The most frequent causes of vestibular and auditory abnormalities are attributed to dysfunctions in the metabolism of carbohydrates, thyroid affections, supra adrenal, and other different metabolic disorders. Among glucose metabolism disorders, diabetes mellitus is the affection most commonly related with auditory disorders1.

Diabetes mellitus is a genetically determined metabolic disorder associated with absolute or relative impairment of insulin and in complete clinical manifestation is characterized by metabolic affections, vascular and neuropathic complications. The main objective of treating patients with diabetes is the prevention of chronic complications because the disease is not curable but only controllable. In Brazil, the incidence of chronic complications of diabetes is quite high. It is estimated that there are five million subjects with diabetes and half of them are not aware of the diagnosis. A large number of subjects, especially children and adolescents, have diagnosis of diabetes made in face of complications, especially infections2.

One of the morphological aspects more constant in diabetes mellitus is diffuse thickness of basal membrane, which may also happen with vascular endothelium, and it is named diabetic microangiopathy. It is more evident in skin capillaries, skeletal muscles, retina, renal glomeruli, and renal medulla. Its pathogenesis is still obscure, but it is clearly associated with hyperglycemia. Other morphological affections refer to impairment of both motor and sensorial nerves of lower limbs, characterized as Schwann cell lesions, degeneration of myelin and axon damage. The cause of neuropathy is still very controversial, and it may be related with diffuse microangiopathy that would affect nourishment of peripheral nerves3.

Neuropathy and angiopathy are common affections in diabetes mellitus. Angiopathy has been observed in small arteries and skin capillaries, muscle, kidney, retina and peripheral nerves. The factors that may cause neuropathy are metabolic disorders (glucose metabolism, lipid metabolism defects and vitamins). Some researchers referred to the fact that vascular affections in interfascicular or intrafascicular branches of vasonbervorum contribute to neuropathy. Atherosclerosis, however, very common in diabetes mellitus, can also contribute to neuropathy, owing to interference in rate of nutrient transfer4.

Angiopathy may occur both in direct way, interfering with supply to the cochlea by reducing transport through the thickened walls of capillaries, and indirectly by the reduction of flow in vascular pathways, or still, because of secondary degeneration of 8th cranial nerve5.

Concerning diabetes mellitus, there are different opinions about the pathological affections caused in the auditory system.

According to Nageris et al., the association between hearing loss and diabetes mellitus has been debated since it was first reported by Jordao in 18576.

In the literature, there are many different types of hearing loss found in diabetic patients. One of them is progressive, gradual bilateral sensorineural loss, affecting especially high frequencies and the elderly. It would be similar to presbyacusis, but with more severe losses than those expected by aging5–8. Conversely, there are authors that report the possibility of having early sensorineural loss7 and others that reported hearing loss in low and medium frequencies9,10. Some studies described diabetes as the possible cause of unilateral sudden loss4,10,11,, but other authors did not find the same association12.

As to incidence of hearing loss in patients with diabetes mellitus, we observed that there is no consensus in the literature either, ranging from zero to 93%6.

In view of the reported data, we decided to perform a literature review to check whether there is really a hearing loss inherit to diabetes mellitus and to which factors it is related.

As presented in the literature review, there is not enough evidence to solidly define diabetes mellitus as a cause of hearing loss. There is no consensus on any aspect of this topic, either audiologically or histopathologically. Clinical experience, in many studies, showed direct correlation between hearing loss and diabetes, as reported in Table 1. However, there are other studies with more subjects and better designed that did not identify this association (Table 2).

Affection of blood vessels that irrigate the inner ear and affections that occur in stria vascularis of diabetic patients are unquestionable facts confirmed by different authors that believe in the correlation between hearing loss and diabetes, and these affections are strong evidences that diabetes may cause hearing loss10,13–15. However, there are others that do not believe in this association24.

Some studies indicate that decrease in hearing acuity, similarly to the one presented in presbyacusis, is greater than what is expected for the age in elderly and diabetic patients8,10,12,22,24. According to them, it is more likely that we can associated factors that would lead to hearing loss, such as consequence of vascular abnormalities caused by diabetes mellitus.

Up to 1960, there were 3 theories about the pathogenesis of hearing loss caused by diabetes mellitus: neuropathy, angiopathy and association of both. Currently, there are significant reasons to believe that the angiopathy found in diabetic patients contributes to greater evidences of hearing loss. In favor of this hypothesis we can rely on histopathological findings of microvascular lesions of the inner ear, such as thickness of basal membrane of stria vascularis capillaries, as well as the fact that diffuse vascular affections caused by diabetes mellitus are well known4,13–15,25,26.

It is known that the main difficulty to conduct inner ear study is related with difficulty to access histological assessment, especially in human beings. Moreover, the steps to preparation of material can also have artifacts, provoking misinterpretations. The localization of inner ear circulation makes it difficult to study homeostasis, as well as the consequences of metabolic disorders. Other factors that can induce misinterpretation of statistical data are use of different populations for comparison purposes, lack of good control group, need to conduct longitudinal studies (owing to the fact that in transversal studies many times it is not possible to know if diabetes mellitus precedes hearing loss or vice-versa). Other important aspect is size of studied group and the huge universe of variables such as other diseases, use of drugs, noise exposure, genetic affections. It is possible that these are the reasons for discrepancy in results detected among similar studies conducted by different authors.

Finally, we observed growing interest in hearing loss caused by genetic affections, many of them related to mitochondrial DNA mutations. They correspond to 0.5% to 1% of all genetically-based hearing losses, which may be associated with syndromic presentations. One of the syndrome presentations is diabetes mellitus associated with maternal-inherited hearing loss, amounting to 1.5% of all cases of diabetes in Japan and The Netherlands19.

The investigation of genetic causes has been the answer to many questions and disagreeing results.

In view of current knowledge, there is evidence that diabetes mellitus may cause hearing loss, but we can not state that there is clear cause-effect correlation. It is known that a series of variants may favor the association between both diseases, but it is necessary to conduct more careful studies to define the true role of these factors. As seen before, diabetes mellitus and hearing loss can be dependent components, or even components of a genetic syndrome and not dependent one on the other.

According to the analyzed studies, we can conclude that many questions will be answered by multicenter longitudinal studies with large populations and strict inclusion and exclusion criteria.