Hidden auditory dysfunction in tinnitus with normal audiograms: selective and chronicity-dependent ABR changes in middle-aged adults.

Tinnitus can occur in adults with normal audiograms, suggesting subtle auditory dysfunction beyond conventional threshold measures. This study investigated whether middle-aged adults with tinnitus and clinically normal hearing show evidence of peripheral neural dysfunction and duration-related brainstem changes. Seventy-four tinnitus patients (101 ears) and 15 healthy controls (30 ears) with normal audiograms and preserved distortion-product otoacoustic emissions were enrolled; the tinnitus group was subdivided into acute (<6 months) and chronic (≥6 months) tinnitus. Click-evoked auditory brainstem responses were recorded from 45 to 90 dB nHL, and absolute wave latencies and amplitudes, interpeak latencies, intensity-response slopes, the wave V/I amplitude ratio, and wave V ABR growth ratio were analyzed using linear mixed-effects models. Pure-tone thresholds, wave V thresholds, and DPOAE measures did not differ across groups. Chronic tinnitus showed prolonged wave I latency at 55, 75, and 80 dB nHL, while both acute and chronic tinnitus showed reduced wave I amplitude at 45 dB nHL. Chronic tinnitus also showed a steeper wave V latency-intensity slope and a higher V/I ratio at 80 dB nHL, whereas interpeak latencies and wave V ABR growth ratio showed no robust group differences. These findings support selective ABR abnormalities in normal-audiogram tinnitus, centered on wave I changes, with additional suprathreshold brainstem alterations in chronic tinnitus.