Protective Effect of Isoflurane on Noise-induced Hearing Loss in Mice

Background: It is well known that the loud noise exposure can lead to noise-induced hearing loss (NIHL). Drilling during mastoid surgery may result in NIHL. The noise level produced by drilling of the mastoid bone can exceed 125 dB HL (hearing level); therefore, mastoid surgery itself is associated with a lower incidence of NIHL than expected. The aim of this study was to analyze the effects of isoflurane on NIHL and hair cell morphological changes. Methods: BALB/c mice were divided into 2 groups; a control group (n = 20) and an isoflurane group (n = 20). The mice of both groups were exposed to 120 dB SPL (sound pressure level) broadband white noise for 3 hours per day, for 3 consecutive days. The mice in the isoflurane group were anesthetized with isoflurane while exposed to the noise. The auditory brainstem response (ABR) thresholds were determined 1 day before and after the noise-exposure and then again after 7 days. Both cochlea were removed and stained using fluorescent isothiocyanate (FITC) phalloidin. Results: 1 day prior to noise-exposure, the ABR thresholds were those of a normal hearing level in both the control and isoflurane groups. In the control group, the mean hearing threshold was 78.0 2.6 dB HL after 1 day of noise-exposure and 81.5 3.4 dB HL after 1 week; in the isoflurane group, the mean hearing threshold was 49 11.7 dB HL after 1 day and 30.5 9.3 dB HL after 1 week. The hearing thresholds after noise exposure in the control were significantly higher than those in the isoflurane group (P ＜ 0.05). Conclusions: The occurrence of NIHL decreased and the hair cell damage suppressed in the mice exposed to intense noise while anesthetized by isoflurane.