What Did You Say? The Link between Hearing Loss and Developing Dementia
According to recent studies, older adults with hearing loss are vulnerable to developing Alzheimer’s disease and dementia, compared to those with normal hearing. As the hearing further deteriorates, the risk increases exponentially.
Those with mild hearing loss are nearly twice as likely to develop dementia compared to those with normal hearing. The risk increases threefold for those with moderate impairment, and five-fold for those with severe impairment.
More than 48 million Americans have some degree of hearing loss, according to the Hearing Loss Association of America. The risk of dementia increases among those with a hearing loss greater than 25 decibels.
In one study with participants over the age of 60, 36 percent of the risk for dementia was associated with hearing impairment. By the time Americans reach their 70’s, two thirds have hearing loss.
Etiology of Hearing Loss
Hearing loss is classified as conductive or sensorineural. The underlying etiologies of conductive hearing loss are problems in the external or middle ear that interferes with transmitting sound and its conversion to mechanical vibrations. Examples of these obstructions include:
- Cerumen (ear wax)
- Foreign body
- Debris from otits externa (swimmer’s ear)
- Large exotoses (surfer’s ear)
- Perforated tympanic membrane
- Otitis media with effusion
- Otosclerosis (abnormal growth of bone near the middle ear)
Sensorineural hearing loss involves problems with converting mechanical vibrations to electrical potential in the cochlea and/or in auditory transmission to the brain. It is usually caused by permanent damage in the organ of Corti.
More than 90 percent of older persons with hearing loss have age-related sensorineural hearing loss, which is gradual, symmetric loss of hearing that is exacerbated in noisy environments. Older persons may have both conductive and sensorineural hearing loss, in addition to cognitive impairment impacting sound interpretations.
Approximately 50 percent of the susceptibility to age related hearing loss may be genetically determined. Noise exposure contributes to the onset, but not the progression of age-related hearing loss. Other risk factors for sensorineural hearing loss include:
- Alcohol abuse: Low or moderate consumption has no effect, but may interact with vitamin B12
- Family history
- Hormones: Progestin may increase risk
- Illicit drug use: Ecstasy linked to ototoxicity
- Industrial chemicals
- Male sex: younger age of onset and greater loss in men
- Medical conditions: diabetes, renal failure, atherosclerosis, immunosuppression, head injury
- Medications: gentamicin, chemotherapy, high doses of aspirin, other pain relievers, anti-malarial drugs, loop diuretics
- Tobacco use
Results of Recent Studies
In a 2011 study funded and conducted by Johns Hopkins and the National Institute on Ageing, 639 participants hearing and cognition were tested. At the start of the study approximately one-fourth had some hearing loss but no evidence of dementia. The volunteers were closely monitored with repeat assessments every one to two years over a 12 -18 year period, and by 2008, 58 of them had developed dementia.
In a 2013 study, Frank Lin, an otologist and epidemiologist at Johns Hopkins University in Baltimore, tracked the overall cognitive abilities; concentration, memory and planning skills of nearly 2,000 older adults with an average age of 77.
After six years, those who began the study with hearing loss severe enough to interfere with conversation were 24 percent more likely than those with normal hearing to find their cognitive abilities decline. The researchers concluded that that hearing loss appeared to escalate age related cognitive decline.
Taking into account other factors associated with the risk of developing dementia; diabetes, hypertension, age, sex and race, hearing loss and dementia are still strongly connected. Dr. Lin states, “There hasn’t been much cross talk between otologists and geriatricians, so it’s been unclear whether hearing loss and dementia are related.”
4 Ways Hearing Loss Can Lead to Dementia
Lin suggests four possibilities for hearing loss to contribute to cognitive decline:
The first possibility is that a common physiological pathway contributes to both conditions such as high blood pressure. However, Lin and other researchers have used statistical methods to account for the factors known to be associated with both conditions, and the data does not support this explanation.
A second possibility is referred to as “cognitive load” or “effortfulness hypothesis” where the strain of decoding sounds over time may overwhelm the brains of those with hearing loss, leaving them more vulnerable to cognitive impairment.
Sound signals become more distorted in hearing loss, especially in the high frequency range, leading to greater effort in perceiving sound. The extra load could be at the expense of encoding and processing speech into memory.
According to Arthur Wingfield, professor of neuroscience at Brandeis University, “if you put a lot of effort just to comprehend what you’re hearing, it takes resources that would otherwise be available for encoding what you hear in memory.”
This effect on a short-term basis has been documented in Wingfield’s lab. Nevertheless, the big question is whether years of drawing resources away from the brain functions such as working memory will eventually drain the brain’s resilience.
A third possibility is that hearing loss may affect brain structure in a way that impacts cognitive decline. Lack of sensory input and difficulty processing may lead to changes in brain structure and function.
According to Wingfield, brain imaging studies demonstrate that older adults with hearing loss have less gray matter in the part of their brain that receives and processes sounds from the ears.
When certain structures of brain cells don’t receive enough stimulation, they can shrink. Evidence from cross sectional studies suggests that hearing loss is associated with a reduction in the cortical volume, specifically the superior, middle and inferior temporal gyri, brain structures responsible for processing sound and speech.
Wingfield questions whether getting clearer speech signals to the brain through modern hearing aids might allow these brain structures to recover their prior size and function. Further studies are underway to determine if hearing aids can delay, or even prevent the onset of dementia and Alzheimer’s by improving patients’ hearing.
The fourth factor, social isolation can play a role in cognitive decline. Hearing loss can result in isolating people from each other. Struggling to engage in conversation can make it difficult to socialize with others or participate in community activities. Social isolation in itself is a risk factor for cognitive decline and dementia.
Hearing Aids Reduce the Risk of Dementia
A recently published study in the Journal of the American Geriatrics Society revealed for the first time that use of hearing aids reduces cognitive decline associated with hearing impairment.
The study followed 3,670 adults and older over a 25-year period. Researchers compared the trajectory of cognitive decline among older adults who were using hearing aids with those who were not.
The study conducted by Columbia University Medical Center found a direct correlation between hearing aid use and cognitive decline performance in older adults (aged 80-99) with hearing loss. The study indicates that people with hearing loss who used hearing aids performed significantly better on cognitive tests than those who did who did not use hearing aids.
Furthermore, people with hearing loss who use hearing aids have the same risk for age-related cognitive decline as people without hearing loss. Cognitive decline is accelerated for those with hearing impairment that do not use hearing aids.
The lead investigator of the study, Dr. Anil Lawlwani, professor of otolaryngology at CUMC said the following of their findings: “Our study suggests that using a hearing aid may offer a simple, yet important way to prevent or slow down the development of dementia by keeping adults with hearing loss engaged in conversation and communication.”
Cochlear implants might benefit those individuals with severe to profound hearing loss. Kathleen Pichora-Fuller, a psychologist and former audiologist from the University of Toronto, is conducting research to determine if treating hearing loss in those with dementia will help optimize communication and quality of life.
According to Pichora-Fuller,” I have no doubt that a cochlear implant makes it easier for a person to listen, and then they will be able to spend more of their power to do other cognitively demanding tasks.”
A recent small retrospective study showed that even in adults older than 70 years, cochlear implants improved quality of life and reduced tinnitus. Greater use of cochlear implants and other surgical treatments for older persons will depend on evidence that the potential for enhanced quality of life can be obtained despite the surgical risk and cost.
The Importance of Education and Awareness
After arthritis and hypertension, hearing loss is the most common chronic health problem among older adults. As baby boomers age, the impact of hearing loss will grow because of the age specific prevalence of the hearing loss and the number of people aging.
Due to the insidious progression of hearing loss, most individuals do not recognize that they have a problem, therefore, they do not seek medical treatment until the hearing loss reaches moderate to severe levels. Furthermore, the majority of individuals with hearing loss are unaware of the connection between hearing loss and the risk for developing dementia.
Healthcare professionals who are interacting with older adults have an opportunity and responsibility to educate their clients on the importance of hearing health for prevention of dementia, and the deleterious effects of social isolation. Although cost and stigma are two of the major barriers for hearing aid usage, resources can often be found to address these challenges.