In Vivo Cochlea Hair Cell Rescue after Noise‐Induced Hearing Loss Using Low‐Level Laser Therapy

Abstract

Objectives: There is no study regarding the effects of low level laser therapy (LLLT) on cochlear after noise-induced hearing loss (NIHL). The aims of this study were to (1) elucidate the ability of LLLT applied to noise-induced damaged cochlea to recover NIHL and (2) demonstrate histopathological changes of remaining outer hair cells (OHCs). Methods: A total of 6 rats (C group) for control and 9 rats (N group) as the noise-exposure group were included. Nine rats were exposed to noise once and the left ears (NL side) were treated with low-level laser daily. Right ears (N side) were the control sides. Hearing thresholds were observed through auditory brainstem response recordings. Thresholds were monitored before noise exposure and also after the twelfth irradiation. Cochlear pathology and number of OHCs were observed by scanning electron microscopy. Results: The initial average hearing threshold levels were 26 ± 4, 24 ± 5, 24 ± 5, 24 ± 3, 24 ± 5 dB SPL in 4, 8, 12, 16, 32 kHz, respectively. After noise exposure, N group thresholds were 63 ± 15, 64 ± 16, 71 ± 11, 73 ± 15, 67 ± 14 dB SPL. After 12 irradiations, NL side thresholds recovered significantly (27 ± 4, 26 ± 4, 28 ± 8, 30 ± 9, 27 ± 6 dB SPL, P < .05). N side thresholds were 46 ± 16, 51 ± 13, 63 ± 16, 50 ± 12, 48 ± 14 dB SPL. OHC number was significantly larger in the NL side than the N side. In the C group, OHCs counts were 176 ± 16, 146 ± 5, 154 ± 6 cells for apical, middle, basal turns, respectively. N side counts were 122 ± 2, 113 ± 2, and 116 ± 19. NL side counts displayed promising results (167 ± 27, 140 ± 29, 129 ± 26). Conclusions: This study demonstrates that LLLT promotes recovery of hearing and OHCs survival from apoptotic changes by NIHL.