The effect of hyperinsulinemia on cochlear functions

Recently, some early signs of diabetic complications has been found in the prediabetes, which diabetes has not clinically settled. There are limited number of studies on the effects of hyperinsulinemia in the inner ear. In the animal studies, it has been shown that hyperinsulinemia has adverse effects on cochlea. The aim of this study is to investigate the effects of hyperinsulinemia and dysglycemic states on hearing. The study was conducted at Baskent University Medical Faculty Departments of Internal Medicine and Otorhinolaryngology. Is is designed as prospective controlled clinical study. Local ethics committee approved the study, and informed consents were obtained from all of the participants. Fasting plasma glucose, fasting plasma insulin, HbA1c, creatinine, LDL cholesterol, HDL cholesterol, triglyceride, TSH, free T4 levels of patients referring to outpatient clinics of Department of Endocrinology, all of whom were between 18-64 years age, were measured. Each patient was given 75 gr oral glucose solution, and blood samples were received at 30., 60., and 120. minutes for plasma glucose and insulin measurements. Those who did not meet exclusion criteria, whose otorhinolaryngological examination was normal, and who had insulin resistance (IR) were included. Thereafter, electrocochleography(EcochG), pure tone audiometry at 125, 250, 500, 1000, 2000, 4000, 6000, 8000 Hz and transient evoked otoacustic emission (TEOAE) measurements at 1.00; 1.42; 2.00; 2.83; 4.00 kHz were performed. 76 patients had HOMA-IR values ≥2.5 and constituted the active case group. 88 cases had HOMA-IR <2.5, and became the control group. The case and control groups did not differ according to age and sex distribution. As for pure noise audiometry, the comparison of the case and control groups in both ears revealed that, the threshold values of the IR group at 500, 1000, 2000, 4000 Hz were significantly higher. There was no difference between the groups in EcochG. In TEOAE, the signal to noise ratio (SNR) was significantly lower in IR group compared to the control group at only 2.83 kHz (p<0.05). HbA1c and audiometry comparisons have shown that auditory thresholds had significantly increased and speech discrimination scores had decreased at 4 and 8 kHz in IR group (p<0.05). TEOAE and EcochG tests showed no difference in this respect. As for dysglycemic states, auditory thresholds had significantly increased in both ears at 4, 6, 8 kHz, speech discrimination scores had decreased, and SNR had decreased at 1.00 and 1.42 kHz in TEOAE. These findings suggest that auditory functions are affected in chronic hyperinsulinemia. It has also been shown that dysglycemic states (IFG/IGT) are associated with auditory dysfunction before Type 2 Diabetes Mellitus is diagnosed. It has been determined that pure noise audiometry plays an important role in the evaluation of metabolic and neural effects in prediabetes. Key words: Hyperinsulinemia, prediabetes, insulin resistance, hearing, cochlea