statistically control these variables to assess whether hearing loss is an independent risk factor for dementia. A large epidemiological study also allows us to overcome other limitations of earlier studies. By following participants over a period of many years rather than just at one point in time, we can observe if the hearing loss preceded cognitive changes (and hence may have contributed to it) rather than following after cognitive impairment. A carefully designed epidemiological study of older adults can also benefit from precise measures of hearing and cognitive function. Such studies often incorporate calibrated tests of the peripheral auditory system rather than relying solely on participants’ subjective ratings of their hearing, which may be highly variable depending on personality, lifestyle, and other factors. Rigorous protocols for the assessment of cognitive function and dementia also guard against cognitive results being biased by hearing loss. Testers are specifically trained to communicate in ways that ensure the participant’s ability to hear and understand them, and testing batteries incorporate a wide range of tests (both auditory and non-auditory) to precisely assess different aspects of cognitive function such as memory, processing speed, and language abilities. In collaboration with researchers at the National Institute on Aging, we carried out such an epidemiological study in 2010 to see if we could replicate the intriguing results of the 1989 JAMA study. Using data from the Baltimore Longitudinal Study of Aging, the longest ongoing study of older adults in the U.S., we investigated whether hearing levels in a sample of adults measured in the early 1990s were associated with the risk of being diagnosed with dementia over the subsequent 15 years. In the study, we found that individuals with mild, moderate, and severe hearing loss, respectively, had a two-fold, three- fold, and five-fold increased risk of dementia, compared to those with normal hearing. These results were surprising to us in the magnitude of the associations—we weren’t seeing a modest 5-10 percent increase in dementia risk but an astronomical 100, 200 and 400 percent increase. Such an effect for a single risk factor was extremely large, and we didn’t quite believe our findings (and neither did others) when the study was published in 2011. Since then, the wheels of science have continued to turn, and other investigators who have conducted similar studies using large epidemiological datasets and precise measures of hearing and cognitive function have found comparable results. This research is perhaps best summarized in a report published in 2017 by a commission on dementia convened by the journal Lancet to review the literature on major, potentially treatable risk factors for dementia. In this review, which was updated in 2020 , the authors concluded that age-related hearing loss may account for nine percent of all cases of dementia, making it the single largest, potentially modifiable risk factor for the condition, far outstripping the contribution of In 2017, the federal Over-the-Co the hearing aid mar
From Theory to Evidence: Correlation vs. Causation These mechanistic theories linking hearing loss to dementia are plausible, consistent with clinical observations, and supported by half a century of research across all fields. But even if ongoing findings continue to bear them out, does it really matter? Do these mechanisms exert a meaningful enough impact on the risk of cognitive decline and dementia over time that hearing loss is something to worry about? To answer this question, we must draw on large epidemiological studies that follow older adults living in the community for many years. Such studies allow us to account for factors that might explain in simple terms the correlation between hearing and dementia. For example, they may be linked by advanced age, which is strongly associated with both. Likewise, cardiovascular risk factors such as hypertension and diabetes, or general predictors of health, such as low levels of education, could potentially have negative consequences for both hearing and the brain. In a large epidemiological study consisting of hundreds to thousands of participants, we can
Hearing Loss in Adults
24 DANA FOUNDATION CEREBRUM | Fall 2020
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