Cochlear synaptopathy (CS), defined as the loss of ribbon synapses between inner hair cells and auditory nerve fibers due to aging or loud sound exposure, is considered an early lesion in hearing loss development. Therefore, its detection may serve as a key to the early diagnosis. CS preferentially affects low-spontaneous rate (low-SR) fibers, while high-SR fibers are relatively preserved. We proposed a method using the auditory brainstem responses (ABRs) to paired-click stimuli, for a noninvasive assessment of this selective neural damage in humans. We focused on the fact that low-SR fibers, compared to high-SR fibers, are less affected by adaptation and exhibit delayed firing. It was expected that the activity of low-SR fibers is reflected to root mean square values for the response within a range delayed from the wave I peak (RMSpost-w1) for the second click. In Experiment 1, the RMSpost-w1 values significantly declined with age. In Experiment 2, young adults frequently exposed to loud sounds through music practice had significantly lower values than age- and hearing-matched controls. These findings suggest that ABRs using the paired-click paradigm reflect age- and noise-related CS, supporting their potential utility as a noninvasive diagnostic marker.
Fujihira et al. (2025) studied this question.