Tinnitus in Normal-Hearing Individuals: Is Outer Hair Cell Dysfunction the Mechanism?

Background/Objectives: Cochlear "injury" is thought to be a significant cause of tinnitus in patients with hearing loss. Interestingly, individuals with normal hearing may also experience tinnitus. This study evaluates otoacoustic distortion product emissions (DPOAEs) in individuals with normal hearing who experience tinnitus perception. Methods: In this prospective study, the tinnitus group (TG) consisted of 34 subjects with tinnitus (four unilaterally) and normal hearing (threshold ≤ 25 dBHL at 0.25-8 kHz). The control group (CG) comprised 10 healthy volunteers (20 ears) without tinnitus and normal hearing. Medical history was recorded, and all participants underwent a complete otolaryngological examination, pure tone audiometry, and DPOAE recording (DP-gram, L1 = 55 dB, L2 = 65 dB, for F2: 619-10,000 Hz). Moreover, participants in the TG completed a detailed tinnitus history (with self-rated loudness scoring) and the Tinnitus Handicap Inventory (Greek-version THI-G) and underwent tinnitus analysis. Results: The recorded mean DPOAE values during the DP-gram of the CG were significantly larger in amplitude at low (t-test, Bonferroni-corrected p < 0.09) and high frequencies (t-test, Bonferroni-corrected p < 0.02) compared with the TG. Tinnitus assessment showed tinnitus pitch matching at the frequency area in the DP-gram, where the acceptance recording criteria were not met. There were no statistically significant differences in tinnitus onset, self-rated loudness scores of >70, and severe disability (THI-G > 58) for TG subjects in whom DPOAEs were not recorded at frequencies of ≤1000 Hz. Participants with abnormal DPOAEs at around 4000 Hz had tinnitus of sudden onset and severe disability (THI-G > 58). Finally, those with pathological recordings of DPOAEs at ≥6000 Hz had gradual onset tinnitus (Pearson Chi-square test, p < 0.05). Conclusions: DPOAEs in normal hearing individuals with tinnitus show lower amplitudes in low and high frequencies compared with normal hearing individuals without tinnitus. The tinnitus matched-frequency coincided with the frequency area where DPOAEs were abnormal.