Steroids in aminoglycoside-containing ear drops: do they reduce cochlear toxicity?

OBJECTIVES/HYPOTHESIS: To determine whether betamethasone (BM) reduces the cochlear toxicity of otic gentamicin (GM) if given together. STUDY DESIGN: Controlled animal study. METHODS: Thirty-four mice were assigned at random to receive intratympanic injections of either 0.1 % BM (11 mice), 0.3% GM (13 mice), or a combination of both (GM/BM) with benzalkonium chloride (10 mice) in the left ear (treated) and saline on the right (untreated). Six injections were given on alternate days. Auditory brainstem response thresholds were assessed at 1 month, 2 months, and >2 months. RESULTS: There was a significantly greater degree of hearing loss in the BM-treated ears compared to the untreated ears (6.48 dB hearing loss, P = .007) and in the GM-treated ears compared to untreated ears (6.59 dB hearing loss, P = .010,). However, otic GM/BM and benzalkonium chloride did not cause significant additional hearing loss compared with the untreated ears (3.56 dB hearing loss, P = .242). CONCLUSIONS: Our data suggest that hearing loss caused by GM otic drops may be reduced by the inclusion of BM and benzalkonium chloride. Our finding that BM alone was associated with hearing loss suggests that the benzalkonium chloride may be the protective agent in combination otic drops.

Addressing animal model issues in auditory research.

OBJECTIVE: To better understand the differences in auditory systems across species. METHODS: Auditory brainstem response (ABR) click thresholds were obtained from normal 3- to 6-week-old animals including 15 guinea pigs, 62 mice, and 6 rats. Pure-tone ABR thresholds were obtained in 7 guinea pigs, 6 mice, and 13 rats. Threshold variability was then considered a function of basilar membrane length, mean body weight, basal metabolic rate, and longevity as identified in the literature. RESULTS: Interspecies variability of auditory thresholds for normal-hearing animals is not explained by differences in mean body weight, metabolic rate, or longevity. Simple linear models appear to adequately describe threshold variability across the parameters studied. Click thresholds, with significant low-frequency content, suggest that mice have better hearing than rats or guinea pigs. CONCLUSION: In spite of wide variations in cochlear anatomy and metabolic factors, different species have evolved similar auditory thresholds across species in normal, young animals.

Does sodium thiosulphate delay presbyacusis?

INTRODUCTION: This study compares the effects of sodium thiosulphate (STS) and normal saline on the prevention of hearing loss. SETTING: Animal research laboratory. METHODS: Sixteen mice were randomized to receive intraperitoneal injections of either normal saline or STS. Auditory brainstem response testing was used to determine baseline and posttreatment hearing thresholds over the course of 1 year. RESULTS: Compared with saline, treatment with STS resulted in a statistically significant improvement in click and pure-tone thresholds until the end of the year. CONCLUSION: STS can significantly delay hearing loss associated with age in this murine model.