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 fro m the sequential stimulation of all paired combinations of 22 electrod es. In almost all patients the amplitude of the surface potentials inc reased with the distance between the stimulating electrodes along the scala tympani and decreased with the distance between the basal electr ode and the round window. However, in two patients whose cause of deaf ness was otosclerosis, the largest surface potentials were seen when t he stimulation was applied to electrodes which were approximately half a cochlear turn apart. These highly regular patterns suggested two di fferent pathways for the currents that generated surface potentials: ( 1) through the fluid along the scala and not through the dense cochlea r 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 unpl easant sensations in the head (four patients) did not give rise to abn ormal surface potential amplitudes. Two patients who suffered from fre quent threshold and comfort level changes were tested repeatedly. As t he recorded amplitudes did not change significantly over time, neuroph ysiological changes were a more likely cause than fluctuations of the stimulator output.