The effects of round window membrane injury and the use of a model electrode application on hearing in rats.

ABSTRACT

We conducted an animal experiment to investigate the effects of mechanical trauma to the round window with the placement of a model electrode inserted into the scala tympani on the cochlear reserve, and to determine the efficacy of topical steroids in preventing hearing loss in such a situation. Our subjects included 21 male Wistar albino rats that were assigned into three groups of 7 each. In all three groups, an initial mechanical injury to the round window was created. At that point, group 1 received no further treatment, group 2 received a dexamethasone injection into the cochlea, and group 3 underwent implantation of a multichannel cochlear implant guide followed by dexamethasone administration. After a few minutes, the round window opening was obliterated with muscle, and the incision was sutured with 4-0 Vicryl Rapide polyglactin in all 3 groups. Distortion-product otoacoustic emissions were obtained before and immediately after the surgical injury, and again on postoperative day 7. Mean signal/noise ratios (S/Ns) obtained at 2, 3, and 4 kHz were calculated, and datasets were compared with nonparametric statistical tests. We found that the early postoperative mean S/N values were significantly lower than the preoperative values in groups 1 and 2, but there was no difference between the mean preoperative values and those obtained on postoperative day 7 in those two groups. In group 3, there were statistically significant differences among the mean preoperative, early postoperative, and postoperative day 7 S/N values. We observed that an electrode insertion into the cochlea via the round window subsequent to mechanical trauma seemed to cause a progressive hearing loss. Therefore, we conclude that special care must be taken to avoid injury to the round window membrane during placement of a cochlear implant electrode, as well as during surgery for chronic otitis media.