A silastic positioner for a modiolus-hugging position of intracochlear electrodes: electrophysiologic effects.

Abstract

It was postulated that an electrode array that achieved a close modiolar proximity would result in reduced threshold levels and amplitude slopes, as measured with electrically evoked auditory brainstem responses (EABRs). Quality and quantity of auditory information transmitted by a cochlear implant to patients with sensorineural hearing loss depend on spatial and temporal resolution achieved by the electrical intracochlear stimulation. To improve spatial resolution, a new electrode system was developed by Advanced Bionics Corp., with the intention of obtaining greater modiolar proximity. The implant version specified for animal experiments consists of a straight electrode array of seven embedded platinum discs and a so-called Silastic-positioner. The Silastic positioner is shaped to follow the dimensions of the scala tympani with a concave (triangular) inner side, which fits the form of the electrode array. The aim of the study was to evaluate the influence of a modiolus-hugging electrode position in contrast to a conventional electrode position on EABR in short-term animal experiments. Short-term electrophysiologic studies were performed on six adult cats. After local intracochlear application of neomycin solution (50 mg/mL), electrodes were inserted into the scala tympani. Electrically evoked auditory brainstem response threshold levels and EABR amplitude slopes were systematically investigated with and without the positioner. Electrically evoked auditory brainstem response measurements revealed a distinct apicobasal threshold shift, with increasing thresholds toward the basal end of the electrode. After insertion of the positioner, this shift diminished or was inverted and EABR thresholds and amplitude slopes were reduced significantly. Threshold and amplitude slope data emphasize the functional benefit of the positioner system, especially for the stimulation of electrodes in the more basal channels.