Jennifer Jones Lister and Jerri Edwards
The relationship between hearing loss and cognitive impairment is quite complex and poorly understood; however, understanding this relationship is of critical importance to the profession of clinical audiology. As is the case for most disease processes, early identification of cognitive impairment prior to dementia onset such as Alzheimer's disease (AD) is necessary for early intervention, and early intervention means more treatment options and better outcomes. Research suggests that an auditory indicator of early-stage cognitive impairment exists, although it has yet to be defined. Audiologists are well-positioned to establish and develop tools in this area.
The Alzheimer's Association estimates that more than 5 million Americans have AD, which is but one form of cognitive impairment, and that AD is the sixth leading cause of death. In addition, dementia is currently the most expensive medical condition in the United States, costing $259 billion in 2017. The high cost of AD includes the great economic impact of the professional health care needed by those with AD, of course—but there is also a high cost to the health and well-being of the more than 15 million unpaid caregivers, many of whom have to give up wages to care for a loved one with dementia.
Much like hearing loss, cognitive impairment increases in prevalence with age and has a gradual progression, with symptoms becoming increasingly apparent over several years or even decades. It is believed that both hearing loss and cognitive impairment are experienced by a large number of Americans. The Hearing Loss Association of America (HLAA, 2017) estimates that 48 million Americans have significant hearing loss, and that hearing loss is the third most prevalent health condition in older adults, behind arthritis and heart disease. This co-occurrence—and the fact that some individuals compensate remarkably well for the decline in hearing and/or cognition—makes diagnosis difficult in some cases.
To provide a little background, cognition is measured across multiple domains, including executive function, memory, and speed of processing. Within each domain, a variety of tests are available to quantify function. Cognition may also be quantified using global measures of cognitive status such as the Montreal Cognitive Assessment (MoCA; Nasreddine et al., 2005). Two terms often used to describe diminished cognition are mild cognitive impairment (MCI) and dementia. MCI is thought to be a transitional state between normal cognitive aging and dementia. It is defined as a clinical condition with significant impairment in one or more cognitive domains, in the presence of otherwise normal everyday functioning; it is not severe enough to warrant a diagnosis of dementia. A diagnosis of dementia encompasses a greater cognitive impairment with noticeable effects on daily life. An estimated 12% of persons with MCI convert to dementia every year, compared with approximately 1%–2% of those without MCI. AD is the most common form of dementia.
A number of studies have described a significant relationship between hearing and cognitive status. Most of those fall into the category of behavioral studies—that is, examining the relationship between (a) pure-tone hearing or auditory processing and (b) one or two measures of cognition. For example, in multiple studies, Frank Lin and colleagues (Lin, 2011; Lin, Ferrucci, et al., 2011; Lin, Metter, et al., 2011; Lin, Thorpe, Gordon-Salant, & Ferrucci, 2011) have shown a small but significant relationship between a pure-tone average in the better hearing ear and one to four measures of cognition. Lin and colleagues (Lin et al., 2013) have also shown that 1,162 older adults with baseline hearing loss had, over a 6-year time period, rates of decline on two measures of cognition that were 30%–40% greater than for older adults with normal hearing at baseline. Older adults with hearing loss were found to be at a 24% greater risk of cognitive impairment than those without hearing loss.
In our lab (Harrison Bush, Lister, Edwards, Lin, & Betz, 2015), the relationship between pure-tone hearing and the most comprehensive battery of cognitive measures to date was examined. Pure-tone hearing was significantly related to 10 of 11 cognitive measures, including measures of processing speed, executive function, memory, and global cognitive status. However, the percentage of variance in the cognitive measures accounted for by hearing thresholds was small across domains of cognition, ranging from 0.4% to 2.2%. When the participants with normal hearing (n= 595) and participants with hearing loss (n= 299) were analyzed separately, it was clear that the significant effects were being driven by the participants with hearing loss. Although a relationship between pure-tone hearing and cognition undoubtedly exists, its clinical utility is limited due to the small size of the effect. This is a case of statistical significance not corresponding to clinical significance. Audiologists will have to look further than a standard audiogram to find an auditory indicator of early-stage cognitive impairment.
Stronger relationships may be found between measures of auditory processing and cognition, including dementia incidence. George Gates and colleagues (Gates, Anderson, Feeney, McCurry, & Larson, 2008; Gates et al., 1995, 1996, 2010) have shown, in multiple studies, that performance on tests of speech understanding in competition and binaural processing is poorer among those with cognitive impairment than those without. Gates and colleagues (Gates, Anderson, McCurry, Feeney, & Larson, 2011; Gates, Beiser, Rees, D'Agostino, & Wolf, 2002) also demonstrated that deficits on these auditory processing tests are predictive of a later diagnosis of AD.
In our lab (Edwards et al., 2017), older adults with and without MCI were compared on several measures of auditory processing. We found that older adults with MCI performed significantly more poorly on tests of speech understanding in competition, binaural processing, and temporal processing than did adults without MCI. Group differences were not found for pure-tone hearing thresholds. Consistent with previous studies, our study revealed that older adults with MCI have difficulty with speech understanding in competition and binaural processing. Expanding upon previous results, we showed that older adults with MCI also have difficulty with temporal processing. Of the three types of tests, the strongest effect was found for binaural processing; this suggests that tests of binaural processing may be a useful indicator of early cognitive impairment.
Fewer studies in this area have used electrophysiological measures such as auditory event-related potentials (ERPs), although there are several advantages to doing so. Behavioral methods are susceptible to bias due to differences in listener motivation, response criteria, or other external factors, making it difficult to draw conclusions about underlying neurophysiological mechanisms. In contrast, ERPs are objective, noninvasive methods of documenting neural function at various levels of the auditory system through wave amplitude, latency, and spatial activation patterns. ERPs index real-time activity related to sensory and cognitive processes, and they are sensitive enough to detect underlying neural changes years prior to behavioral manifestation.
Studies in our lab have focused on two auditory ERPs: the P1-N1-P2 complex and the P3b. The P1-N1-P2 complex is reflective of synchronous neural activity of thalamocortical structures of the central auditory system in response to sound. It is considered exogenous (i.e., an obligatory response to sound) and has a latency of 50–200 ms following the onset of a sound. Results from our lab suggest that, of the components of the P1-N1-P2 complex, only P2 amplitude varies significantly with cognitive status (Lister et al., 2016). Significantly smaller P2 amplitudes were found for older adults with MCI than for those without MCI for both pure-tone and speech stimuli. This result suggests that P2 amplitude may be useful as an indicator of early-stage cognitive impairment.
In contrast to P1-N1-P2, the P3b response is considered endogenous and has a latency of 300–800 ms after the onset of a sound. The P3b is viewed as an indicator of memory or context updating, stimulus discrimination, response preparation, and duration of stimulus evaluation. In a recently completed study, we found that, like the P2, the auditory P3b component is significantly reduced in amplitude among older adults with MCI, as compared with healthy older adults for both pure-tone and speech stimuli. A follow-up analysis indicated that, among older adults, P3b amplitude was significantly related to several measures of cognition and auditory processing. When the P3b was evoked by pure tones, significant correlations were found between P3b amplitude and executive function, memory, and speech understanding in competition. For P3b evoked by speech, a greater number of significant correlations were found; P3b amplitude was significantly correlated with executive function, auditory temporal processing, memory, speech understanding in competition, binaural processing, and a global measure of cognitive status. In all cases, higher P3b amplitude was associated with better performance across measures. Like P2 amplitude, P3b amplitude may be a useful early indicator of cognitive impairment.
Several potential mechanisms have been suggested to underlie the hearing–cognition relationship (for a review, see Fulton, Lister, Harrison Bush, Edwards, & Andel, 2015). These include the following:
The simple overdiagnosis of cognitive impairment among those with hearing loss because most cognitive assessments are given via live voice with minimal control of background noise and other auditory interference
A widespread age-related neural degeneration that affects both hearing and cognition
A hearing loss–related sensory degradation/deprivation that results in poor input to higher cognitive processes
The possibility that hearing loss and cognitive decline result in changes in neural resource allocation and/or depletion of neural resources
The possibility that social isolation and depression that often accompany hearing loss contribute to cognitive decline.
Although it is likely that a combination of these hypothetical mechanisms is at work and that these mechanisms could vary across individuals, research in our lab suggests strong support for a depletion of neural resources by hearing loss that leaves fewer resources for cognitive processing.
In summary, our study shows that a number of auditory measures—including pure-tone thresholds, auditory processing measures, and ERPs—are predictive of cognitive impairment and decline. The strongest predictors appear to be an auditory processing measure of binaural processing and the amplitude of two ERPs: P2 and P3b. Clearly, the simple presence of hearing loss, auditory processing disorder, or reduced ERPs does not mean that dementia is looming in one's future; losing your hearing does not mean you are about to lose your mind. However, the potential for an auditory indicator of early-stage cognitive decline remains strong. Auditory measures have the potential to serve as efficient, noninvasive, cost-effective indicators of future cognitive decline and current cognitive impairment.
Jennifer Jones Lister, PhD, CCC-A, is a professor and chair of the Department of Communication Sciences and Disorders at the University of South Florida (USF). She is a faculty affiliate with the Byrd Alzheimer's Institute, the Global Center for Hearing and Speech Research, and the School of Aging Studies at USF. She is an audiologist and hearing scientist with more than 15 years of expertise in the areas of aging and event-related potentials (ERPs). Her research experience spans aging, hearing, speed of processing, psychophysical methods, and ERPs.
Jerri Edwards, PhD, is a professor in the Department of Psychiatry and Behavioral Neurosciences at the University of South Florida. She is faculty affiliate of the Byrd Alzheimer's Institute, the Department of Communication Sciences and Disorders, the Global Center for Hearing and Speech Research, and the Center for Urban Transportation Research. Edwards is a developmental psychologist who completed postdoctoral training in applied gerontology. She is an internationally known expert in cognitive intervention for older adults, particularly as applied to older drivers.
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