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Evaluation of the cochlear microphonic for the diagnosis of noise-induced cochlear synaptopathy Tran, Natalie


Purpose: Conventional audiometry, used to detect elevated hearing thresholds, is insensitive to cochlear pathologies involving cochlear synapse degeneration in the absence of hair cell or nerve fiber loss. Patients with this pathology—known as cochlear synaptopathy (CS)— report symptoms such as tinnitus and speech in noise difficulty despite clinically normal hearing thresholds. Current methods for identifying CS in animals require immunohistochemistry, however this invasive technique cannot be translated to clinical diagnostics. The present study aims to evaluate the diagnostic potential of the cochlear microphonic (CM) for noise-induced CS via electrocochleography. Design: The CM in response to 95 dB nHL broadband clicks was recorded via tympanic membrane electrocochleography. 18 music students (n = 32; mean age = 21.7) with normal hearing thresholds (≤ 25 dB, HL 250-8000 Hz) and 19 normal hearing controls (n = 35; mean age = 22.5) were recruited for the study. Lifetime noise exposure was obtained using the NESI. A repeated-measures ANOVA was used to analyze the effects of music background and rater on CM/CAP amplitude, CM duration, and CM area under the curve. Correlation tests were performed between NESI scores and liberal or conservative CM power calculations. Inter-rater reliability was tested using a multiple regression design. Inter-class correlation tests were performed on liberal and conservative CM/CAP amplitude and CM duration values. Results: There were no significant group differences on any of the electrocochleography measures. Lifetime noise exposure scores were not significantly different between groups. Conclusion: The present study found no evidence that CM/CAP amplitude, CM duration, or CM area under the curve are associated with noise exposure. Results suggest 1) no effect of CS on the cochlear microphonic, 2) electrocochleography is insensitive to CS, or 3) noise exposure is too low to detect CS. It is possible the effects of noise exposure may be observed in individuals with greater lifetime noise exposure than those recruited for the study. Use of tympanic membrane electrocochleography to assess CS in humans remains inconclusive. Additional research is needed to develop a clinical diagnostic protocol for early cochlear damage that precedes hair cell loss.

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