Evaluation of afferent and efferent auditory systems in tinnitus patients with normal hearing and mild hearing loss

Abstract Objective Tinnitus is the perception of sound without an external source, and its pathophysiology—particularly in individuals with normal hearing thresholds—remains unclear. This study evaluates the interaction between tinnitus and outer/inner hair cells, the efferent auditory system, and potential dead regions based on Jastreboff’s 'mismatched damage' theory. Methods The study included 32 healthy individuals aged between 20 and 40 years, matched with 32 individuals with normal and mild hearing loss and tinnitus complaints. Participants were subjected to acoustic impedance, acoustic reflex, pure tone audiometry, high-frequency audiometry, speech reception and discrimination tests, Threshold Equalizing Noise test, Transient Evoked Otoacoustic Emissions, Distortion Product Otoacoustic Emissions, Medial Olivocochlear Reflex tests and tinnitus frequency and intensity matching tests. Results There were frequency-dependent and lateralized differences in pure tone audiometric thresholds between the tinnitus and control groups, with more widespread threshold elevation in the tinnitus-right ear. DPOAE amplitudes were significantly reduced across all tested frequencies in the tinnitus group, whereas TEOAE amplitudes were significantly reduced in the mid-to-high-frequency range (1400–4000 Hz). After correction for multiple comparisons, no significant differences were observed between groups in the TEN test. MOCR measurements revealed significantly reduced suppression in the tinnitus group at 1400, 2000, and 2800 Hz. Conclusion The findings indicate that tinnitus in individuals with normal hearing and mild hearing loss is associated with subtle outer hair cell–related dysfunction and frequency-specific alterations in efferent auditory modulation, in the absence of extensive inner hair cell or neural damage. This pattern is consistent with the concept of afferent–efferent mismatch described in the “mismatched damage” theory. However, given the cross-sectional design of the study, these associations should not be interpreted as causal.