Auditory Nerve Responses to Electric Pulse Trains as Measure of Neural Health

Background: Numerous studies in cochlear implant (CI) users have indicated only limited clinical value of electrically evoked compound action potentials (eCAPs). In contrast, in animal models advanced stimulation paradigms including pulse trains have yielded promising results in that derived eCAP measures correlated well with neural survival of the auditory nerve (Ramekers et al., 2015, Hear Res 321:12-24).

Objective: To assess the value of eCAPs in response to pulse trains by testing the same recording paradigms in CI users and in deafened guinea pigs.

Methods: Ototoxically deafened guinea pigs were implanted with a MED-EL PULSAR CI in the basal cochlear turn. A 100-ms pulse train with varying inter-pulse interval (IPI; 0.4 – 16 ms) was presented; eCAPs were recorded to the initial ten pulses and to the final ten pulses. Using pulse trains with similar parameter settings, we recorded eCAPs in human subjects with a MED-EL CI at both basal and apical electrodes, peroperatively or postoperatively.

Results: In guinea pigs the eCAP amplitudes modulated with successive pulses for IPIs between 0.5 and 1 ms, i.e., amplitudes were larger to odd than to even pulses. These modulations were negatively correlated to neural survival. In human subjects we found similar eCAP amplitude modulations in the IPI range of 0.5-1 ms. However, some individuals did not tolerate pulse train stimulation at the current level required to evoke eCAPs.

Conclusion: Useful information about the condition of the auditory nerve in CI users may be derived from eCAPs recorded to a pulse train with short IPIs.