome of frailty or progressive overall disability, suggested James T. Pacala, MD, a professor and the associate head of family medicine and community health at the University of Minnesota Medical School in Minneapolis. The more researchers look into this, “the more they uncover associations of hearing loss with many other conditions that produce functional decline,” he said.
EARLIER COGNITIVE DECLINE
A recent study from Dr. Lin's group found that older adults with hearing loss had a 24-percent higher risk of cognitive impairment compared with normal-hearing individuals and that they experienced such decline sooner.
The research, published online first by JAMA Internal Medicine, doesn't prove that hearing loss causes impaired mental capacity, but it does show a clinically significant association between the two (see FastLinks).
Dr. Lin and colleagues studied 1,984 adults age 70 to 79 enrolled in an 11-year prospective, observational study called the Health, Aging, and Body Composition (Health ABC) Study. Participants had audiometric testing, and cognitive testing was done using the Modified Mini-Mental State Examination (3MS), which measures overall cognitive functioning, and the Digit Symbol Substitution (DSS) test, which measures executive function—the skills needed to pay attention, focus, plan and organize, remember information, problem solve, and adapt one's behavior. None of the participants had impaired mental abilities at the start of the study, but 1,162 of them had some degree of hearing loss.
Participants were followed up for six years. Those with hearing loss had annual rates of cognitive decline, based on 3MS and DSS test scores, that were 41% and 32% greater, respectively, than the adults with normal hearing. On average, people with hearing loss showed a significant cognitive decline (as measured by a five-point drop on the 3MS test) after 7.7 years. Normal hearing individuals experienced such a decline after 10.9 years.
Dr. Lin and colleagues also have examined just how much cognitive performance erodes with hearing loss. In a study of 347 participants from the Baltimore Longitudinal Study of Aging, a 25-dB hearing loss was equivalent to the reduction in cognitive performance associated with a 6.8-year age difference. This finding was published in Neuropsychology (2011;25:763).
Data from the Baltimore Longitudinal Study of Aging were again used to uncover the link between hearing loss and the kind of severe memory loss that interferes with daily life in a study published in Archives of Neurology (2011;68:214), which is now called JAMA Neurology. Although some of the 639 participants had hearing loss at the beginning of the study, none had dementia at that time point.
By the end of the study period, however, a total of 58 cases of dementia had been diagnosed. After adjusting for sex, age, race, education, and certain health variables such as smoking, diabetes, and hypertension, Dr. Lin's research team found that hearing loss was independently associated with new cases of dementia. In fact, the risk of dementia increased with the severity of hearing loss. For people older than 60, more than one-third (36.4%) of the risk of dementia was associated with hearing loss.
SENSORY DISORDERS AND COGNITION
“Hearing scientists have long been interested in the connections between hearing and cognitive functioning,” noted Karen J. Cruickshanks, PhD, a professor of ophthalmology and visual sciences and of population health sciences at the University of Wisconsin–Madison, who studies the health problems of aging.
Research into hearing and cognition has, in fact, spawned a thriving interdisciplinary field dubbed cognitive hearing science. This June, researchers from around the globe will convene in Linköping, Sweden, for the Second International Conference on Cognitive Hearing Science for Communication.
“Dr. Lin's papers are consistent with earlier studies that have shown strong associations between hearing problems in aging and more general age-related changes in cognitive functioning,” Dr. Cruickshanks noted, adding that similar associations have been reported between other sensory changes in aging—olfaction or vision, for example—and risk of cognitive dysfunction.
“It seems that how well the brain interprets sensory signals, whether sound, light, or odors, may be an important indication of cognitive function,” she added. “It isn't clear whether these associations mean that sensory disorders cause cognitive changes or whether the damage in sensory systems and cognitive function are caused by the same factors, such as inflammation, vascular damage, or the accumulation of certain proteins in several regions of the brain.”
CAUSE OR CONSEQUENCE?
There could be some common neurologic or physiologic pathway underlying both hearing loss and cognition, Dr. Lin hypothesized.
Another possible explanation for the association between hearing loss and poorer cognitive function and dementia is that when the brain has to expend more resources to decode speech, an ever-present cognitive load is imposed, he explained. Or, it could be that people with hearing loss have difficulty communicating and, therefore, suffer social isolation and loneliness that lead to cognitive decline and dementia over time.
Much more research is needed to determine whether hearing loss is an early marker of cognitive decline or a modifiable risk factor.
“For now, a critical point is for hearing healthcare professionals not to over-interpret the correlation because there is no good evidence that, for example, wearing hearing aids will prevent dementia,” noted M. Kathleen Pichora-Fuller, PhD, a professor in the department of psychology at the University of Toronto Mississauga and an adjunct scientist at the Toronto Rehabilitation Institute.
She added, however, that “it is reasonable to suggest that hearing rehabilitation, possibly including the use of technology, will help all people with hearing loss, including those with dementia, to maintain communication and social interaction, which are important to our well-being.”
* Learn more about Dr. Frank Lin's research: www.linresearch.org.
* Read the recent JAMA Internal Medicine study: http://bit.ly/JAMA-IM.
* Get information on the Second International Conference on Cognitive Hearing Science for Communication: www.chscom2013.se.
* Find out about the burgeoning field of cognitive hearing science: http://1.usa.gov/CognitiveHS.
* Click and Connect! Access the links in The Hearing Journal by reading this issue on our website or in our iPad app, both available at thehearingjournal.com.
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