Aging is correlated with both hearing loss and cognitive decline. “Inner ear hair cells, spiral ganglion cells, and cortical neurons can be lost with aging and generally do not regenerate,” said Joseph Breen, MD, assistant professor of otolaryngology–head and neck surgery at the University of Cincinnati College of Medicine in Ohio. “The long-held assumption has been that the metabolic processes of aging are causative for both cognitive decline and hearing difficulties, but more recent research has explored a direct relationship between these two problems, independent of normal aging.”
Explore This IssueSeptember 2019
Data from the National Health and Nutrition Examination Survey report that more than 50% of adults in their 70s have some degree of bilateral hearing loss, and that number rises to more than 80% for adults 80 and older (Am J Public Health. 2016;106:1820–1822). Researchers found a greater than 20% rate of mild cognitive impairment in adults over 70, in addition to the 5% to 7% of adults in that same age group who have dementia (Alzheimers Dement. 2019;11:248–256).
In 2017, The Lancet International Commission on Dementia Prevention, Intervention, and Care estimated that hearing loss will increase risk for dementia by 94%, with risk escalating as hearing loss severity increases (Lancet. 2017;390:P2673–P2734). “Because so many older adults have hearing loss, prevention or treatment of hearing loss is estimated to have the greatest potential for dementia prevention compared with any other modifiable dementia risk factors,” said Jennifer Deal, PhD, assistant scientist in the departments of epidemiology and otolaryngology–head and neck surgery and core faculty with the Johns Hopkins Cochlear Center for Hearing and Public Health in Baltimore, Md. According to the commission, up to 9% of dementia cases in the world could be prevented if hearing loss were treated; however, this assumes that hearing loss is a cause of dementia, information that is currently unknown.
Sharon G. Curhan, MD, ScM, a physician and epidemiologist with Brigham and Women’s Hospital and Harvard Medical School in Boston, and her colleagues examined the longitudinal relationship between hearing loss and a decline in subjective cognitive function, a measure that appraises cognitive concerns that reflect changes in memory or thinking experienced by an individual who may not demonstrate abnormal performance on standard cognitive tests (Alzheimers Dement. 2019;15:525–533). “A decline in subjective cognitive function may capture an individual’s self-perceived experience of cognitive decline before cognitive impairment becomes apparent on clinical neurocognitive tests and may be an early manifestation of preclinical dementia,” she said. The eight-year study examined 10,107 men aged 62 and older.
The researchers found that hearing loss was associated with higher risk of subjective cognitive function (SCF) decline, Dr. Curhan said. Compared with men who did not experience hearing loss, the multivariable-adjusted relative risk of incident SCF decline was 30% higher among men with mild hearing loss, 42% higher among men with moderate hearing loss, and 54% higher among men with severe hearing loss who didn’t use hearing aids. Among men with severe hearing loss who did use hearing aids, the risk was somewhat attenuated (37% higher) but wasn’t statistically significantly different from the risk among those who didn’t use hearing aids.
Although we don’t know yet if hearing loss causes dementia, if it does, it is something that we can treat in late life—with the potential to have a great impact on delaying dementia. —Jennifer Deal, PhD
How Hearing Loss Might Cause Cognitive Decline
Several theories exist as to how hearing loss might be a causative factor of cognitive decline. One possible mechanism relates to the additional cognitive load placed on the brain when it has to work harder to process impoverished auditory input from the ear, said Marlan R. Hansen, MD, professor in the departments of otolaryngology–head and neck surgery and neurosurgery at the University of Iowa, in Iowa City. In this theory, the brain of someone with hearing loss reallocates resources that could otherwise be used for memory, thinking, or other functions to simply try to make sense of distorted sound and speech signals. Recent studies have shown that hearing loss places additional demands on the brain, including recruitment of areas of the brain not normally activated by sound processing in normal hearing patients (Ear Hear. 2018;39:204–214; Ear Hear. 2016;37 Suppl 1:5S–27S; J Neurosci. 2003;23:3423–3431).
A second possible mechanism by which hearing loss may cause dementia and cognitive decline is through changes in brain structure and function, Dr. Deal added. Neuroimaging studies suggest that hearing loss may affect the brain, even in regions outside the primary auditory cortex (J Neurosci. 2011;31:12638–12643; Front Hum Neurosci. 2018;12:172). Individuals with hearing loss appear to recruit executive networks and show evidence of cross-modal plasticity between the somatosensory and auditory systems for compensatory processing of degraded acoustic signals. Hearing loss has also been associated with lower gray matter volume in the primary auditory cortex and with faster rates of brain atrophy over time in the temporal lobe and whole brain.
A third possible link between hearing loss and cognitive decline is the observation that significant levels of hearing loss lead to decreased social interactions, Dr. Hansen said. This can further exacerbate structural or functional changes in the brain due to hearing loss. Social isolation negatively impacts a variety of health conditions, including cardiovascular health, that are likewise linked to cognitive decline and dementia.
Dr. Breen said that he frequently hears reports from patients and their family members that progressive hearing loss leads to patients isolating themselves, avoiding social situations, and forgoing activities they previously found enjoyable and stimulating. Correlations between depression and cognitive decline have been reported as well.
Sensory cells for balance are close cousins to the hearing auditory cells. If they become degenerated or lack connections, patients become unsteady and can fall, which can happen with age. —Rick A. Friedman, MD, PhD
Risk Factors for Cognitive Decline and Falls
A range of potential risk factors for cognitive decline and dementia exist, including advancing age, genetic factors (including one or more apolipoprotein ε4 alleles), and several health conditions and lifestyle factors, Dr. Curhan said. The Lancet International Commission on Dementia Prevention, Intervention, and Care identified nine potentially modifiable risk factors that contribute to the risk of cognitive decline and dementia: level of education, hypertension, obesity, smoking, depression, physical inactivity, social isolation, diabetes, and hearing loss.
Risk factors for falls in older adults include older age, muscle weakness, history of falls, gait or balance deficit, visual deficit, arthritis, depression, cognitive impairment, and fear of falling, Dr. Deal said. Some studies have found that hearing loss might be a potential risk factor for falls, but hearing loss isn’t a traditional risk factor for falls and is not typically addressed in fall intervention programs (Laryngoscope. 2016;126:2587–2596).
Rick A. Friedman, MD, PhD, professor of otolaryngology and neurotology and director of the UCSD Acoustic Neuroma Center at the University of California in San Diego, said there is good evidence that genetic variation exists within the population for genes that are critical to balance function, because not every elderly person is unsteady and not every elderly person falls. Falls are a major health risk in the elderly and have a fairly high mortality rate when they result in hip fractures. “Sensory cells for balance are close cousins to the hearing auditory cells,” he said. “If they become degenerated or lack connections, patients become unsteady and can fall, which can happen with age.”
Identifying Patients at Risk
Risk factors for age-related hearing loss include older age, male sex, white race, and history of noise exposure, Dr. Deal said. Some medications can cause hearing loss as well, and it’s probable that some vascular factors, such as hypertension, diabetes, and smoking, may slightly increase risk for hearing loss in seniors.
A hearing test is probably the single best way to identify hearing loss. However, many patients do not seek out a hearing evaluation until significant hearing loss has occurred, Dr. Hansen said. Therefore, clinicians should be proactive in exploring the possibility of hearing loss, just as they are for high blood pressure, diabetes, and other common problems.
It’s difficult for clinicians to perceive a significant hearing impairment in one-on-one conversations because patients develop adaptive strategies and often manage quite well in quiet, private settings, even if they have fairly significant hearing loss. “Oftentimes, spouses or other family members begin to notice the increased hearing difficulties as they interact with patients in more real-world settings,” Dr. Hansen said. “This commonly manifests as misunderstanding of conversations, especially in noisy environments.”
If possible, it’s helpful for physicians to ask the patients’ family members or close friends if they have noticed any hearing difficulties. Furthermore, certain occupations or prior activities such as farming, construction work, manufacturing work, military service, night clubbing, or riding motorcycles carry a higher risk of noise damage and hearing loss, Dr. Hansen added.
Best Treatments for Older Patients with Hearing Loss
A variety of auditory rehabilitation devices and strategies exist to help patients with hearing loss. The best choices for an individual patient depend on a variety of factors, including the nature and extent of hearing loss and the patient’s social, occupational, and lifestyle demands, Dr. Hansen said.
For the vast majority of older patients with hearing loss, hearing aids are the most appropriate option. However, for those with very poor word understanding and unsatisfactory performance with hearing aids, cochlear implants can be considered, Dr. Breen said. Candidacy for cochlear implants is primarily based on results from aided speech testing, where physicians measure word and sentence understanding, while patients use their optimally-fit hearing aids.
Despite the benefits of cochlear implants, fewer than 10% of patients in the U.S. who could benefit actually receive one, Dr. Hansen said. Additionally, most adults who ultimately receive cochlear implants do so after a delay of several years from the time they were first identified as good candidates. Many studies confirm that the best outcomes with cochlear implants result when they are placed shortly after hearing loss occurs (JAMA Otolaryngol Head Neck Surg. 2015;141:442–450; Clin Interv Aging. 2018;13:701–712; Sci Rep. 2017;7:16900). “While normal acoustic hearing continues to decline with age, hearing with a cochlear implant tends to improve or at least remains steady over time,”
Hearing loss has only recently been recognized as a potential risk factor for dementia and cognitive decline. “What we know from observational studies in humans is that the relationship appears to be strong and consistent across different populations,” Dr. Deal said. “Many risk factors for dementia occur in early life or midlife. I think what’s key is that although we don’t know yet if hearing loss causes dementia, if it does, it is something that we can treat in late life—with the potential to have a great impact on delaying dementia.”
Karen Appold is a freelance medical writer based in Pennsylvania.