Using Evoked Compound Action Potentials to Assess Activation of Electrodes and Predict C-Levels in the Tempo+ Cochlear Implant Speech Processor

Abstract

In this study, we analyze how electrically evoked compound action potential (ECAP) responses can be used to assess whether electrodes should be activated in the map and to estimate C levels in the Med-El Tempo+ Cochlear Implant Speech Processor. ECAP thresholds were measured using the ECAP Recording System of the Pulsar CI implant. Twenty-one postlingually and 28 prelingually deafened patients participated in this study. The relationship between ECAP responses and the activation of electrodes was analyzed. Because an error in the estimation of T levels (behavioral thresholds) has less effect on hearing quality than an error in the estimation of C levels in the Tempo+ cochlear implant speech processor (maximum comfort levels), correlation and regression analyses were performed between ECAP thresholds and C levels. The observation of an evoked potential generally implied that the electrode was activated because only 3.5% of electrodes that yielded measurable evoked responses were deactivated, because of collateral stimulations or an unpleasant hearing sensation. In contrast, the absence of an evoked potential did not imply that an electrode should be deactivated, because 20% of these electrodes provided a useful auditory sensation. ECAP responses did not predict the absolute behavioral comfort levels because of the excessive error between behavioral C levels and those derived from ECAP thresholds (the mean relative error is 43.78%). However, by applying a normalization procedure, ECAP measurements allowed the C-level profile to be predicted with a mean relative error of 6%; that is, they provided useful data to determine the C level of each electrode relative to the average C level of the patient. ECAP is a reliable and an useful objective measurement that can assist in the fitting of the Tempo+ cochlear implant speech processor. From results presented in this work, a protocol is proposed for fitting this cochlear implant system. This protocol facilitates appropriate cochlear implant fitting, particularly for children or uncooperative patients.