In the past couple of weeks (Part I and Part II) we have been reviewing Diabetes.  This week, in Part III, Hearing International will look at the connection of diabetes with hearing loss. As noted previously, patients with diabetes usually have a myriad of serious medical complications.   The American Speech-Language-Hearing Association (ASHA) indicates that the majority of adults with diabetes experience cardiovascular disease risk factors such as hypertension and high cholesterol. Also, the risk of a stroke or fatal cardiac event is two to four times higher among adults with diabetes than those without diabetes. Diabetic retinopathy is the leading cause of blindness in the United States. Among Americans, diabetes is also the leading cause of kidney failure. Severe forms of diabetic nerve disease often necessitate amputation of the lower extremities. Diabetic autonomic neuropathies can affect cardiovascular, gastrointestinal, bladder, and erectile function.

Since diabetes is a systemic disease that affects multiple organ systems, it is reasonable to inquire if the auditory system is among those affected.  Although there has been evidence since the mid-19th century linking diabetes with hearing loss, controversy has surrounded this association. Early attempts to establish an association between diabetes and hearing impairment were not very convincing, probably due to subject selection, design or other errors. ASHA reports that historical  studies that have attempted to relate diabetes to hearing loss.  Researchers examined the data collected in the National Health and Nutrition Examination Study in the early 1980s.  In that study, pure-tone air conduction thresholds were examined at 500, 1000, 2000, and 4000 Hz by diabetes status. It found a higher threshold among diabetic persons only at 500 Hz (Ma, Gomez-Marin, Lee, & Balkany, 1998).

Others have looked at the Framingham Cohort Study, a leading population-based epidemiological study.  In their review, the researchers averaged pure-tone air conduction thresholds at 250, 500, and 1000 Hz as well as 4000, 6000, and 8000 Hz in the better ear and worse ear.  Their definition of hearing loss, however, was a high pure-tone average of  >40 dB HL and, probably due to this strict definition, researchers found no association with diabetes (Gates, Cobb, D’Agostino, & Wolf, 1993; Ma et al., 1998).

Only the Epidemiology of Hearing Loss Study, a community-based investigation conducted in Beaver Dam, Wisconsin, detected a modest increase in occurrence of hearing impairment among adults with diabetes compared to those without diabetes. It defined impairment as the pure-tone average above 500, 1000, 2000, and 4000 Hz as being >25 dB HL in the worse ear (Dalton, Cruickshanks, Klein, Klein, & Wiley, 1998). Other studies to identify such a link were conducted in industrial or military settings and could not rule out the possibility that the effect of diabetes was limited to individuals who were predisposed to hearing loss due to occupational noise exposure (Ishii, Talbott, Findlay, D’Antonio, & Kuller, 1992; Sakuta, Suzuki, Yasuda, & Ito, 2007).

Conflicting evidence has prevented diabetes-related hearing impairment from receiving much attention among research scientists and much acceptance among health care professionals.  Recently, though, research findings suggest that impaired hearing is not only very common among older and middle-aged people with diabetes, but that hearing loss also affects young people with diabetes more than those with normal hearing.

A recent study by a Japanese team, Horikawa, et al (2013) found that, in general, diabetics were 2.15 times more likely to have hearing loss than people without the disease. When their results were broken down by age, people under 60 had 2.61 times the risk while people over 60 had 1.58 times higher risk.  Horikawa and colleagues collected information from 13 previous studies examining the link between diabetes and hearing loss which were published between 1977 and 2011. Altogether, the data covered 7,377 diabetics and 12,817 people without the condition.  Some scientists, however, caution that this kind of study does not prove that diabetes is directly responsible for the greater rate of hearing loss among those with the disease.

A major study that supports audiologists’ suspicions that diabetes is linked to hearing loss was conducted by Bainbridge, Hoffman, and Cowie (2011). They examined the risk factors of low/mid-frequency and high-frequency hearing impairment among a nationally representative sample of diabetic adults.  Data came from 536 participants, aged 20–69 years, with diagnosed or undiagnosed diabetes who completed audiometric testing during 1999–2004 in the National Health and Nutrition Examination Survey (NHANES).  Bainbridge et al. (2011)  defined “hearing impairment” as a pure-tone average of greater than 25 dBHL for low and mid frequency pure-tone thresholds (500; 1,000; and 2,000 Hz) and high frequencies pure tone thresholds (3,000; 4,000; 6,000; and 8,000 Hz) and then identified independent diabetic risk factors using logistic regression, controlling for age, race/ethnicity, and marital status, income poverty ratio, and healthiness.  Audiogram A depicts those with low high-density lipoproteins (HDL) having better hearing and those with high HDL having worse hearing.  The top audiogram in figure B shows those without coronary heart disease and the worse hearing is among those with coronary heart disease. Figure C suggests the difference in hearing for those with peripheral neuropathy, with the peripheral neuropathy patients having the worse of the two audiograms presented.  Finally Figure D presents the healthiness of the patients, with those in poor health having worse hearing.

1.  Low HDL or high-density lipoprotein cholesterol, coronary heart disease, peripheral neuropathy, and having poor health are potentially preventable correlates of hearing impairment for people with diabetes.

2.  Some degree of hearing impairment affects over two-thirds of diabetic adults, about twofold higher than that of the non-diabetic population.

3.  Data that would have provided evidence that diabetes severity is associated with the likelihood of hearing impairment was inconclusive.

4.  Two-thirds of subjects with diabetes had high-frequency hearing impairment.

5.  Twenty-six percent also had low/mid-frequency hearing impairment.

Bainbridge et al (2011) conclude their investigation with an interesting summary:

Hearing impairment is not widely recognized as a complication of diabetes despite its occurrence in as many as two-thirds of our diabetic sample. Among U.S. adults with diabetes, we identified a greater likelihood of hearing impairment for those who are older, non-Hispanic white, male, or who have lower income. Diabetic subjects with low HDL, a history of coronary heart disease, symptoms of peripheral neuropathy, or those who report poor health also exhibited a greater likelihood of hearing impairment.

So….What Does All this Mean For Audiology

The prevalence of diabetes for all age-groups worldwide was estimated to be 2.8% of the world population in 2000 and 4.4% in 2030. The total number of people with diabetes is projected to rise from 171 million in 2000 to 366 million in 2030. The prevalence of diabetes is higher in men than women, but there are more women with diabetes than men since men do not live as long. The urban population in developing countries is projected to double between 2000 and 2030. The most important demographic change to diabetes prevalence across the world appears to be the increase in the proportion of people >65 years of age.  These findings indicate that the “diabetes epidemic” will continue even if levels of obesity remain constant. Given the increasing prevalence of obesity, however,  it is likely that these figures provide an underestimate of future diabetes prevalence.

While some of the diabetes related hearing impairment can be prevented, these data suggest that all diabetics should have their hearing screened to determine if their disease affects their hearing is compromised by the disorder.  As audiologists we need to encourage physicians worldwide to refer patients with diabetes for hearing evaluations to rule out a related hearing impairment.