The Link Between Hearing Loss And Dementia

Recent research has indicated that older adults with hearing loss are significantly more likely to experience cognitive decline than those with normal hearing. With the prevalence of dementia expected to double every two decades, identifying strategies to combat this debilitating condition has become a public health priority. If hearing loss is a risk factor for cognitive decline, effectively treating hearing loss could be a way to help fight dementia.

Recent Research Findings

Dr. Frank Lin, epidemiologist and otolaryngologist at Johns Hopkins University, recently led two major studies that found an independent association between hearing loss and dementia [3, 4]. Adults with hearing loss in both studies were significantly more likely to experience cognitive decline than those with normal hearing.

Lin’s 2011 study included 639 subjects, ranging from 36 to 90 years old. Those who suffered from hearing loss at the beginning of the study were more likely to develop dementia, and the likelihood was greater if the hearing loss was more severe. For every 10 decibels of hearing lost, the likelihood of developing Alzheimer’s disease increased by 20 percent [3].

Lin and his team found similar results in their 2013 study, which followed 1,984 subjects, all in good health and with no evidence of cognitive impairment when the study began. Over the course of six years, individuals with hearing loss were 24% more likely to develop incident dementia than those with normal hearing, reinforcing the results found in 2011. This latter study also measured the rate of decay and found that people with hearing loss typically experienced memory loss around 30-40 percent faster, or 3.2 years sooner, than those without hearing problems [4].

What Is Causing The Link?

Scientists have offered four hypotheses to explain the mechanisms underlying the observed link between hearing loss and dementia. One is that a common pathological process, such as vascular disease, causes both hearing loss and dementia. A second hypothesis is that of “cognitive load,” which postulates that hearing loss overloads the brain, leaving depleted resources for other cognitive tasks. Thirdly, hearing loss might actually cause changes in brain structure, resulting in cognitive decline. Lastly, sensory deprivation associated with hearing loss may contribute to cognitive deterioration.

A Common Physiological Pathway

The most obvious explanation for the link between hearing loss and dementia is a common physiological pathway that contributes to both conditions —such as high blood pressure. Both Johns Hopkins studies, however, used statistical methods to account for factors known to be associated with both conditions, such as cardio-vascular conditions including diabetes, stroke history, and hypertension. Risk factors for cognitive decline that have not been found to be linked to hearing loss, such as alcohol consumption, were also controlled for. Researchers concluded that a shared etiologic pathological mechanism underlying both hearing loss and cognitive decline is unlikely [3, 4].

Cognitive Load

Perhaps a more plausible explanation is the theory of “cognitive load,” which refers to the increased effort required by people with hearing loss to listen and decode sounds. The excess strain on the brain makes it vulnerable to cognitive decline. Several researchers have suggested that this effortful listening causes people with hearing loss to draw upon resources they might otherwise use for cognitive tasks like memory and thinking [4, 7, 8]. Imagine being at a cocktail party, with multiple speakers conversing at once. For someone with hearing loss, trying to understand what one person is saying while others are also speaking is challenging. The primary task of listening to one particular person may deplete an individual’s cognitive capacity and leave no resources for the secondary task of following the conversation of other speakers. In Lin’s own words, redirecting cognitive sources to help with processing sound “comes at the loss of something else” [4]. Not surprisingly, fatigue is one of the most frequent complaints among people with hearing loss [8].

Changes to Brain Structure

A third possible explanation for the link between hearing loss and dementia is that hearing loss affects the structure of the brain in a way that contributes to cognitive decline. Research using MRIs has shown that individuals with hearing loss experienced increased activity in the prefrontal cortex when engaged in effortful listening, suggesting that they were taking resources away from this region of the brain, which is responsible for working memory [6]. Furthermore, even moderate hearing loss was correlated to decreased neural activity and a reduction of gray matter volume in the auditory cortex. This reduction in gray matter, according to scientists, is not necessarily reflective of lost brain cells, but could be due to shrinkage of brain cells caused from lack of stimulation.

Reduced Social Stimulation

In fact, lack of social stimulation is the fourth hypothesis that researchers have postulated to explain the link between hearing loss and dementia. A common result of hearing loss is the tendency to withdraw from social activities and isolate from others. Previous research has already established that social isolation has been associated with an increased risk for dementia [2]. In addition, loneliness, or perceived isolation, was found to be an even stronger predictor of cognitive decline than objective social isolation. Those who ranked themselves in the top 10% on loneliness scales were twice as likely to develop Alzheimer’s disease than those who self-ranked in the lowest 10%. People with hearing loss, often in situations where they are surrounded by people yet unable to hear a word, very frequently experience perceived isolation [1].

Conclusion

Whatever the underlying cause, the finding that hearing loss is linked to an increased likelihood of developing dementia is tremendously important, given the rapidly growing aging population. Early identification and intervention of hearing loss could have far-reaching effects. According to Lin, 1 in 30 people nationwide will have dementia by 2050. Delaying the onset by just one year would reduce prevalence by 15%, resulting in billions of dollars in health care savings [1].

In short, anyone with hearing loss should seek intervention, whether that means getting a hearing aid, or seeking alternative means of care. Regular audiologic screenings are essential, but focus should also be on training and counseling in adaptive technology and tools, public health awareness, and greater availability of hearing assistive devices. Given that 2 out of 3 people over age 70 have a clinically significant hearing loss, the importance of addressing effective interventions cannot be understated, especially if those interventions can indeed help to delay the onset of dementia and Alzheimer’s disease.

Sources

1. Bouton, K. (2013, February 11). Straining to hear and fend off dementia. The New York Times. http://well.blogs.nytimes.com/2013/02/11/straining-to-hear-and-fend-off-dementia/?_php=true&_type=blogs&_php=true&_type=blogs&_r=1

2. Hawkley, L.C., & Cacioppo, J.T. (2010). Loneliness matters: A theoretical and empirical review of consequences and mechanisms. Annals of Behavioral Medicine, 40, 218-227.

3. Lin, F.R., Metter, J.E., O’Brien, R., Resnick, S.M., Zonderman, A.B., & Ferrucci, L (2011). Hearing loss and incident dementia. Archives of Neurology, 68(2), 214-220.

4. Lin, F.R., Yaffe, K., Xia, J., Xue, Q-L., Harris, T.B., Purchase-Helzner, E., Satterfield, S., Ayonayon, H.N., Ferrucci, L., & Simonsick, E.M. (2013). Hearing loss and cognitive decline in older adults. JAMA Internal Medicine, 173(4), 293-299.

6. Peelle, J. E., & Wingfield, A. (2012). How does hearing loss affect the brain? Aging Health, 8(2), 107.

7. Rudner, M., Lunner, T., Behrens, T., Thorén, E.S., & Rönnberg, J. (2012). Working memory capacity may influence perceived effort during aided speech recognition in noise. Journal of American Academy of Audiology, 23(8).

8. Tun, P.A., Williams, V.A., Small, B.J., & Hafter, E.R. (2012). The effects of aging on auditory processing and cognition. American Journal of Audiology, 21, 344-350.