Extended High-frequency Audiometry and Distortion Product Otoacoustic Emissions in Normal-hearing Patients with Unilateral Tinnitus

Some tinnitus patients have normal hearing on the conventional audiogram. It has been suggested that the presence of a limited area of damaged outer hair cells (OHCs) with intact inner hair cells, which is not detected on the audiogram, results in unbalanced neural activity between Type I and Type II fibers leading to tinnitus. Distortion product otoacoustic emissions (DPOAEs) provided ambiguous data of OHC function in normal-hearing tinnitus patients when compared to non-tinnitus controls. It is known that hearing loss in the extended high frequency (EHF) region may decrease DPOAEs evoked at lower frequencies. Results of EHF audiometry in tinnitus patients are limited. The aim of the study was to evaluate DPOAEs and EHF thresholds in normal-hearing patients reporting unilateral tinnitus in left ear. Thus, each subject acted as their own control. Data were obtained for 25 subjects with bilateral hearing thresholds 3dB. Median audiometric data showed that thresholds in the left ears were significantly higher than those in the right ears at all four EHFs. Mean DPOAE levels of the left ears were lower than those of the right ears in the frequency range above 1 kHz. Additionally, a paired-comparison test of DPOAE levels of each patient’s right and left ear revealed significant differences at 6, and 8 kHz. The results indicate that: 1. OHC impairment in the most basal region reduces contribution to more apically generated DPOAEs; 2. OHC impairment in a limited area, which may be revealed by DPOAEs but not by conventional audiometry, can contribute to tinnitus generation; and 3. patients with unilateral tinnitus and normal hearing on the conventional audiogram are likely to demonstrate hearing loss in the EHF region