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Deafness and Diabetes from an Epidemiology Standpoint

Diabetes mellitus is condition where the metabolism of glucose is altered. Consequentially, levels of blood glucose remain persistently high. The classification of Diabetes is generally broken down into two types: Type 1 diabetes; the actual production of insulin is lowered, whereas type 2 diabetes is characterized by a cellular de-sensitivity to insulin and thus resulting in the same elevation of blood glucose. While millions of Americans have been diagnosed with Diabetes, it is estimated there are millions who suffer from the condition but have not had an official diagnosis.

A variety of complications may be experienced by patients that have been diagnosed with Diabetes. Most adults are at risk of cardiovascular disease, high blood pressure and elevated cholesterol levels, resulting in an increased likelihood (4 times greater!) of experiencing cardiac stroke compared to healthy folks.

Blindness is another prominent complication among Diabetics due to acute damage of the retina (retinopathy). In fact, Diabetic retinopathy is the number one cause of said blindness. For Americans who suffer from Diabetes Mellitus, renal failure is also all too common. Diabetic Neuropathy (a form of peripheral nerve disease) may ultimately lead to the amputation of lower extremities. Because Diabetic Neuropathy attacks peripheral nerves systemically, various organ systems such as cardiovascular, gastrointestinal and urological/sexual networks are adversely affected. As you can see, there is a pattern here. Diabetes is a systemic condition that has the ability to cause multi organ pathologies. As such, we can reasonably look into Diabetes affecting the auditory system.

Earlier attempts at establishing a connection between diabetes and deafness weren’t necessarily persuasive.

Constructing epidemiological proof between diabetes and hearing impairment is contingent on a vigorous definition of the disease; that way, cross referencing conclusions among other studies can provide a clearer picture of the results and thus, leave less room for interpretation. Nevertheless, deafness does not have a clear cut definition to this day.

Detailed examination was performed on the associations across varying social demographic attributes. The higher pervasiveness of deafness with persons suffering from diabetes developed in several age groups and among different educational backgrounds and not limited to men or women, nor was it limited to ethnicity.

Moreover, occurrence of deafness did not limit itself to people with a predisposition to hearing impairment, i.e. smokers, those exposed to loud noises that we might find in certain occupations and those taking pharmaceuticals high in ototoxicity. Additionally, researchers did not find a higher prevalence of hearing impairment between those diagnosed with Diabetes and those without a diabetic diagnoses (but had consistently high levels of blood glucose after fasting).

However, upon closer inspection of the data, a more robust association between diabetes and hearing impairment was found in younger people than in older age folks. This information helps explaining why preceding studies that enlisted older persons were not able to establish a connection or reported just mild association.

With a myriad of overlapping factors that a person accrues during their lifespan that contribute to age-associated deafness, the disparity in pervasiveness between fully grown adults with Diabetes Mellitus and those without suggests to decrease the older a person becomes (see Figure 2a). This idea is backed when the brink of pure-tone audiometry are analyzed by different group ageing. For people aged 30–39 years and with Diabetes, average thresholds at a frequency between 3000–8000 Hz are 10–16 dB HL greater. With each consecutive decadal age group, there is a rise in thresholds for the two groups. However, the discrepancy among thresholds of the diabetes group versus the non-diabetes group becomes narrower. Analysis of persons aged between 50–59 years old and 60–69 years old, the average thresholds at 3000–8000 Hz just 4–6 dB HL greater for the diabetic group. In other words, Diabetes can cause premature ageing of the ear.

For now, the evidence for diabetes-associated deafness is hypothetical as the evidence has not pointed to a conclusive answer. Complications that arise as a result of Diabetes are chiefly rooted in vascular causes; these include nephropathy, peripheral vascular diseases and retinopathy. Diabetic neuropathy affects tangential sensitivity and other autonomic behaviors. The pathology of change that follows diabetes can cause corresponding damage to the vasculature and/or the neural network of the inner ear. Histological proof of vascular or neural involvement collected from autopsies of diabetic persons include internal auditory artery sclerosis, increased vessel wall thickness of the stria vascularis and of the basilar membrane, myelin loss of the auditory nerve, and atrophy of the spiral ganglion (Makishima & Tanaka, 1971). Loss of outer ear hair cells was also a common observation in patients with diabetes (Fukushima et al., 2006).

While research has been pointing towards a connection between Diabetes and hearing loss, further epidemiological data needs to be collected for clearer epidemiological evidence. Promising researches that are devised to examine if hearing loss commences quicker with the presence of Diabetes than without would secure the advancement of the thesis that hearing loss should be anticipated earlier than not with the occurrence of Diabetes.

Kathy Bainbridge
National Institute on Deafness and Other Communication Disorders.

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