Hearing impairment and risk of Alzheimer's disease: a meta-analysis of prospective cohort studies.

Observational studies suggested an association between hearing impairment and cognitive disorders. However, whether hearing impairment is an independent risk factor or a harbinger of Alzheimer's disease remains controversial. Our goal was to assess the association between hearing impairment (HI) and the risk of Alzheimer's disease (AD) by conducting a meta-analysis of prospective cohort studies. We comprehensively searched the PubMed, Embase, Web of Science and Cochrane Library databases on January 19, 2016 to incorporate all the prospective cohort studies meeting the inclusion criteria to perform a systematic review and meta-analysis. Four prospective cohort studies with comparison between hearing impairment and normal hearing were incorporated, with 7461 participants. The outcomes of three studies were the incidence of Alzheimer's disease and the outcome of the fourth study was the incidence of mild cognitive impairment. The overall combined relative risk of people with hearing impairment to develop Alzheimer's disease was 4.87 (95% CI 0.90-26.35; p = 0.066), compared with the control group. Since both Alzheimer's disease and mild cognitive impairment are cognitive disorders, we incorporated all the four studies and the overall combined relative risk was 2.82 (95% CI 1.47-5.42; p = 0.002), indicating that the difference was significant. This meta-analysis suggests that hearing impairment significantly increases the risk of cognitive disorders and future well-designed prospective cohort studies are awaited to confirm the association between hearing impairment and risk of Alzheimer's disease.