Electrode features for hearing preservation and drug delivery strategies

Reducing the risk of hearing loss after cochlear implantation requires optimization of the electrode array to minimize the physical trauma caused by insertion and placement. Furthermore, the electrode design must be optimized for atraumatic surgical approaches. Even greater levels of protection may be achieved by the use of a drug during and after implantation. The electrode array offers a potential vehicle for drug delivery.

This article reviews the laboratory and clinical data available thus far relating to the importance of electrode design parameters for trauma minimization, and the possibility of further reduction through pharmaceutical intervention. Candidate drugs were identified through literature review and laboratory evaluation. The most promising have been investigated in vitro and in animal models of implantation trauma. Three delivery devices are currently under development to satisfy the specific demands of different therapy regimes. The delivery profiles of each were evaluated through both modelling and bench testing and the concepts investigated in vitro and in vivo.

Current evidence favours a thin, flexible electrode array with wires in a zigzag shape. Steroids and an apoptosis inhibitor (AM111) performed well in animal models of electrode trauma and are both good drug candidates for reduction of the risk of hearing loss after implantation. Semi-chronic dexamethasone elution, acute drug delivery by intracochlear catheter, and longer-term delivery through diffusion from a reservoir were all shown to be feasible.

An extensive programme focussed on minimizing hearing loss through device optimization and the development of new technologies has yielded positive results and new concepts for further development and clinical application.