Cochlear implant generated surface potentials: current spread and side effects.

Surface potentials were recorded in 16 users of the Cochlear (Nucleus) Mini System 22 in order to investigate the current flow resulting from the sequential stimulation of all paired combinations of 22 electrodes. In almost all patients the amplitude of the surface potentials increased with the distance between the stimulating electrodes along the scala tympani and decreased with the distance between the basal electrode and the round window. However, in two patients whose cause of deafness was otosclerosis, the largest surface potentials were seen when the stimulation was applied to electrodes which were approximately half a cochlear turn apart. These highly regular patterns suggested two different pathways for the currents that generated surface potentials: (1) through the fluid along the scala and not through the dense cochlear bone, leaving the cochlea only at the basally located openings; (2) through the very permeable cochlear bone in the case of otosclerosis. Stimulation of any electrodes that caused facial twitching and/or unpleasant sensations in the head (four patients) did not give rise to abnormal surface potential amplitudes. Two patients who suffered from frequent threshold and comfort level changes were tested repeatedly. As the recorded amplitudes did not change significantly over time, neurophysiological changes were a more likely cause than fluctuations of the stimulator output.