Long-term stabilisation of electrode impedance with implantation of a dexamethasone-eluting intracochlear array

Local steroid delivery via implantation of drug-eluting intracochlear electrode arrays is an emergent strategy for improving postoperative outcomes in cochlear implant recipients. Corticosteroids like dexamethasone (DEX) have been shown effective in reducing formation of fibrotic tissue and minimising electrode impedance. However, the benefits of this approach differ in preclinical implant models with the extent of electrical stimulation applied and the type of impedance measured. Where daily electrical stimulation is used to model human clinical scenarios, steroid-eluting arrays have been shown beneficial in maintaining stable four-point electrode impedance over the course of 5 weeks. Here we examine fibrosis and impedance after implantation of a drug-eluting intracochlear array and daily electrical stimulation over an extended period of 15 weeks. Adult guinea pigs were bilaterally implanted with a fully implantable DEX-eluting array (right ear) and a standard non-eluting array (left ear), with electrode impedance assessed before and after daily electrical stimulation. Four-point impedance remained stable for the duration of the study in the presence of a DEX-eluting array, whilst significantly increasing in the presence of a standard array. Assessment of a trans-impedance matrix also revealed consistently lower impedance measures in the DEX-eluting array. This was accompanied by lower levels of fibrotic tissue (particularly loose fibrous tissue) in cochleae implanted with a steroid-eluting array, and correlation between four-point impedance and the extent of fibrosis. The outcome of this extended study supports four-point impedance as an indicator of local fibrosis, and the importance of sustained intracochlear release of dexamethasone to benefit the tissue-electrode interface.