Therapeutic effect of magnesium on noise-induced hearing loss.

This study examined the therapeutic effect of magnesium (Mg) on noise trauma in anesthetized guinea pigs exposed to an impulse noise series (1/s) of Lpeak 167 dB (Leq,1s 127 dB) for 38 min. The permanent hearing threshold shift (PTS) was measured 1 week post-exposure, using auditory brain stem response audiometry (frequency range, 0.5-32 kHz). The total Mg concentrations of perilymph, cerebrospinal fluid and plasma were analyzed by atomic absorption spectrometry. In a first series, animals maintained on physiologically low Mg received subcutaneous injections of either different Mg doses (0.11-0.33 mmol MgSO4/100 g per day) for 3 days and drinking water with an additive of 39 mmol MgCl2/l for 1 week or saline as a placebo and tap water alone. The treatment began immediately after the impulse noise exposure. The dose of 0.29 mmol Mg/100 g per day was found to be most effective and reduced the hearing loss by 13-20 dB compared to placebo. The PTS and the perilymph Mg level showed a close negative correlation, suggesting that the intracochlear Mg level plays an important role in bringing about these protective effects. In a second series, we tested the therapeutic efficacy as a function of the post-exposure time of onset of the optimal Mg treatment (1 min, 2 and 4 hours), using normal Mg animals. The therapeutic effect decreased with the length of time elapsed between the end of exposure and the beginning of treatment. In a parallel scanning electron microscopic test, we also found a Mg-related difference in the susceptibility of hair cell stereocilia to impulse noise exposure.