Our report evaluates the association between diabetes and audiometrically assessed hearing impairment in the U.S. non-institutionalized population using nationally representative data. We estimate a prevalence of low/mid frequency hearing impairment of mild or greater severity of 28.0% among people with diabetes. The prevalence of hearing impairment was higher among individuals with diabetes in both sexes, all groups of race/ethnicity, education, income-to-poverty-ratio, and all but the oldest age group. The higher prevalence was not limited to possibly pre-disposed subgroups such as those who smoke, those with occupational or leisure time noise exposure, or those taking potentially ototoxic medications. The association between diabetes and hearing impairment remained in analyses that adjusted for other factors that may contribute to impairment.
The strength of association of diabetes with hearing impairment that we observe is comparable to that of two previous population-based studies. We report an odds ratio of 1.82 (95% CI 1.27, 2.60) for low/mid frequency hearing impairment of mild or greater severity assessed in the worse ear, while Helzner et al.
) and Dalton et al.
) reported odds ratios of 1.41 (95% CI 1.05, 1.88) and 1.42 (95% CI 1.10, 1.83), although the outcome of the latter study was based on a pure tone average in the worse ear of >25 dB HL over frequencies of 500, 1000, 2000, and 4000 Hz. For the purposes of comparison, we replicated the definition of hearing impairment used by Dalton et al
, and observed an odds ratio of 1.89 (95%CI 1.27, 2.81). Our definition of diabetes differed from the one used by Dalton et al.
, who included cases of undiagnosed diabetes and attempted to exclude individuals with type 1 diabetes. Our analysis focused on people reporting a diabetes diagnosis. Due to the self-reported nature of our assessment, we were unable to restrict to people with type 2 diabetes, although 90–95% of the diabetes in our nationally representative sample of adults with diabetes is likely to be type 2 in origin (9
Differences in age composition might account for the modest differences in the strength of association among these population-based studies. The adults in the sample of Helzner et al.
were age 73–84 years, and those studied by Dalton et al. were of target age 43–84 years. The relative contribution of diabetes to hearing impairment may be stronger among our substantially younger sample (aged 20–69 years) before the cumulative effects of aging, noise exposure, and other factors have made substantial contributions to hearing impairment. Indeed, our graphic analysis of mean pure tone thresholds suggested the separation in pure tone thresholds by diabetes status was smaller in those aged 60–69 years. In addition, the ratio of the age-specific prevalence estimates presented in appeared to be smaller for older versus younger participants, suggesting the relative contribution of diabetes may be less as one ages. Evidence from another relatively young sample of Japanese men in the military demonstrated a 87% increased odds of hearing impairment (using the Dalton et al.
definition) for those reporting diabetes, which is consistent with our findings (25
Gates et al.
did not find a statistically significant difference in pure tone average (over 250, 500, and 1000 Hz or over 4000, 6000, and 8000 Hz) by diabetes status, although, with a mean age of 73 years, this Framingham cohort may be sufficiently old whereby any effect of diabetes is less likely to be observed (26
). Ma et al.
, examined mean pure tone thresholds at 500, 1000, 2000, and 4000 Hz using data from the Hispanic Health and Nutrition Examination Survey and observed a higher mean threshold for Mexican American adults with diabetes, but only at 500 Hz (27
Diabetes-related hearing loss has only been described as progressive, bilateral, sensorineural impairment with gradual onset predominantly affecting the higher frequencies (15
). We observed generally stronger associations between diabetes and high frequency hearing impairment than low/mid frequency hearing impairment. No consistently stronger associations were observed with hearing impairment assessed in the better ear (bilateral impairment) or when assessing greater severity levels. When we examined hearing thresholds at specific frequencies, we observed higher thresholds at every frequency for people with diabetes compared to people without diabetes. This pattern held across all age groups. These observations are consistent with a report of higher hearing thresholds across all frequencies among patients with diabetes aged 40 and younger compared to healthy age-matched controls, even though the thresholds in either group were not in the range to be considered hearing impaired (28
Several biological mechanisms might explain an association between diabetes and hearing impairment. Well-established complications of diabetes, such as retinopathy, nephropathy, and peripheral neuropathy involve pathogenic changes to the microvasculature and sensory nerves (14
). These pathologic changes may plausibly include the capillaries and sensory neurons of the inner ear, but evidence from human studies is limited. Post-mortem observations of diabetic patients include thickening of capillaries within the stria vascularis (13
) and demyelination of the eighth cranial nerve, one branch of which transmits auditory signals from the cochlea to the brain stem (12
). Pathologic changes specific to the cochlea also include thickened walls of the vessels of the basilar membrane and greater loss of outer hair cells in the lower basal turn (30
). Compromised cochlear function has been measured using evoked otoacoustic emissions, a non-invasive method to assess damage to the outer hair cells of the cochlea, among patients with diabetes relative to healthy controls (31
). Other vascular changes include narrowing of the internal auditory artery (32
). A number of rare genetic syndromes such as Alström syndrome (33
), Wolfram syndrome (34
), and “maternally inherited diabetes and deafness”(35
) feature diabetes and hearing impairment as characteristics. It is possible that more common, but yet to be identified, genetic factors predispose to both diabetes and hearing impairment.
Potential limitations of the analysis include recall-based assessments of leisure time and occupational noise exposure; self-reported noise exposure is subject to measurement error, so we cannot rule out residual confounding as contributing to some of the association we observe. This limitation may be more of a factor for high frequency impairment because this outcome incorporates pure tone thresholds observed across 3000–6000 Hz, where injury from excessive noise stimulus is most notable (36
). Similarly, for most of our analyses, the assessment of diabetes was based on self-report, and persons with undiagnosed diabetes were considered to be nondiabetic. Given the greater prevalence of hearing impairment that we observed with greater dysregulation of glucose, we have likely under-estimated the overall measures of association. Also, we are unable to distinguish between type 1 and type 2 diabetes, however, almost all participants in this study have type 2 diabetes. In addition, our measure of ototoxic drug use does not incorporate information on dose or use in the past. Last, we are able to make inferences only to the U.S. non-institutionalized adult population. It is likely that the prevalence of both diabetes and hearing impairment is even greater among institutionalized adults.
In summary, these data suggest that hearing impairment may be an under-recognized complication of diabetes. Although this analysis does not focus on possible mechanisms for the association of diabetes and hearing impairment, we have identified an important public health problem that can be addressed. With the high prevalence of hearing impairment occurring among diabetic patients, screening for this condition may be justified (37