![]() ![]() This has led to confusion about the feasibility of using the ABR as an indicator of synaptopathy. Although a large number of human studies have been conducted to investigate the relationships between ABR measurements and risk factors or predicted perceptual consequences of synaptopathy, differences in the study design, methodology, and statistical approach have resulted in mixed findings across studies. Although several factors, such as sex, skull thickness, and tissue conductivity, can impact ABR wave I amplitude ( Trune et al., 1988 Irimia et al., 2013), much of the variability in wave I amplitude may result from differences in cochlear synaptic integrity. Note that Arabic numerals (i.e., 1, 2, 3, 4, 5) are typically used when referring to the ABR waves in animals, whereas Roman numerals (i.e., I, II, III, IV, V) are used when referring to the ABR waves in humans. The finding that auditory brainstem response (ABR) wave 1 amplitude is correlated with cochlear synapse numbers in animal models (e.g., Kujawa and Liberman, 2009 Sergeyenko et al., 2013) has led to renewed interest in this classic audiological test, particularly in ABR wave I amplitude, which was previously overlooked due to the large degree of variability associated with the measurement. Synaptopathy is of particular interest in the field because it has been suggested as a possible explanation for the paradoxical situation of auditory complaints that accompany a normal audiogram such as tinnitus, hyperacusis, and difficulty with speech perception in noise ( Kujawa and Liberman, 2015). The discovery of cochlear synaptopathy in mice ( Kujawa and Liberman, 2009), the loss of the synapses between the inner hair cells (IHCs) and their afferent auditory nerve fiber targets, has motivated a search for noninvasive physiological measurements that can detect this condition in humans. This review summarizes the animal and human studies that have investigated the ABR as a measure of cochlear synaptic function, discusses factors that may have contributed to the mixed findings and the lessons learned, and provides recommendations for future use of this metric in the research and clinical settings. However, these studies have yielded mixed results, leading to a lack of consensus about the utility of the ABR as an indicator of synaptopathy. ![]() This has led to many studies investigating the use of the ABR as a metric of synaptopathy in humans. However, animal models indicate that the amplitude of the first wave of the ABR, a far-field evoked potential generated by the synchronous firing of auditory nerve fibers, is highly correlated with synaptic integrity. Whereas modern imaging now allows for the noninvasive detection of vestibular schwannomas, imaging technology is not currently capable of detecting cochlear synaptopathy, the loss of the synaptic connections between the inner hair cells and afferent auditory nerve fibers. Although clinical use of the auditory brainstem response (ABR) to detect retrocochlear disorders has been largely replaced by imaging in recent years, the discovery of cochlear synaptopathy has thrown this foundational measure of auditory function back into the spotlight. ![]()
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