Vol.100 No.5 contentsPRACTICA OTO-RHINO-LARYNGOLOGICA
Vol. 100 No. 5 May 2007
Recent Findings in Acoustic Trauma
Naohiro Yoshida and Toshimitsu Kobayashi
(Tohoku University)
Acoustic trauma remains a most important issue in technologically advanced countries. Overexposure to intense sound damages the inner ear sensory cells and can lead to temporary threshold shift (TTS) or permanent threshold shift (PTS) if exposure is sufficiently intense or prolonged. This review includes the recent findings of histological, metabolic, physiological changes after sound exposure and the proposed mechanisms for protection from acoustic injury.
Cats, guinea pigs and mice are widely used as experimental models for hearing research. Recently, mice have been more frequently used because their genomic information and lower degree of inter-animal variability in the response to sound exposure. Overexposure damages the cochlea in two ways, i.e. mechanical and metabolic pathways. In mice, intense exposure (over 116 dB SPL 2 hours, 8 kHz octave band noise) simultaneously damages the organ of corti and stria vascularis by mostly mechanical pathways. However, sound exposure (112 dB SPL 2 hours, 8 kHz octave band noise) damaged the organ of Corti within a week by mechanical and metabolic pathways. The first row of outer hair cells is the most vulnerable to sound. Sound exposure (100 dB SPL 2 hours, 8 kHz octave band noise), which showed damage to only the first row of outer hair cells in histological findings, causes 40 dB PTS.
To prevent and protect PTS from sound exposure, some factors or reagents have been identified in animal models, 1) efferent reflex via the olivocochlear bundle (OCB); animals with a large OCB reflex are resistant to sound. 2) conditioning effect; "conditioning" the ear by pre-exposure to a moderate level, non-traumatic sound or heat stress, restraint can dramatically reduce the development of PTS by subsequent sound exposure. 3) Reactive oxygen substances (ROS) scavenger, glutathione; oxidative stress damages the tissue, while ROS scavenger can protect the ear from sound exposure. 4) blocker of glutamate, or neutrophine, 5) steroid. Some reagents have been examined in humans as clinical trial in the US.
Key words : acoustic trauma, protection, conditioning, reactive oxygen substance