Age Related Hearing Loss and Cognitive Decline II: Mechanisms and Effect of Cochlear Implantation

Background

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An expanding literature indicates that age related hearing loss (ARHL) is associated with age related cognitive decline (ARCD).  This was discussed in the April 2016 blog in this series suggesting that memory and concentration both decline 30 – 40% faster in older adults who have hearing loss and that the risk of dementia increases proportionately with the degree of hearing loss.  In an aging mouse model, deafened mice had poorer cognitive function over time than normal hearing controls—inferring that the relationship may be causative. (References cited in April 2016 Blog.)  

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What's New

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The Lancet Commission on Dementia recently concluded that appropriate treatment of hearing loss may reduce the risk of dementia by up to 9% and that hearing loss is the leading treatable risk factor for dementia. (1)   Uchida et al. have described three mechanisms (and an artifact) by which ARCD and cognitive decline may be causally related. (2)  

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Possible Mechanisms

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1. Cognitive load hypothesis (Causal relationship between ARHL and ARCD).  Cognitive load refers to the amount of information processing required to perform a task.  If a task requires too much processing, performance is reduced because of a capacity limitation thought to be in working memory.  Older adults with hearing loss expend more attention and concentration (greater cognitive load) in listening than do their normal hearing counterparts.  This may deplete cognitive reserve required for other cognitive employment. (3)    

2. Cascade hypothesis (Causal relationship).  ARHL is associated with changes of brain morphology (neural plasticity) in older adults.  For example, ARHL subjects have smaller brain volume than age-matched normal-hearing peers (4,5) as well as accelerated rates of brain atrophy when followed over time.  (6)  In fMRI studies, the degree of hearing loss predicted the volume of gray matter activation in the primary auditory cortex, thalamus, and brainstem. (7) Thus, in cascade theory, hearing loss may lead to changes in brain structure and function which may lead to ARCD, depression, social isolation, and behavioral involution.  Brain atrophy may also cascade into cognitive load issues.  

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3. Common cause hypothesis (Non-causal relationship).  Common neurodegenerative process(es) in the aging brain may be responsible for both ARHL and ARCD.  Leading suspects include microcirculatory insufficiency, general health, oxidative stress, smoking, diabetes, cardiovascular and large vessel cerebrovascular disease, and genetic factors.  For example, the APOE gene encodes for Apolipoprotein E protein that has been associated with both neurodegenerative conditions such as central hearing loss and Alzheimer disease.  In this theory, ARHL and ARCD may be caused by the same underlying processes but one does not cause the other. (8)

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4. Overdiagnosis    (Non-existent relationship)  It remains possible that hearing loss results in poor performance on neuropsychological tests leading to misdiagnosis of ARCD.  This notion is similar to that of culture-unfair IQ tests leading to misrepresentation of low intellect.  In a study that supports this theory, use of ear plugs on older adults with normal cognitive test results caused artifactual ARCD.  

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Cochlear Implants And ARCD

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Lin et al (9) demonstrated that the rate of ARCD is directly related to the severity of hearing loss.  It follows that rehabilitation of severe-profound HL with cochlear implants may have a greater effect on ARCD than rehabilitation of mild-moderate HL with hearing aids.

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Mosnier et al. (10) reported results of 37 implanted older adults who had abnormal scores on 2 or 3 of 6 cognitive tests (i.e.: ARCD) before surgery.  At 1-year post-op, 30 (81%) showed improvement with abnormal cognitive scores on 0 or 1 of the 6 tests. Thirty-one ARCD/implanted subjects were re-tested > 5 years post-op.  Ten (32%) returned to normal cognitive function, 19 (61%) remained stable and 2 (6%) progressed to dementia. (11)
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Recently, Claes et al. (12) described the 12-month results of a planned 10-year prospective cohort study of cochlear implantation and ARCD in 20 severely hearing-impaired subjects > 55 years of age.  Significant improvements were found in total cognitive scores (p < 0.001), immediate (p < 0.005) and delayed memory (p < 0.002) and attention (p < 0.047).  However, the authors also found that after one year, the implanted patients showed significantly greater ARCD than normal hearing controls and suggested routine auditory and cognitive rehabilitation for implanted patients with ARCD. (13)

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Take Home

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Age related cognitive hearing loss is related to cognitive decline and is considered to be the leading treatable cause of dementia.  Three hypothetical mechanisms for the relationship of ARHL and ARCD include Cognitive Load, Cascade, and Common Cause.  Early reports of longitudinal cohort studies indicate a beneficial effect of cochlear implantation on cognitive decline in older adults with ARHL.  Further studies of the effects of cognitive and language training after CI in older adults are called for.

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References

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1. Livingston G, Sommerlad A, Orgeta V, Costafreda SG, Huntley J, Ames D, et al. Dementia prevention, intervention, and care. Lancet 2017;390 (10113):2673–734.
2. Uchida Y, Sugiura S, Nishita Y, Saji N, et al.  Age-related hearing loss and cognitive decline—The potential mechanisms linking the two.  Auris Nasus Larynx. (2018) In press. https://doi.org/10.1016/j.anl.2018.08.010
3. Sweller J, Ayres PL, Kalyuga S.  Cogniive load theory.  New York; London: Springer, 2011.
4. Lin FR, Ferrucci L, An Y, et al.    Association of hearing impairment with brain volume changes in older adults. Neuroimage (2014) 15: 84-92.  
5. Rigters SC, Bos D, Metselaar M, Roshchupkin GV, Baatenburg de Jong RJ, Ikram MA, et al. Hearing impairment is associated with smaller brain volume in aging. Front Aging Neurosci 2017;9.  
6. Golub JS. Brain changes associated with age-related hearing loss. Curr Opin Otolaryngol Head Neck Surg 2017;25(5):347–52.
7. Peelle JE, Troiani V, Grossman M, Wingfield A. Hearing loss in older adults affects neural systems supporting speech comprehension. J Neurosci 2011;31(35):12638–21643.
8. Mener DJ, Betz J, Yaffe K, et al. Apolipoprotein E allele in older adults. Am J Alzheimers Dis Other Demen 2016;31(1):34–9)  
9. Lin FR, Yaffe K, Xia J, et al. Hearing loss and cognitive decline among older adults. JAMA Intern Med 2013;173:293–9.
10. Mosnier I, Bebear JP, Marx M, Fraysse B, Truy E, et al. Improvement of cognitive function after cochlear implantation in elderly patients. JAMA Otolaryngol Head Neck Surg 2015;141 (5):442–50.
11. Mosnier I, Vanier A, Bonnard D, et al. Long-Term Cognitive Prognosis of Profoundly Deaf Older Adults After Hearing Rehabilitation Using Cochlear Implants. J Am Geriatr Soc. 2018 Aug;66(8):1553-1561.  
12. Claes AJ, Van de Heyning P, Gilles A, et al.  Cognitive Performance of Severely Hearing-impaired Older Adults Before and After Cochlear Implantation: Preliminary Results of aProspective, Longitudinal Cohort Study Using the RBANS-H.  Otol & Neurotol, Vol. 39, 2018.
13. Claes AJ, Van de Heyning P, Gilles A, et al.  Impaired Cognitive Functioning in Cochlear Implant Recipients Over the Age of 55 Years: A Cross-Sectional Study Using the Repeatable Battery for the Assessment of Neuropsychological Status for Hearing-Impaired Individuals (RBANS-H).  Front Neurosci. 2018 Aug 24;12:580. doi: .3389/fnins.2018.00580. eCollection 2018.

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