PREOPERATIVE ELECTRICAL-STIMULATION FOR COCHLEAR IMPLANT SELECTION - THE USE OF EAR CANAL ELECTRODES VERSUS TRANSTYMPANIC ELECTRODES

Preoperative electrostimulation tests were performed on 43 postlingual ly deaf and 20 prelingually deaf cochlear implant (CI) candidates. The stimulating electrode was placed at three different locations, i.e. t he round window, the promontory and the ear canal and the results were compared. The evoked sensations were reported to be of auditory origi n by most of the postlingually deaf CI candidates. Prelingually deaf s ubjects could not always distinguish clearly and reliably between ''he aring'' and ''feeling''. The percentage of stimulated ears of postling ually deaf subjects in whom hearing sensations were evoked was almost identical for the three locations of the stimulating electrode. Howeve r, in 5 out of the 7 ears without hearing sensations during ear canal stimulation (ECS), hearing sensations could be evoked during either pr omontory stimulation (PS) or round window stimulation (RWS). The mean threshold level for ''hearing'' at a stimulation frequency of 62 Hz wa s lowest during RWS, 7.7 dB higher during PS and 35.8 dB higher during ECS. The mean electrical dynamic range at 62 Hz was most favourable d uring RWS (23.9 dB), smaller during PS (15.6 dB) and smallest during E CS (10.0 dB). All differences were statistically significant. Placemen t of the ear canal electrode was easier, less frightening for the pati ent and required less time than insertion of the needle electrode for PS or RWS. Therefore we recommend the use of ECS to examine whether th e CI candidate can be stimulated, and of RWS if more detailed informat ion is required.