Chronic conditions such as age-related hearing loss and senile dementia are becoming more prevalent and costly in terms of healthcare burden and expenditures. An estimated 5.2 million older adults had Alzheimer's disease in 2012, with prevalence projected to rise to 6.7 million by 2025.1,2 Like age-related hearing loss, detection of dementia is a problem in routine medical practice, with the diagnosis often missed.3 Dementia and age-related hearing loss have numerous other features in common, as shown in table 1.
Neuroimaging studies have demonstrated independent associations among hearing impairment and decreased cortical volumes in the auditory cortex, reduced gray matter in the auditory cortex, and accelerated rates of lateral temporal lobe and whole brain atrophy.4,5 Notably, the temporal regions where anatomical changes appear to take place are important for spoken language processing, semantic memory, and sensory integration. These are also the areas involved in the early stages of mild cognitive impairment or early Alzheimer's disease.6,7
People with a mild, moderate, or severe hearing impairment have a twofold, threefold, or fivefold increased risk, respectively, of incident all-cause dementia compared with people who have normal hearing.8 Among those older than 60, more than one-third of the risk of incident all-cause dementia is associated with hearing loss.
Further, longitudinal studies in community-dwelling older adults have demonstrated an independent relationship between hearing impairment and a 30 percent to 40 percent accelerated rate of cognitive decline on auditory and nonauditory tasks.9
The devastating effects and medical expenditures associated with dementia care have served as the impetus for research devoted to uncovering modifiable determinants and risk factors. Treating hearing impairment may in fact serve as a buffer against onset of cognitive decline.
Jennifer A. Deal, PhD, and colleagues recently reported on data from a pilot study of hearing and cognitive impairment initiated within the Atherosclerosis Risk in Communities (ARIC) Study, a population-based prospective cohort study.10 The mean age of participants at the time of the hearing assessment was 77; 37 percent had mild hearing loss. In addition, 34 percent of participants presented with moderate or severe hearing loss, and their mean age was 79.4.
A comprehensive neurocognitive battery including tests of memory, language, and processing speed/attention was administered upon initiation of the hearing pilot study. Neurocognitive tests in the same three domains also were administered on two prior occasions approximately 20 years before the audiologic assessment.
There was a dose-response relationship between severity of hearing loss and cognitive decline over time. Compared with participants who had normal hearing, people who had moderate or severe hearing loss declined at a faster rate on the memory task and the global composite score, an average of the domain-specific scores.
Dr. Deal and coauthors compared cognitive task performance among participants with moderate or severe hearing loss who were hearing aid users (length of hearing aid use varied from less than one year to 48 years) versus hearing aid nonusers. The findings were noteworthy.
The performance of participants with moderate or severe hearing impairment decreased in all three cognitive domains of function, with the greatest declines in the memory task and for hearing aid nonusers. Hearing aid nonusers also had more comorbidity, such as hypertension and diabetes, at a younger age than did people with the same amount of hearing loss who reported wearing hearing aids.
The findings of Dr. Deal and colleagues are especially important in light of the data of Kim M. Kiely, PhD, and coauthors from the Australian Longitudinal Study of Aging and the Blue Mountains Eye Study.11 Dr. Kiely and coinvestigators explored the interaction between hearing loss and probable cognitive impairment in a group of older adults, nine percent of whom had probable cognitive impairment at baseline and six percent of whom developed incident cognitive impairment over the 11 years of the study.
Neurocognitive decline, as defined by a score of 23 or lower on the Mini-Mental State Examination, was independently associated with lower levels of and accelerated decrease in peripheral hearing ability. Probable cognitive impairment at baseline was related to poorer initial pure-tone averages (PTA = 0.5 kHz, 1 kHz, 2 kHz, and 4 kHz) and faster rates of change in the four-frequency PTA. The oldest participants experienced quicker decline and were more likely to have moderate hearing loss.
ROLE OF COCHLEAR IMPLANTS
To further explore whether hearing interventions may be protective against cognitive decline, Isabelle Mosnier, MD, and colleagues conducted a longitudinal, multicenter study of the effects of cochlear implantation on neurocognitive performance in older adults with severe to profound hearing loss.12
Mean age at implantation was 72, and the participants were deafened postlingually. Of the cochlear implant recipients, 57 percent continued to use their hearing aid in the contralateral, non-implanted ear post-activation.
All participants underwent intensive aural rehabilitation after cochlear implant activation. The rehabilitative tasks were designed to engage memory, attention span, speed of processing, and mental flexibility.
At baseline and at six months and one year after cochlear implant activation, neurocognitive tests of attention, memory, orientation, executive function, mental flexibility, and fluency were administered.
Speech perception in noise improved dramatically six months after implantation, with scores stabilizing between six months and one year. In contrast, speech perception in quiet steadily improved over the 12 months of this prospective study.
Prior to cochlear implantation, 25 percent of participants obtained normal scores on six cognitive tasks, 31 percent had one abnormal test score, 24 percent had two abnormal test scores, and 20 percent had three abnormal test scores. The mean age at implantation was comparable for those with and without abnormal test scores.
Twelve months post-activation, 40 percent of participants had normal scores on the six cognitive tasks, 22 percent had abnormal scores on two tasks, and five percent had abnormal scores on three of the cognitive tasks (see table 2).
Among those with abnormal test scores preimplantation, significant improvement was noted as early as six months postimplantation in performance on the Mini-Mental State Examination and on the memory and visual attention tasks.
Prior to implantation, 59 percent of participants offered responses suggesting the absence of depression; 12 months following activation, 76 percent of participants emerged without self-reported depression.
WHERE TO GO FROM HERE
Hearing loss and senile dementia are ubiquitous and under-recognized, and the consequences are exceedingly costly in terms of healthcare expenditures and toll on family and society. With 5.1 million people age 65 and older reportedly affected by Alzheimer's disease this year, the direct costs of caring for them are projected to be as high as $226 billion, with half of the costs borne by Medicare (Alzheimer's Association, March 2015http://bit.ly/AlzFigures). By 2050, these costs could reach $1.1 trillion.
Hearing loss prevalence is expected to rise from 36 million today to 53 million by 2050.13 The collective monetary cost of untreated hearing loss could exceed $100 billion annually, given the condition's association with repeat hospitalizations, reduced physical activity, and increased fall risk.
Treatments for Alzheimer's disease and related dementias remain elusive, so interventions that could forestall cognitive impairment, the onset and progression of dementia, and dementia's devastating consequences are critically important.
Preliminary data regarding the relationship among cognitive status, hearing impairment, and hearing healthcare interventions—ranging from hearing aids to cochlear implants and including auditory-based cognitive rehabilitation—are encouraging.
Audiologists should consider including cognitive measures in routine audiologic assessment, the hearing healthcare intervention fitting process, and auditory–cognitive rehabilitation.
Given the ease of administration, any of the three screening tests listed in table 3 could easily be integrated into practice as baseline and outcome measures.14-16 Similarly, the visual reading span task, which measures storage and processing capacity, can help guide the hearing aid selection process, especially when deciding on compression speed.17
Further, information on cognitive status can be used in patient counseling when discussing how amplification could reduce the burden on cognitive processing, especially in challenging listening environments.17
Finally, when collaborating with physicians on the management of people with age-related hearing loss, it is important for us to share the outcomes of our services. Interventions like hearing aids and cochlear implants are multifaceted solutions to communication challenges posed by cognitive impairment. Audiologists must assume responsibility for sharing clinical evidence that restoration of communication contributes to successful aging.