The relationship between hearing impairment and cognitive function: a meta-analysis in adults

Objective: Hearing loss affects over 1.23 billion people globally. It has been proposed that hearing impairment negatively impacts on cognition. Some studies have demonstrated a faster rate of decline in cognition, and increased risk of incident all-cause dementia. This finding is not ubiquitous. This study used meta-analysis to examine the evidence-base regarding the relationship between hearing and cognition. Design: A systematic review of the literature and meta-analyses of study findings were conducted. Published and grey literature was reviewed. Papers were included if they studied the relationship between hearing and cognition in adults with and without hearing impairment. Main outcome measures: Meta-analyses examined evidence for and against seven questions. Is cognition poorer in individuals with normal hearing compared to (i) untreated or (ii) treated hearing impairment, is cognition associated with degree of hearing impairment in (iii) untreated and/or (iv) treated hearing, is cognition (v) different in untreated compared to treated hearing impairment, (vii) does cognition improve after intervention, and (vii) how is hearing impairment differentially associated with cognitive ability across six domains of cognition?. Results: The 33 included studies contributed 40 samples, with a total of 602 participants with untreated hearing impairment, 672 participants with treated hearing impairment, 176 healthy controls, and 4260 individuals with a range of hearing impairment with/without treatment. The results demonstrated that cognition is significantly poorer in (i) individuals with untreated hearing and remains poorer in (ii) treated hearing impairment compared to normal hearers. The degree of cognitive deficit is significantly associated with the degree of hearing impairment in both (iii) untreated and (iv) treated hearing impairment. Furthermore, (v) hearing intervention significantly improves cognition. Finally, (vii) hearing impairment impacted on all domains of cognition. Conclusions: This meta-analysis suggests that hearing impairment is associated with cognitive problems. However, due to diversity within studies, small sample sizes, the failure to control for premorbid and other health factors, this conclusion may be premature.