Hearing Loss May Signal Increased Dementia Risk

Megan Brooks

February 16, 2011

February 16, 2011 — Hearing loss in older adults is independently associated with an increased risk of developing dementia over time, new findings from the Baltimore Longitudinal Study on Aging suggest.

Dr. Frank R. Lin

"Whether hearing loss is a marker for early-stage dementia or is actually a modifiable risk factor for dementia deserves further study," concludes the study team in the February issue of Archives of Neurology.

"We'll need to do more research to figure out what the exact mechanism may be and whether interventions such as hearing aids or cochlear implants could have an effect on delaying cognitive decline," first study author Frank R. Lin, MD, PhD, of the Center on Aging and Health, Johns Hopkins Medical Institutions, in Baltimore, Maryland, told Medscape Medical News.

"Including hearing in a neurologic assessment is long overdue," said George A. Gates, MD, of the University of Washington in Seattle, who was not involved in the study. Dr. Gates' own research has also shown a link between hearing difficulties and dementia.

Prospective Association Study

Dr. Lin's team studied associations between hearing loss and incident dementia in 639 initially dementia-free adults who underwent audiometric testing between 1990 and 1994 and were followed up for the development of dementia or Alzheimer's disease through May 31, 2008.

Hearing loss was defined by the pure-tone average of hearing thresholds at 0.5, 1, 2, and 4 kHz, with tones presented by air conduction in the better-hearing ear. Normal hearing was defined as a pure-tone average of less than 25 dB. Of the 639 study subjects, 125 had mild hearing loss (25-40 dB), 53 had moderate hearing loss (41-70 dB), and 6 had severe hearing loss (>70 dB).

During a median follow-up of 11.9 years, 58 subjects developed dementia, including 37 who developed Alzheimer's disease. "The risk of incident all-cause dementia increased log linearly with the severity of baseline hearing loss (1.27 per 10-dB loss; 95% confidence interval [CI], 1.06 – 1.50)," the investigators report.

The association was independent of sex, age, race, education, diabetes, smoking, and hypertension and remained "robust" in multiple sensitivity analyses, they point out.

Table 1. Risk for Incident Dementia by Hearing Status

Hearing Status Hazard Ratio (95% CI)
Normal Reference
Mild hearing loss 1.89 (1.00 – 3.58)
Moderate hearing loss 3.00 (1.43 – 6.30)
Severe hearing loss 4.94 (1.09 – 22.40)

CI = confidence interval

Among individuals older than 60 years, the researchers estimated that more than one-third of the risk for incident all-cause dementia was associated with hearing loss.

The risk of developing Alzheimer's disease specifically also increased with hearing loss. In analyses restricted to the 37 cases of incident Alzheimer's disease, hearing loss was associated with an excess risk of 1.20 per 10 dB of hearing loss (95% CI, 0.94 – 1.53). "This result is comparable to the risk seen for all-cause dementia but with a wider CI, possibly owing to the smaller sample size," the investigators note.

Dr. Gates commented that "prior studies have shown no relation between pure-tone thresholds and dementia, so this is new. Unfortunately, there is not a 'cut point' that separates those going to become demented from the others. So it can't be used for case detection or screening," he said.

Possible Mechanisms

Dr. Lin and colleagues say a number of mechanisms "may be theoretically implicated in the observed association between hearing loss and incident dementia." Dementia may be overdiagnosed in people with hearing loss; alternatively, people with cognitive impairment may be overdiagnosed as having hearing loss. It's also possible that the 2 conditions share an underlying neuropathologic process.

Another option is that hearing loss may be causally related to dementia, "possibly through exhaustion of cognitive reserve, social isolation, environmental deafferentation, or a combination of these pathways," the researchers say.

They note that hearing loss is "highly prevalent" in older adults and often "remains untreated." Dr. Lin said it will be “very important” to see whether the findings can be replicated in other independent study populations.

In the current study, self-reported use of hearing aids was not associated with a significant reduction in dementia risk, but data on other key factors that would have an impact on this relationship were not gathered, including the type of hearing aid used and the duration of use. "Consequently, whether hearing devices and aural rehabilitative strategies could affect cognitive decline and dementia remains unknown and will require further study," they say.

Dr. Lin is involved in a study at Johns Hopkins that is looking at the effects of hearing aids and cochlear implants on the cognitive functioning of adults older than 50 years. The study is called Studying Multiple Outcomes after Aural Rehabilitative Treatment (SMART).

"With colleagues at the National Institute on Aging, we're also currently studying the association between hearing loss and [magnetic resonance images] of the brain to see how hearing loss may affect brain function," Dr. Lin noted.

The study was supported by the Intramural Research Program of the National Institute on Aging and a grant from the National Institute on Deafness and Other Communication Disorders. The study authors and Dr. Gates have disclosed no relevant financial relationships.

Arch Neurol. 2011;68:214-220. Abstract

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