The Baltimore Longitudinal Study of Aging, the longest running study of human aging in the United States, recently uncovered an association between hearing loss and dementia. The exact relationship between the two, however, remains unclear. One hypothesis asserts that there is a shared neurological pathology between the two, and another suggests that dementia is an outcome of hearing loss due to social isolation or cognitive burden. A recent analysis of the National Health and Nutrition Examination Survey (NHANES) shows an association between hearing loss and cognitive functioning (Lin, 2011).
The NHANES provides ongoing data and uses a weighted sample generalizable to the United States’ population. Hearing loss data used in NHANES is methodologically and technologically rigorous, and is coded according to World Health Organization and American Speech-Language Hearing Association guidelines. Cognitive function in the NHANES is assessed using the Digit Symbol Substitution subtest (DSST) of the Wechsler Adult Intelligence Test, which is the most widely-used and accepted of all “IQ tests.” The DSST is useful to test the relationship between hearing and cognition because it is a nonverbal assessment of executive functioning and psychomotor speed. In addition, it is a test of nonverbal performance, thus, the discrepancies between actual hearing loss and test administration is minimized.
This study also reduced conflicting variables by incorporating relevant demographic and medical risk factors measured using the NHANES. Also included was self-reported hearing aid use, though only 13 out of 605 participants from the sample used hearing aids. The analysis was based on NHANES participants between the ages of 60 and 69 who had received hearing and cognitive testing.
The analysis found that there is a clinically significant association between hearing loss and cognitive function. Based on the findings, controlling for relevant variables, a 25 decibel (dB) increase in hearing loss is the equivalent of the reduction in cognitive performance occurring over seven years. Self-reported hearing aid use was associated with higher cognitive functioning, but the sample size of participants with hearing aids was too small to confidently interpret this finding. It may suggest, however, that hearing aid use is protective against dementia, and if this association between hearing aid use and cognitive functioning could be further supported longitudinally, it would indicate both that hearing loss contributes to reduced cognitive functioning, and that hearing aid use may prevent this.
This study used cross-sectional data, and because of this, it is not clear whether hearing loss leads to reduced cognition (or vice-versa), or whether there is a shared cause of the two. These are still useful findings that demonstrate an association between hearing loss and cognition within a nationally representative sample, and suggest that hearing aid use may help preserve cognitive function.
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