Chronic Electrical Stimulation of the Auditory Nerve using Non-charge-balanced Stimuli

Abstract

This study was designed to evaluate the pathophysiological response of the cochlea following long-term intracochlear electrical stimulation using a poorly charge-balanced stimulus regime, leading to direct current (DC) levels >0.1 microA. Four normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods up to 2200 h. Stimuli consisted of 50 micros monophasic current pulses presented at 2000 pulses per second (pps) per channel, and resulted in DC levels of 0.4-2.8 microA. Both acoustic and electrical (EABR) evoked potentials were periodically recorded during the stimulation program. Frequency-specific stimuli indicated that an extensive and widespread hearing loss occurred over the 4-24 KHz region in all stimulated cochleae, although the 2 KHz region exhibited thresholds close to normal in some animals, despite long-term implantation and chronic stimulation. Longitudinal EABRs showed a statistically significant increase in threshold for three of the four animals. Histopathological evaluation of the cochleae revealed a highly significant reduction in ganglion cell density in stimulated cochleae compared with their controls. Spiral ganglion cell loss was significantly correlated with the degree of inflammation, duration of electrical stimulation, and the level of DC. In conclusion, the present study highlights the potential for neural damage following stimulation using poorly charge-balanced stimuli.

Safety of multi-channel stimulation implants: a single blocking capacitor per channel is not sufficient after single-fault failure

“…The DC is highly dependent on time, stimulation amplitude, and stimulation frequency but not significantly on the distance between the electrodes. These levels of DC would most certainly lead to tissue damage as they are typically one or two orders of magnitude higher than the 2-3 lA level for which damage has already been seen [10,18].…”

“…For example, it has been reported that DC levels as low as 2-3 lA cause pathological changes in rat spinal cords [10] or in cat auditory nerves [18]. Therefore, both the U.S. Food and Drug Administration (FDA) and E.U.…”

An energy-efficient, dynamic voltage scaling neural stimulator for a proprioceptive prosthesis

“…Charge balanced stimulation pulses are important for patient safety and preventing electrode degeneration. DC current flows of 100 nA across an electrode have been correlated with tissue damage in animal models [8] and industry targets a DC error of < 25 nA [26]. The stimulator proposed here is capable of stimulating at over 1500 pulses per second, but will in practice only be used to deliver up to 80 stimulations per second per channel (based on maximum observed firing rates of human proprioceptors [37], [38]).…”

“…In practice safe stimulation means that the packet of charge delivered to the tissue also needs to be removed -giving a charge balanced stimulation. Unbalanced stimulations give rise to DC currents flowing across the electrode / tissue interface and have been linked with tissue damage and deterioration of the electrode [8].…”

Analysis of passive charge balancing for safe current-mode neural stimulation

“…One of the original numbers (below 100 nA DC current error) was cited for a cochlear implant study [12] and has been cross-cited numerous times also for other applications [5]. This number has also been used by the European cochlear implant standard [13].…”