Are Electrocochleographic Changes an Early Sign of Cochlear Synaptopathy? A Prospective Study in Tinnitus Patients with Normal Hearing

The mechanism of tinnitus accompanied by a normal audiogram remains elusive. This study aimed to investigate evidence of primary neural degeneration, also known as cochlear synaptopathy, in tinnitus patients with normal hearing thresholds. We analyzed the differences in electrocochleography (ECochG) measurements between normal-hearing subjects with and without tinnitus. Forty-five subjects were enrolled in this study: 21 were in the tinnitus group, defined by chronic tinnitus of over two months’ duration with normal audiometric thresholds, and 24 were in the control group, defined by a lack of tinnitus complaints. Electrocochleograms were evoked by 1, 4, 6, and 8 kHz alternating-polarity tone bursts at sound pressure levels (SPLs) of 90–110 dB. The tinnitus group had smaller action potential (AP) amplitudes than the control group for 1, 4, 6, and 8 kHz tone bursts and showed significant amplitude reduction at 1 kHz 110 dB SPL (p < 0.01), 1 kHz 90 dB SPL (p < 0.05), and 4 kHz 110 dB SPL (p < 0.05). There were no significant differences in the summating potential/action potential (SP/AP) amplitude ratios across the four tested frequencies. A trend of reduced AP amplitudes was found in the tinnitus group, supporting the hypothesis that tinnitus might be associated with primary neural degeneration.