Although several investigators have studied the effects of aminoglycosides on experimental endolymphatic hydrops (EH), it remains unclear whether these drugs have any effect on the formation of EH because of the difficulty in measuring EH. In the present study, we evaluated the effects of gentamicin (GM) on the formation of EH in guinea pigs with experimentally induced EH by measuring the endolymphatic spaces and the length of Reissner's membranes in the temporal bone sections using a device composed of a microscope, videocamera, and computer. EH was induced by cauterizing the endolymphatic sac with a 10% silver nitrate solution. The EH animals were divided into 2groups; one group received GM systemically by intramuscular injection, and the other group received GM topically by applying it to the tympanic cavity through the tympanic membrane. Each group was subdivided into 3 subgroups based on the time of GM administration. All animals were euthanized 6 weeks after cauterization.
1) The ratio of the dilated endolymphatic space (Sh) to the original endolymphatic space (Sc) of the animals treated with systemically or topically administrated GM was smaller than that of untreated animals (control group).
2) The subgroup treated with GM 4 weeks after cauterization showed the smallest Sh/Sc ratio, while the subgroup treated with GM before cauterization showed the largest Sh/Sc ratio next to the untreated group.
3) The cochlear turns which showed smaller Sh/Sc ratios showed atrophic changes of the stria vascularis together with folding of Reissner's membrane, suggesting a decrease in endolymph secretion. To clarify the mechanisms of this drug-induced suppression of endolymph secretion, we introduced a new parameter, a reduction in the rate of EH, which allowed us to evaluate the degree of folding in Reissner's membrane.
4) In the treated animals, the reduction in the rate of EH was higher in the subgroups which showed smaller Sh/Sc ratios.
5) From these results, it would appear that systemically or topically administrated GM effects the stria vascularis, resulting in a reduction of endolymph secretion, and folding of Reissner's membrane due to the reduction of EH.

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