Disorders of hearing in diabetes mellitus
The sense of hearing is one of the most important tools of social communication. Impaired function of the hearing organ leads to a significant deterioration of the social and emotional well-being of the affected person. Hearing loss is significantly more prevalent among diabetic patients in comparison with a general population. However, in the minds of many doctors, even those involved in diabetes care, hearing impairment is rarely connected with diabetes. Moreover, the clinical importance of the audiological complications of diabetes is frequently underestimated and undervalued. Abnormal auditory organ function was found in both type 1 and 2 diabetes. These abnormalities can be assessed by a pure-tone audiometry (PTA), which reflects the function of the auditory organ as a whole, by an otoacoustic emissions (OAE) evaluation, which determines the status of the cochlear micromechanics, and by an auditory brainstem response (ABR) audiometry which measures the function of the retrocochlear auditory pathway up to the brainstem level.
Hearing function in diabetes
The studies assessing associations between diabetes and hearing function have a long history . A growing amount of epidemiological data indicates that both type 1 and in type 2 diabetes are associated with auditory organ dysfunction ,. Abnormalities of hearing were demonstrated in all audiological evaluations: PTA, OAE and ABR. This indicates involvement of both the peripheral (mainly cochlea) and the central (auditory pathway) parts of the auditory system, and the clinical picture of diabetes-related hearing organ impairment is usually sensorineural hearing loss (SNHL).
Pure tone audiometry
Pure-tone audiometry is used to determine a hearing threshold, i.e. the softest sound audible to the studied person across a wide range of frequencies. Results are presented as an audiogram, a graph which presents hearing threshold as a function of frequency. This measure reflects the auditory organ function as a whole, with both its peripheral and its central parts (see figure 1).
Figure 1. Hearing threshold in pure-tone audiometry (mean ± SD) in diabetic and control subjects Abnormalities in PTA were found even in children, adolescents and young adults with type 1 diabetes, and with relatively short duration of the disease. In this group the hearing impairment is usually subclinical, with hearing threshold only slightly elevated. However, the differences between patients with diabetes and their healthy comparators are significant, predominantly at high frequencies (≥ 3,000 Hz) (Fig. 1) . Type 2 diabetes is associated with a roughly 2-fold higher prevalence of hearing loss compared to people without diabetes . In this case differences in hearing threshold can be found throughout the whole spectrum of frequencies, however, in this group they are likewise more pronounced at high frequencies.
Figure 2. Mean amplitude of TEOAE at band range 1.2–3.5 kHz (bars) and at particular frequencies of TEOAE spectrum (presented as TE-gram) (mean ± SD) in diabetic and control groupsOtoacoustic emissions are generated by the outer hair cells (OHCs) in the spiral organ of Corti as a kind of “echo” in reaction to an acoustic stimulus of 80-85 dB intensity. These can be evoked by clicks (TEOAEs – transiently evoked otoacoustic emissions) or by a pair of pure tones (DPOAEs – distortion-product otoacoustic emissions). OAEs can be measured and their amplitude reflects the status of the cochlear micromechanics. In patients with diabetes this measurement differentiates between the sensory and neural components of SNHL. Decreased amplitude or lack of OAEs can be found in both type 1 and type 2 diabetes . These observations indicate an increased prevalence of impaired function of the cochlea in patients with diabetes. An example of differences found in TEOAE is presented in Figure 2.
Auditory brainstem response audiometry
Figure 3. Schematic presentation of differences in ABR between patients with type 1 diabetes and non-diabetic group.Auditory brainstem response (ABR) audiometry is a neurological test to assess the function of the retrocochlear part of the acoustic pathway up to the level of the brainstem. An ABR is generated in response to an acoustic click stimulus of 70-90 dB intensity. Results are presented as a waveform (waves I – VII) graph. From a clinical point of view the most important is wave V, which is generated probably in the vicinity of the inferior colliculus. Both in type 1 and in type 2 diabetes, the latency of wave V is usually delayed and the interval between waves I – V is usually longer compared to subjects without diabetes. These findings indicate the presence of central auditory neuropathy in patients with diabetes (Figure 3).
Risk factors and pathogenesis of hearing impairment in diabetes
Apart from known risk factors responsible for hearing loss in the general population such as age, occupational noise exposure, smoking and BMI, several other factors linked to diabetes may influence hearing function in diabetic patients. These include metabolic control of diabetes, disease duration, antidiabetic medications, and other factors . The exact mechanisms involved in the pathogenesis of diabetes-related hearing loss are not yet fully elucidated. However, similar to other diabetic complications, factors like oxidative stress, generation of advanced glycation end-products (AGEs), activation of the polyol pathway, and accelerated development of atherosclerosis of the vessels supplying the auditory organ seem to play an important role . This is supported by histopathological findings in the temporal bones of people with diabetes.
In a few histopathological studies conducted in diabetic populations, abnormalities in the microvasculature as well as atherosclerotic changes in the auditory arteries were found. The capillary walls of the stria vascularis, the basilar membrane and the endolymphatic sac were thickened. Moreover, atrophy of the stria vascularis, and loss of outer hair cells, predominantly in the lower basal cochlear turn (which is responsible for hearing at high frequencies) were present. Also changes in the neural part of the auditory system were observed: atrophy of the spiral ganglion, and demyelination of the cranial nerve VIII .
Duration of diabetes
Diabetes duration appeared to have an impact on hearing performance. However, the progression of hearing deterioration was significantly more pronounced in recently diagnosed patients with diabetes compared to non-diabetic subjects. Among patients with longer-standing disease the difference with healthy controls was less pronounced .
Metabolic control of diabetes
Data regarding the impact of HbA1c level on hearing function in diabetes are conflicting. Surprisingly, the majority of studies do not show that poor metabolic control has a negative impact on hearing. Data regarding the direct impact of acute hyperglycemia and hypoglycemia on hearing function are scarce. Interestingly, hyperglycemia seems associated with an improvement in OAEs testing, while hypoglycemia was associated with prolonged conduction through the retrocochlear part of the auditory pathway.
Diabetic neuropathy, retinopathy, as well as nephropathy are associated with more pronounced hearing impairment. In this case the existing complications can be considered as an overt clinical manifestation of endothelial injury and systemic vascular damage, which, non-surprisingly, can be present also in the vasculature of ears.
Type of antidiabetic medications
In type 1 diabetes the only therapeutic choice is treatment with exogenous insulin. In type 2 diabetes a wide range of oral and injectable agents can be used. The existing evidence shows no direct impact on hearing function regardless of agent used.
There are very few data regarding the impact of high blood pressure on hearing function in diabetes. In one study no association was found. Blood lipids seem to be associated with hearing function in diabetic patients. The existing evidence indicates the potential deleterious role of low HDL-cholesterol and elevated triglycerides on auditory organ function in diabetic patients. Higher BMI, similarly to the non-diabetic population, also appears to be associated with abnormalities in audiological evaluations among people with diabetes . Finally, hearing impairment was also associated with the presence of cardiovascular disease.
The sense of hearing is one of the most important tools of social communication. Hearing impairment is associated with worse quality of life and with depression. There is a growing evidence linking diabetes to impaired function of the auditory organ. Many risk factors are associated with hearing loss in diabetic patients. Some of them are modifiable. Maintaining good metabolic control can prevent vascular injury and complications associated with diabetes. Weight control and maintaining blood lipids in the recommended range can be considered as the tools to preserve satisfactory hearing function in diabetes. The higher prevalence of hearing impairment in patients with diabetes indicates the need for early screening for auditory organ involvement. Regular hearing evaluation, at least pure-tone audiometry, could be considered as a part of routine diabetes care, similarly to eye fundus examination, microalbuminuria testing, or touch sensation assessment.
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