A comparison of efferent suppression of compound action potentials by simultaneous and non-simultaneous contralateral noise paradigms.

The intent of this study was to compare the effectiveness of continuous versus gated contralateral noise in producing efferent suppression of CAPs in guinea pigs. The average contralateral suppression was 2.0 dB when using gated noise, with ipsilateral tone-pip rates of 2 s(-1), and 1.4 dB when using continuous noise at an ipsilateral tone-pip rate of 31 s(-1). When the ipsilateral tone-pip rate employed in the simultaneous paradigm with continuous noise was reduced to 2 s(-1), a rate equivalent to that employed in the non-simultaneous paradigm, the average attenuation was 2.1 dB, a magnitude comparable to the suppression achieved using gated noise. The present data suggest that, when equated for ipsilateral tone-pip pulse rate, simultaneous and non-simultaneous contralateral masking paradigms produce comparable levels of efferent suppression.

Topical gentamicin and ethacrynic acid: effects on cochlear function.

OBJECTIVE: To determine whether concurrent intravenous administration of the loop diuretic ethacrynic acid potentiates the toxicity of the aminoglycoside antibiotic gentamicin applied topically on the round window. STUDY DESIGN: The authors studied the effects on cochlear sensitivity of co-administered intracardiac ethacrynic acid (40 mg/kg) and high-dose topical gentamicin solution (100%) applied to the round window. Comparisons were made with animals receiving ethacrynic acid plus systemic gentamicin (100 mg/kg); topical gentamicin alone; systemic gentamicin alone; and intravenous ethacrynic acid alone. METHODS: Experiments were carried out on pigmented guinea pigs weighing 400 to 500 g. Changes in cochlear function were characterized by monitoring shifts in compound action potential (CAP) thresholds by use of chronic indwelling electrodes implanted at the round window, vertex, and contralateral mastoid. RESULTS: After 20 days animals receiving ethacrynic acid in combination with topical gentamicin to the round window failed to demonstrate a significant deterioration in cochlear sensitivity, whereas all animals receiving systemic gentamicin plus ethacrynic acid experienced profound increases in CAP thresholds. CONCLUSIONS: This study supports the contention that ethacrynic acid potentiates aminoglycoside ototoxicity by facilitating the entry of the antibiotics from the systemic circulation into the endolymph. In addition, this study answers important clinical concerns regarding the safety of the use of topical aminoglycoside agents in combination with loop diuretics.