Transient evoked otoacoustic emissions after vestibular nerve section in chinchillas

Abstract Problem: The most well-known function of the medial efferent system is that of cochlear response inhibition, achieved through electric stimulation of the floor of the fourth ventricle. However, studies investigating the effects of lesion-related de-efferentation have obtained contradictory results. In fact, there is evidence that de-efferentation causes the relationship between transient evoked otoacoustic emissions and distortion product otoacoustic emissions to become unbalanced. To determine the role that the medial efferent system plays in outer hair cell function, transient evoked otoacoustic emissions were recorded before and after vestibular nerve section in chinchillas. Methods: Twenty-two adult male chinchillas, weighing between 500 and 750 g, were divided into 2 groups: (1) experimental group (n = 17), which underwent surgery for right vestibular nerve section; and (2) control group (n = 5), which underwent the same operation but without the right vestibular nerve section. Transient evoked otoacoustic emissions were recorded before and immediately after the surgery in both groups. Responses at frequencies of 0.8, 1.6, 2.4, 3.2, and 4.0 kHz, as well as total emission responses, were analyzed. Results: After the vestibular nerve section, there were significant increases in the amplitudes of the 2.4 and 4.0 kHz responses ( P = 0.004 and P = 0.047) and of the total response ( P = 0.0003). Conclusion: Transient evoked otoacoustic emissions were sensitive in detecting changes of the cochlear mechanics after vestibular nerve section in chinchillas. Significance: These results indicate that changes in cochlear mechanics directly affect transient evoked otoacoustic emissions. Therefore we support the hypothesis that the medial efferent system is involved in the control or modulation of outer hair cell activity. Support: None reported.