MECHANISMS OF NOISE INDUCED HEARING LOSS AND ITS ALLEVIATION

Adelman C (1,2), Tamir S (3), Perez S (3), Weinberger J (4), Sohmer H (2)

(1) Speech and Hearing Center, Hadassah University Hospital; (2) Dept. of Physiology, Hebrew University-Hadassah Medical School; (3) Dept. of Otolaryngology and Head & Neck Surgery, Shaare Zedek Medical Center; (4) Dept. of Otolaryngology and Head & Neck Surgery, Hadassah University Hospital; Jerusalem, Israel.

Exposure to noise can cause a noise induced hearing loss (NIHL), the severity of which is a function of the magnitude and duration of the noise. In this study, pharmacological methods were used to gain insight into the mechanisms of NIHL and to suggest drugs which could alleviate the loss.

In order to enable comparison between drugs, all experiments were conducted on the same species (mice), exposed to the same noise (continuous broadband for 3.5 hours, which causes permanent hearing loss - PTS), with auditory threshold assessed in the same way (click auditory nerve and brainstem evoked response (ABR) thresholds) before and two to three weeks after the noise exposure. The drugs administered in the different groups before or after noise exposure included salicylic acid and furosemide, which depress the cochlear amplifier and hence the active displacements in the cochlea, the anti-oxidant NAC (N-acetyl-L-cysteine), and saline (control). The degree of PTS was calculated as the difference between the ABR threshold two weeks after the exposure and that before exposure. The degree of protection provided by each of the drugs was calculated as the difference between the PTS in the saline control group and that in each drug group.

Salicylic acid and furosemide, administered just before the noise, each provided more protection than NAC, showing that reduction of active mechanics is most effective. This can be due either to a reduction in excessive active vibrations (which can cause direct mechanical structural damage) or to the resulting decrease in metabolic demand (with reduced release of free radicals and indirect metabolic structural damage). The finding that the anti-oxidant NAC also provided some protection is evidence for oxidative damage. Preliminary experiments have shown that it is not likely that the PTS is due to excessive passive displacements, even following exposure to impulse noise.

E-mail: Cahtiaa@hadassah.org.il