A Summary of the Evidence
By Eirini Mihanatzidou, MA(Hons), M.Aud, Aud(C), Reg. CASLPO, and Rhonda Kerlew, RN, BScN, MBA
About the Authors
Eirini Mihanatzidou (far left) is the president and chief audiologist of Brilliant Hearing, a patient-focused audiology practice based in Richmond Hill, Ontario.
Rhonda Kerlew is director of business development with Hearing Solutions, Toronto, ON.
Diabetes Mellitus (DM) is one of the fastest growing chronic diseases of our era. Recent studies suggest that sensorineural hearing loss is more prevalent in diabetic patients than in people without the condition. The aim of this article is to review the existing literature on the relationship between hearing loss and diabetes. Data was obtained by literature search using the MEDLINE, EMBASE and PubMed databases.
Diabetes mellitus is a group of metabolic disorders characterized by an elevated blood sugar and abnormalities in insulin secretion and action.1 This group of disorders disrupts the metabolism of protein fats and carbohydrates rendering the body unable to utilize these nutrients. The resultant hyperglycemia can lead to dysfunction of several organs. Damage is noted in the nervous system, eyes, kidneys, heart and blood vessels.2 In the non-diabetic individual blood glucose levels are controlled by insulin, a hormone produced by the beta-cells of the pancreas.
When glucose levels rise in the blood stream (for example after a meal) insulin is released to normalize glucose levels. In the diabetic patient insulin production is either severely deficient in the pancreas or the pancreas is producing insulin but the body is unable to utilize it.3 There are two major types of diabetes. DM type I results from autoimmune destruction of the beta-cells of the pancreas. Ten percent of all diabetics in the United States are typically diagnosed in childhood or adolescence. Patients with DM I are insulin dependent and require close monitoring of blood sugar levels to ensure blood glucose is controlled throughout the day. This type of diabetes was formerly known as insulin-dependent diabetes mellitus (IDDM).4
DM type II is characterized by resistance such as a lack of response to insulin by the cells of the body (mainly fat and muscle cells), along with increased insulin production by the liver to overcome this resistance. It accounts for 90% of all cases of diabetes. It is typically diagnosed in adulthood and is closely associated with obesity. DM II is managed by diet, weight management, oral medications and/or insulin.5 Type II diabetes was formerly known as non-insulindependent diabetes mellitus (NIDDM), but this term has been abandoned since most of the patients with DM II will require insulin treatment at some point in the course of their condition.6
The prevalence of diabetes among adults within the 20–79 year range was estimated to be 6.4% in 2010, affecting 285 million people worldwide. The prevalence is expected to rise to 7.7% and 439 million adults by 2030.7
Both types of diabetes are associated with a number of chronic complications and co-morbidities. The most prevalent and well known complications include retinopathy, nephropathy, and peripheral neuropathy.8
Each of these complications carries its own set of losses and dysfunction such as blindness, kidney failure, and peripheral vascular disease requiring amputation.9 Another, less well known complication of diabetes is hearing impairment. Accumulating evidence suggests that there is a higher prevalence of hearing loss in the diabetic versus the nondiabetic population.10–12 The hearing loss is bilateral, sensorineural, symmetrical, and tends to affect the high frequencies more than the low/mid ones.13,14
More specifically, Dalton et al. found that 59% of diabetic subjects had a hearing loss as opposed to 44% of non-diabetic subjects.15 The association between diabetes and hearing loss was significant when results were analyzed excluding subjects with non age-related hearing loss. In a study conducted by Bainbridge et al. 68% of patients with diabetes were found to have some high-frequency hearing loss compared to 31% of subjects without diabetes.16
The prevalence of low/mid frequency hearing loss was 28% in the diabetic patients as opposed to 9% in the non-diabetic group. The association between diabetes and hearing loss remained even after controlling for age, race, sex, poverty level, history of noise exposure, ototoxic medication use, and smoking status. The study by Mitchel et al. is in line with the above findings.17 More specifically, hearing loss was found in 50% of diabetic patients ompared to 38% of the non-diabetic subjects after adjusting for multiple risk factors. Furthermore, a study by Uchida et al. found that diabetes may affect the highfrequencies more strongly in the age bracket of 40–64 years of age than at age 65 and above.18
Finally, a study conducted in 2009 by Cheng et al. revealed that the prevalence of hearing loss amongst diabetics has remained high over the decades when compared to non-diabetic persons.19 More specifically, the authors compared the two cross-sectional National Health and Nutrition Examination Surveys of 1971-1973 and 1999-2004 (NHANES I and NHANES II). They discovered that from 1971 to 2004 in adults without diabetes aged 25–69, the unadjusted prevalence of hearing loss decreased by 9% whereas in the diabetic population there was no significant change.
With regards to the risk factors for hearing impairment in the diabetic population, evidence is conflicting. A number of studies have shown that hearing loss is correlated with glycaemic control (i.e. with the blood glucose levels) and duration of disease.20–22 More specifically, Okhovat et al. compared the hearing thresholds of 100 patients with DM I aged 5–18 years.23 They found that 21% of them had a hearing impairment and that the hearing thresholds were positively correlated with poor metabolic control (defined as an annual HbA1C of more than 7.5%).
Furthermore, thresholds were significantly higher in patients with a history of diabetes of more than five years. Additionally, two studies by Lerman-Garber et al. and Konrad-Martin et al. reported a positive association between poor glycaemic control and impaired auditory brainstem responses in DM II patients.24,25 Pudar et al. examined the effects of peripheral neuropathy and retinopathy on hearing impairment in 50 patients with DM I and found that the average sensorineural hearing loss was increased by 73% in the presence of neuropathy, and by 50% in the presence of retinopathy.26 Bainbridge et al. Found a strong correlation between neuropathy, duration of disease and high-frequency hearing impairment.
Mihanatzidou and Kerlew in 536 diabetic patients, whereas Dabrowski et al. found higher mid frequency thresholds in 31 patients with DM I and retinopathy.27,28 However, both of these studies, as well as a third study by Asma et al., failed to find a correlation between glucose levels and hearing loss.29 Recent studies suggest that diabetes may also increase the susceptibility to noise-induced hearing loss and sudden idiopathic sensorineural hearing loss (SISNHL). More specifically, Wu et al. and more recently Fujita et al. reported on an animal study in which diabetic rats had a significantly im hearing thresholds at 4 kHz in 2,612 automobile factory workers were significantly worse in subjects with impaired fasting glucose and diabetes than in non-diabetic subjects.32
Aimoni et al. Studied the prevalence of diabetes in patients with sudden idiopathic sensorineural hearing loss and found that it was almost doubled when compared with the normal hearing subject group.33 It has been suggested that diabetes ca n mediate SISNHL through cerebral microangiopathy and changes in blood viscosity.34,35 The exact mechanism involved in the pathogenesis of hearing loss in diabetic patients remains unknown.
A number of histopathological studies conducted in humans found thickening of the capillary walls of the stria vacsularis, the basilar membrane and the endolymphatic sac, atherosclerotic narrowing of the internal auditory artery, atrophy of the stria vascularis, loss of outer hair cells especially in the lower basal cochlear turn, spiral ganglion neural atrophy, and VIII cranial nerve demyelination.36–39 In all, hearing impairment is one of the less well known complications of diabetes.
More research is needed to delineate associated risk factors and mediators in its pathogenesis. Untreated hearing loss can negatively impact the social and emotional wellbeing of individuals.40–43 The proportion of hearing impairment in the diabetic population in comparison with the non-diabetic population is high. In light of its high prevalence and its detrimental psychosocial effects, health care providers, primary care physicians and endocrinologists should consider referring all diabetic patients for a hearing test. Audiometry should be a routine evaluation in the annual test battery of all diabetic patients.
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