Johnson, Jeremy Colin Spencer; (2021) Hearing in dementia: defining deficits and assessing impact. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The association between hearing impairment and dementia has emerged as a major public health challenge, with significant opportunities for earlier diagnosis, treatment and prevention. However, the nature of this association has not been defined. We hear with our brains, particularly within the complex soundscapes of everyday life: neurodegenerative pathologies target the auditory brain and are therefore predicted to damage hearing function early and profoundly. Here I present evidence for this proposition, based on structural and functional features of auditory brain organisation that confer vulnerability to neurodegeneration, the extensive, reciprocal interplay between ‘peripheral’ and ‘central’ hearing dysfunction, and recently characterised auditory signatures of canonical neurodegenerative dementias (Alzheimer’s disease and frontotemporal dementia). In chapter 3, I examine pure tone audiometric thresholds in AD and FTD syndromes and explore the functional interplay between the auditory brain and auditory periphery by assessing the contribution of auditory cognitive factors on pure tone detection. In chapter 4, I develop this further by examining the processing of degraded speech signals, leveraging the increased importance of top-down integrative and predictive mechanisms on resolving impoverished bottom-up sensory encoding. In chapter 5, I use a more discrete test of phonological processing to focus in on a specific brain region that is an early target in logopenic aphasia, to explore the potential of auditory cognitive tests as disease specific functional biomarkers. Finally, in chapter 6, I use auditory symptom questionnaires to capture real-world hearing in daily life amongst patients with dementia as well as their carers and measure how this correlates with audiometric performance and degraded speech processing. I call for a clinical assessment of real-world hearing in these diseases that moves beyond pure tone perception to the development of novel auditory ‘cognitive stress tests’ and proximity markers for the early diagnosis of dementia and management strategies that harness retained auditory plasticity.

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