Chronic intracochlear electrical stimulation at high charge densities results in platinum dissolution but not neural...

Although there are useful guidelines defining the boundary between damaging and non-damaging electrical stimulation they were derived from acute studies using large surface area electrodes in direct contact with cortical neurons. These parameters are a small subset of the parameters used by neural stimulators. More recently, histological examination of cochleae from patients that were long-term cochlear implant users have shown evidence of particulate platinum (Pt). The pathophysiological effect of Pt within the cochlea is unknown. We examined the response of the cochlea to stimulus levels beyond those regarded as safe, and to evaluate the pathophysiological response of the cochlea following chronic stimulation at charge densities designed to induce Pt corrosion in vivo.

19 guinea pigs were systemically deafened and implanted with a cochlear electrode array containing eight Pt electrodes of 0.05, 0.075 or 0.2 mm² area. Animals were electrically stimulated continuously for 28 d using charge balanced current pulses at charge densities of 400, 267 or 100 µC/cm²/phase. Electrically-evoked auditory brainstem responses (EABRs) were recorded to monitor neural function. On completion of stimulation electrodes were examined using scanning electron microscopy (SEM) and cochleae examined histology. Finally, analysis of Pt was measured using energy dispersive x-ray spectroscopy (EDS) and inductively coupled plasma mass spectrometry (ICP-MS).

Compared with unstimulated control electrodes and electrodes stimulated at 100 µC/cm²/phase, stimulation at 267 or 400 µC/cm²/phase resulted in significant Pt corrosion. Cochleae stimulated at these high charge densities contained particulate Pt. The extent of the foreign body response depended on the level of stimulation; cochleae stimulated at 267 or 400 µC/cm²/phase exhibited an extensive tissue response that included a focal region of necrosis close to the electrode. Despite chronic stimulation at high charge densities there was no loss of auditory neurons (ANs) in stimulated cochleae compared with their contralateral controls. Indeed, we report a statistically significant increase in AN density proximal to electrodes stimulated at 267 or 400 µC/cm²/phase. Finally, there was no evidence of a reduction in AN function associated with chronic stimulation at 100, 267 or 400 µC/cm²/phase as evidenced by stable EABR thresholds over the stimulation program.

Chronic electrical stimulation of Pt electrodes at 267 or 400 µC/cm²/phase evoked a vigorous tissue response and produced Pt corrosion products that were located close to the electrode. Despite these changes at the electrode/tissue interface there was no evidence of neural loss or a reduction in neural function.