Association between adult-onset hearing loss and dementia biomarkers: A systematic review.

BACKGROUND AND OBJECTIVE

People with adult-onset hearing loss (AoHL) are at increased dementia risk. In this study, we explore potential aetiological mechanisms by synthesising the evidence on the association between AoHL and neuropathological, cerebrospinal fluid (CSF), blood and imaging biomarkers of dementia.

METHODS

We systematically searched electronic databases from inception to 30 April 2024 for cross-sectional and longitudinal studies, including quantitative data on the association between AoHL and dementia biomarkers. Study quality was assessed with the Mixed Methods Appraisal Tool (MMAT).

RESULTS

Sixty-six studies reporting 63 cross-sectional and 10 longitudinal analyses were included. Twenty-one studies met all MMAT quality criteria. We report a narrative synthesis due to the heterogeneity of the included studies. In CSF-based or blood-based assays or imaging, five out of six cross-sectional analyses found that AoHL was associated with elevated in vivo tau levels, whilst four out of 17 reported a link with elevated in vivo β-amyloid measures. One longitudinal analysis identified an association between AoHL and a steeper increase of CSF tau, but not Aβ, levels over time. Twenty-five out of 44 cross-sectional and six out of nine longitudinal analyses identified associations between AoHL and grey matter atrophy of the temporal regions, particularly the medial temporal lobe. Studies using other biomarkers had inconsistent findings.

CONCLUSIONS

AoHL was usually associated with more temporal regions grey matter atrophy both cross-sectionally and longitudinally, and elevated in vivo tau but not β-amyloid. Increasing atrophy and higher tau, leading to decreased cognitive reserve may be how hearing loss increases dementia risk.