Systemic immunity influences hearing preservation in cochlear implantation

HYPOTHESIS

To determine whether a systemic immune response influences hearing thresholds and tissue response after cochlear implantation of hearing guinea pigs.

METHODS

Guinea pigs were inoculated with sterile antigen (Keyhole limpet hemocyanin) 3 weeks before cochlear implantation. Pure-tone auditory brainstem response thresholds were performed before implantation and 1 and 4 weeks later. Dexamethasone phosphate 20% was adsorbed onto a hyaluronic acid carboxymethylcellulose sponge and was applied to the round window for 30 minutes before electrode insertion. Normal saline was used for controls. Cochlear histology was performed at 4 weeks after implantation to assess the tissue response to implantation. To control for the effect of keyhole limpet hemocyanin priming, a group of unprimed animals underwent cochlear implantation with a saline-soaked pledget applied to the round window.

RESULTS

Keyhole limpet hemocyanin priming had no significant detrimental effect on thresholds without implantation. Thresholds were elevated after implantation across all frequencies tested (2-32 kHz) in primed animals but only at higher frequencies (4-32 kHz) in unprimed controls. In primed animals, dexamethasone treatment significantly reduced threshold shifts at 2 and 8 kHz. Keyhole limpet hemocyanin led to the more frequent observation of lymphocytes in the tissue response to the implant.

CONCLUSION

Systemic immune activation at the time of cochlear implantation broadened the range of frequencies experiencing elevated thresholds after implantation. Local dexamethasone provides partial protection against this hearing loss, but the degree and extent of protection are less compared to previous studies with unprimed animals.

Round window delivery of dexamethasone ameliorates local and remote hearing loss produced by cochlear implantation into the second turn of the guinea pig cochlea

Application of dexamethasone to the round window has been shown to ameliorate high frequency hearing loss resulting from the trauma of cochlear implantation in experimental animals, but elucidation of the factors influencing protection of the high frequencies has been confounded by the local trauma from electrode array insertion. In this experiment, a second turn cochleostomy and implantation was performed on guinea pigs, to examine protection in the basal turn without the confounding effect of local trauma, as well as to test the efficacy of hearing protection in the second cochlear turn. The implantation resulted in an increase in hearing thresholds across all frequencies examined (2-32 kHz). Local delivery of dexamethasone to the round window prior to implantation protected hearing across frequencies from 2 to 32 kHz. Auditory thresholds improved over the first week after surgery, and then remained stable for the month of the experiment. The protection of hearing in the basal turn increased with longer periods of drug application prior to implantation. The level of hearing protection in the second turn was similar irrespective of the time that the drug was applied, but was greater when a higher steroid concentration was used. It was concluded that steroids protect hearing in the basal turn of the cochlea even when there was no local trauma. The level of hearing protection in the second turn exceeded that expected from models of steroid diffusion through the cochlea, suggesting that inner ear surgery alters the distribution of dexamethasone within the cochlea.