Competing schools of thought exist on the effects of hearing loss on cognitive function. Studies reporting an association assert that declines in sensory acuity limiting the flow of information to the brain cause declines in cognitive abilities. But other studies reporting no association have argued that the decline in sensory acuity, a peripheral decline, does not necessarily affect central function.
In some cases, an individual may become more resourceful and tap into more cognitive resources to compensate for the reduction in sensory input, leaving researchers perplexed about whether there is an effect. From the studies I reviewed, variability in study design appears to contribute to inconsistent results, such as those that included research participants with well-fitted hearing aids that compensate for hearing loss. Results were also confounded by researchers who employed different methods to quantify hearing loss or who failed to account for demographic factors that may have affected cognitive function.
Two recent well-designed studies, however, help us better understand how these factors intersect.
Hearing Loss and Cognition Among Older Adults in the United States
Lin FR J Gerontol A Biol Sci Med Sci 2011;66(10):1131
Frank R. Lin, MD, PhD, recently analyzed data from the National Health and Nutritional Examination Survey to quantify the relationship between hearing loss and cognitive ability in 605 participants 60 to 69. Participants' hearing thresholds were measured using supra-aural headphones or insert earphones in an environment that met the American National Standards Institute guidelines for maximum permissible ambient noise levels. The degree of hearing loss was calculated from the four-frequency pure tone average in the better ear. Cognitive skills were assessed using the nonverbal Digit Symbol Substitution Test, which examined participants' executive function and psychomotor processing ability. Only 7.3 percent of participants had moderate or severe hearing loss, so analysis was carried out using data from participants with normal to mild hearing loss only.
The results indicated that increases in age and degree of hearing loss are related to lower Digital Symbol Substituting Test scores (lower cognitive function). After accounting for age, gender, race, education, income, smoking, diabetes, hypertension, and stroke, the authors estimated that a 25 dB increase in hearing thresholds is equivalent to an increase in seven years of age in this group of participants with normal and mild hearing loss. Further studies are needed to examine if this relationship holds true for people with a higher degree of hearing loss. Dr. Lin's results also suggested a positive change in cognitive function for 13 hearing aid users. These data, however, should be interpreted with caution due to the small sample size.
As an audiologist with expertise in amplification, I'd love to claim that hearing aids increase or help preserve that function. Unfortunately, studies on the effects of hearing aids on cognitive function are inconclusive. One of the challenges is that cognitive tests are extremely sensitive to the practice effect. The test results of studies using within-subject designs, which are often used in hearing aid research, can be compromised. Salthouse reviewed several studies examining the effects of aging on cognitive function, and found that cross-sectional studies including participants in different age groups often exhibit a monotonous decline in cognition function with age. (J Int Neuropsychol Soc 2010;16:754.)
Longitudinal studies that tested the same group of participants over time, however, often showed similar or higher scores with age. The mystery was resolved by a study in which the data of participants tested five years apart were plotted side-by-side with the data from new participants of the same age. (Psychol Aging 2005;20:3.) The results of the first group showed similar scores, but the results from the new participants showed a decline in scores with age.
These findings imply that it is essential for studies that examine cognitive function before and after hearing aid adoption to include a control group tested at the same interval or for studies to include hearing aid users and non-users with similar demographic data and account for other characteristics that might affect cognitive function. The control group is also especially important when the study involves participants with progressive diseases such as dementia to account for the natural progression of the disease.
Is Cognitive Function in Adults with Hearing Impairment Improved by the Use of Hearing Aids?
Choi AY, Shim HJ, et al Clin Exp Otorhinolaryngol 2011;4(2):72
Young Choi and colleagues examined whether hearing aid use improved cognitive function in two groups of participants of similar age. The groups were tested at the beginning of the study using a computerized Korean visual verbal learning test and a word-in-noise test. One group was then fitted with hearing aids.
Both groups received the same tests six months later. The hearing aid group showed significant improvement in short-term memory and learning ability compared with the group with no hearing aids, but neither showed improvement in the word-in-noise test. These results indicate that hearing aid use can improve at least some cognitive functions assessed using a verbal cognitive test. Future studies are needed to examine whether these improvements are observed in other cognitive functions or remain consistent in a larger group of individuals with a wider range of hearing loss.
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