Deep electrode insertion in cochlear implants:: Apical morphology, electrodes and speech perception results

Morphological examination of the human temporal bone in the apical region supports the benefits of deep electrode insertion. Initiation of spikes on peripheral processes close to the basilar membrane would provide improved channel selectivity during electrical stimulation but recruiting of nerve fibres requires a higher current. A clinical study was performed on 10 users of the MED-EL COMBI 40+ implant to evaluate the effect of the insertion depth of the cochlear implant electrode on speech perception. All subjects were implanted with the standard COMBI 40+ electrode with an insertion depth of > 30 mm. Acute speech tests were carried out in which stimulation was restricted to the apical, middle and basal regions of the cochlea in turn, and using electrode arrangements in which contacts were either distributed over the whole length of the cochlea or concentrated at the basal end, thus mimicking an insertion depth of approximate to 20 mm only. The results showed that stimulation of the apical region of the cochlea supports a significant degree of speech understanding, and that distributing the contacts over the whole length of the cochlea improves speech perception in quiet and in noise.