Altered ABR Waveform Patterns in Tinnitus Patients With Normal Full-Frequency Hearing May Be Induced by Abnormal Middle Ear Muscle Activity Via the Trigeminal Nerve System

Importance Evidence regarding the mechanism of tinnitus in patients with normal full-frequency hearing remains limited. This study investigated the auditory brainstem response (ABR) waveform patterns in these patients to address this unmet need, which is of considerable assistance in the clinical management of tinnitus with normal hearing. Objective This study aims to investigate the potential underlying mechanisms of tinnitus with normal full-frequency (125 Hz-16 kHz) hearing threshold by analyzing waveform alterations in ABR among tinnitus patients. Design Cross-sectional study. Setting Otolaryngology outpatient of a tertiary referral hospital. Participants Patients aged 20 to 40 years with unilateral subjective tinnitus and normal hearing (pure-tone thresholds ≤25 dB HL across 0.125-16 kHz) were recruited. Age-matched healthy volunteers with normal full-frequency hearing (0.125-16 kHz) and without tinnitus served as controls. Exposures Participants were categorized according to the presence of unilateral subjective tinnitus. Main Outcome Measures The primary outcomes included pure-tone audiometry testing (0.125-16 kHz) and ABR testing using 80 dB nHL click stimuli with alternating polarity (19.9 Hz, 1024 scans). Results The study included 136 adult subjects with normal hearing at all frequencies, including 60 unilateral tinnitus patients and 76 normal volunteers. Results showed no significant differences in ABR waveform characteristics between affected and unaffected ears in unilateral tinnitus patients (( P > .05). However, compared with normal volunteers, unilateral tinnitus patients exhibited significantly prolonged latencies of waves I, III, and V on the tinnitus side (all P < .05), with median differences (95% CI) of 0.08 (0.04-0.11), 0.07 (0.00-0.12), and 0.15 (0.09-0.20), respectively. In addition, the interpeak latency between waves III and V was also prolonged, with a median difference of 0.08 (95% CI: 0.03-0.13). Conclusion Tinnitus patients with normal full-frequency hearing exhibit differences in the ABR waves, which may be associated with abnormal middle ear muscle activity. Emotional responses may trigger abnormal middle ear muscle activity via the serotonin system, which in turn affects auditory signal transmission through the trigeminal system, leading to tinnitus and abnormal ABR wave patterns. Relevance Future research will focus on the middle ear muscles as a potential breakthrough point, exploring the relationship between abnormal electrical activity in these muscles and tinnitus, as well as the electrical activity of the trigeminal nerve nuclei. Level of Evidence 3.