Gentamicin delivery to the inner ear: Does endolymphatic hydrops matter?

Abstract

Introduction: Middle ear application of gentamicin is a common medical treatment for uncontrolled Ménière's disease. The objective of the study was to evaluate the impact of endolymphatic hydrops on inner ear delivery.

Methods: Perilymph gentamicin concentrations and correlation with endolymphatic hydrops in an animal model were assessed. A group of 24 guinea pigs was submitted to surgical obstruction of the endolymphatic sac and duct of the right ear. Gentamicin was applied either to the right ear's round window niche or through a transtympanic injection. Perilymph specimens were collected at different times. Histologic morphometry was used to evaluate both turn-specific and overall hydrops degree.

Results: In animals with endolymphatic hydrops, lower concentrations of gentamicin were observed after 20 or 120 minutes of exposure and in both types of administration, when compared to controls. This difference reached statistical significance in the round window niche application group (Mann-Whitney, p = 0,007). A negative correlation between perilymphatic gentamicin concentration and hydrops degree could be observed in both groups, after 120 minutes of exposure (Spearman correlation, round window niche p<0,001; TT p = 0,005).

Conclusions: The study indicates that the endolymphatic hydrops degree has a negative interference on the delivery of gentamicin into the inner ear following middle ear application.

Fig 1. Midmodiolar sections of the cochlea.

Second turn of the left (A) and right (B) cochlea from animal #13, under light microscopy and toluidine blue stain. Arrowheads point the dilation of the Reissner’s membrane (B). (SM: scala media, SV: scala vestibuli, ST: scala tympani).

Fig 2

Perilymphatic gentamicin concentration after RW niche application (A) and Transtympanic application (B). Data presented as average ± 2 standard deviation. (RE–right ear; CTRL–control; EH–endolymphatic hydrops; min- minutes; N–number of animals; 20 min–time period of administration of 20 minutes; 120 min–time period of administration of 120 minutes).

Fig 3. Correlation coefficients between Hydrops Ratio and perilymphatic gentamicin concentration.

Correlation coefficients (Spearman correlation) in different times periods and methods of administration. (RWN–round window niche; HR–hydropic ratio).