Influence of a Short-term Overstimulation on eCap- and eABR-Thresholds

Abstract

Increasing upper current limits (C-levels) can lead to better speech perception in cochlea implantees due to the widening of the electrical dynamic range (eDR). Clinical observations showed that C-levels and eDRs are sometimes extremely increased over time. Therefore, it should be clarified if a larger amount of electrical stimulation is tolerated by the remaining peripheral (auditory nerve) and the central auditory pathway. In the present study, the effect of a short-term overstimulation on the excitability of peripheral and central auditory structures was determined. A scala tympani electrode array (eight contacts) was inserted in adult mechanically deafened guinea pigs. C-levels were adjusted to approx. 8 CL (“LOS” – group), 40.00 CL (“MOS” – group) or 60 CL (“HOS” – group) above the electrode specific electrically evoked compound action potential (eCAP) threshold. The stimulation was performed via a CP 810 sound processor in standardized auditory environment. Implanted and not stimulated animals were used as a control group. The results showed a significant eCAP threshold shift in the “HOS”-group directly after eight hours of stimulation at basal electrodes. Electrically evoked auditory brainstem thresholds were stable over time in all stimulated groups. The control group showed a significant lower eABR threshold as the LOS and HOS group after four hours of stimulation. Even short time overstimulation at high current levels reduces the excitability of peripheral auditory structures. Such an adaptation was not found within the auditory brainstem. A changed relationship between peripheral and central thresholds seem to indicate an overstimulation. The results are of utmost importance for a safe CI-processor fitting especially in children or non-compliant patients.