| Summary: | 碩士 === 國立臺北護理健康大學 === 語言治療與聽力研究所 === 104 === Introduction: The physiological mechanisms of tinnitus and spontaneous cochlear mechanical activity may be related, but the underlying cause is complicated and still unknown. Previous studies of outer hair cells’ known mechanical activities related to otoacoustic emissions have indicated that the pathological activity of these cells is linked to tinnitus. This study focused on studying the contralateral suppression of distortion product optoacoustic emissions (DPOAEs) in tinnitus patients with normal pure-tone thresholds but elevated thresholds of extended high-frequency audiograms.
Materials and Methods: A case-controlled study was carried out with 56 participants from the outpatient clinic of the ENT Department of the Tri-Service General Hospital, Taipei. None of the participants had any history of ear disease or vertigo episodes. All subjects initially underwent pure-tone audiometry (tested frequency range: 2508,000 Hz) and extended high-frequency audiometry (9,00014,000 Hz); they were then divided into a normal control group and a patient group. Normal controls had no complaint of tinnitus and normal hearing thresholds on the extended high-frequency audiogram (HFA; 25 dB HL at 911.2 kHz and 40 dB at 12.514 kHz), while patients reported a history of tinnitus for more than 6 months. Patients with tinnitus were further subdivided into two groups according to the hearing threshold of extended HFA: Group 1 (G1) presented normal hearing thresholds of extended HFA, and Group 2 (G2) presented abnormal elevated hearing thresholds of extended HFA. The DPOAEs were evaluated by means of the Bio-logic Scout OAE system in an acoustically insulated room using two pure-tone signals labeled f1 and f2, with an f2/f1 ratio equal to 1.22. The L1 stimulus intensity level was 65 dB SPL, while the L2 level was 55 dB SPL. The tested frequency range of f2 was 750–8,013 Hz, and three frequencies were sampled per octave. For each ear, the baseline distortion product (DP) amplitudes were measured first, and second, the DP amplitudes in response to 60 dB SPL broadband noise presented to the contralateral ear were determined. Data analyses were performed using the DP-gram, geometric mean of frequency f1 and f2, and DP amplitudes with and without contralateral suppression. Estimates of the contralateral DPOAE suppression effect were calculated by subtracting the signal/noise ratio acquired without the contralateral noise from the value of the noise/signal ratio with contralateral noise for each specific frequency. Positive values indicated DPOAE suppression, and negative values or zero indicated nonsuppression.
Results: There was no significant variation in the thresholds between the right and left ears by group. In quiet conditions, the DP amplitudes in the control group were significantly higher than those in the tinnitus groups (G1 and G2), especially at frequencies of 7,250 Hz and 1,027 Hz in the left ear and 4,756 Hz, 4,196 Hz, 3,418 Hz, 2,378 Hz, 2,122 Hz, and 1,818 Hz in the right ear. Regarding the contralateral suppression effect, the DP amplitude in the control group was still significantly higher than the amplitudes in the tinnitus groups were, especially at frequencies of 7,250 Hz, 5,479 Hz, and 1,608 Hz in the left ear and 1,889 Hz in the right ear. There was a significant decrease in the suppression of DP amplitudes at high frequency in the tinnitus groups compared to that in controls.
Conclusions: Tinnitus patients tend to present abnormal extended high-frequency audiometry, suggesting that a defect in the outer hair cells may be involved in the mechanism of tinnitus. In contrast, the decrease in the DP amplitude of the contralateral suppression effect and the diminished presence of the suppression effect in the tinnitus groups may suggest that there is a correlation between diminished effectiveness of the medial olivocochlear bundle and the presence of tinnitus.
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