The electrically evoked whole-nerve action potential: fitting applications for cochlear implant users

Abstract

The goal of this study was to explore the clinical utility of the electrically evoked whole-nerve action potential (EAP), measured with the Neural Response Telemetry (NRT) capabilities of Cochlear Corporation's Nucleus C124M cochlear implant system. Users of the C124M cochlear implant system followed at the University of Washington with at least three months of implant experience were subjects (n = 12) in this study. EAP growth and refractory recovery functions were successfully quantified on each active electrode of every subject. In addition, two measures of loudness growth were explored at 80 Hz (the frequency of the EAP stimuli) and at 250 Hz (the frequency of clinical implant fitting stimuli). The first loudness measure was a repeated ascending and descending method of adjustment using a continuous signal. The second loudness measure was a magnitude estimation technique. Other information including performance, etiology and duration of hearing loss, and individual electrode, impedance was considered. EAP thresholds were found to be highly correlated with threshold loudness responses. The rate of EAP growth with increasing stimulation levels was also found to be correlated with the dynamic range of loudness limits and threshold loudness responses. Inconsistencies were found between the two methods used to measure loudness growth. No relationship was evident between EAP measures and performance. EAP measures can be used to estimate loudness information used in cochlear implant fitting. Suggested cochlear implant fitting protocols based on EAP measures may be more reliable than protocols based only on loudness techniques.