Nowadays, the association between hearing loss and cognitive decline is no longer surprising. Several studies have shown (Lin et al., 2014; Loughrey et al., 2018) not only that hearing loss can predict cognitive decline but can also accelerate it. Despite that, the link between those factors is still unclear, leaving questions like “how can hearing loss cause dementia?” unanswered.
One promising approach to answering this question is related to the role of the auditory brain in cognitive decline; more specifically, how auditory processing mechanisms would be involved in the association between dementia and hearing impairment. This topic was brilliantly explained in a review paper by Johnson and his colleagues (Johnson et al., 2021), using different compelling lines of evidence.
The first one is related to the structural and functional impact of neurodegenerative pathologies on the auditory brain. It is well known that most neurodegenerative diseases are characterised by the progressive spread of pathogenic (or abnormal) proteins in the nervous system (Jucker & Walker, 2018). This pathological mechanism directly affects brain networks, including areas where sound information is processed. Consequently, it leaves the auditory brain more vulnerable than the peripheral auditory system to this structural effect of the disease. This would explain why auditory processing difficulties would be observed as an early functional consequence of those conditions compared to hearing loss.
Another essential point is related to the integration between the peripheral and central auditory system, which is observed through the extensive connections between ascending and efferent pathways. Functionally or clinically speaking, this integration can be noticed through the impact of the hearing loss on central auditory skills (e.g., individuals with a significant hearing loss and, consequently, secondary auditory processing disorder) or vice-versa (e.g., hearing difficulties caused by central auditory difficulties even in the presence of normal hearing sensitivity). The combination of both peripheral and central auditory problems would aggravate even more the functional impact of a hearing loss, leading to more difficulties in challenging listening conditions, e.g., understanding speech in background noise (Murphy et al., 2018).
The third line of evidence is related to the clinical phenotypes of dementia syndromes. As previously mentioned, the spread of pathogenic proteins and, more importantly, the pattern of spread determines the clinical phenotypes of different dementia syndromes. It is known that specific auditory cognitive deficits are also included in this group of clinical phenotypes, and their characteristics correspond to different types of syndromes (e.g. typical Alzheimer’s disease would present with more pronounced auditory scene analysis deficits while frontotemporal dementias would present with problems in terms of perception of pitch, timbre and rhythm).
After considering those lines of evidence, it is almost impossible to ignore the importance of the central auditory system to neurodegenerative diseases. This importance again indicates that our role as audiologists can be much broader than it currently is. From the clinical point of view, it indicates the importance of assessing beyond the ear and start considering hearing as something more complex than just detecting or not detecting a hearing loss. We can do this by going beyond the audiogram and assessing real-world hearing through tests that simulate everyday situations. Also, we can use validated questionnaires to deeply investigate the nature of the hearing concern presented by the individual. This more comprehensive approach allows us to assess the contributions of hearing loss and the central auditory deficit to the hearing concern and, consequently, detect if the central auditory deficit is much more pronounced than expected for that level of hearing loss. Moreover, it would facilitate the observation of red flags for early stages of dementia, as explained by the research. Clinically, those changes would enormously impact hearing rehabilitation since we would not be neglecting the importance of the auditory brain to the hearing process.
References
Lin F, Ferrucci L, An Y et al. (2014) Association of hearing impairment with brain volume changes in older adults. Neuroimage 90:84–92.
Loughrey DG, Kelly ME, Kelley GA, Brennan S, Lawlor BA. Association of age-related hearing loss with cognitive function, cognitive impairment, and dementia a systematic review and meta-analysis. JAMA. Otolaryngol Head Neck Surg 2018; 144: 115–26.
Jeremy C S Johnson, Charles R Marshall, Rimona S Weil, Doris-Eva Bamiou, Chris J D Hardy, Jason D Warren, Hearing and dementia: from ears to brain, Brain, Volume 144, Issue 2, February 2021, Pages 391–401
Jucker, M., Walker, L.C. Propagation and spread of pathogenic protein assemblies in neurodegenerative diseases. Nat Neurosci 21, 1341–1349 (2018).
Murphy CFB, Rabelo CM, Silagi ML, Mansur LL, Bamiou DE, Schochat E. Auditory Processing Performance of the Middle-Aged and Elderly: Auditory or Cognitive Decline? J Am Acad Audiol. 2018 Jan;29(1):5–14.