Effect of increasing duration of stimulation on the electrically evoked auditory brainstem and middle latency responses in pediatric cochlear implant users.

OBJECTIVE

Electrophysiologic responses used to predict behavioural stimulation levels in implant users are typically evoked with a single-pulse stimulus versus higher-rate pulse trains of longer duration. Unfortunately, electrophysiologic measures tend to overestimate behavioural responses. It may be possible to improve the predictive ability of evoked potential thresholds by increasing the duration of the stimulus. We asked if auditory brainstem and middle latency responses can be reliably measured using electrical pulse trains of increasing duration.

DESIGN

Evoked potential testing was performed in five pediatric N24RE cochlear implant users aged 7.6 to 14.9 years.

SETTING

Cochlear Implant Program, Cochlear Implant Laboratory, The Hospital for Sick Children, Toronto.

METHODS

Responses were electrically evoked for durations of 2, 6, and 10 milliseconds by single pulses and pulse trains of 500 pulses per second.

MAIN OUTCOME MEASURES

Evoked auditory brainstem response (EABR) and evoked middle latency response (EMLR) waveform latencies and amplitudes were compared between durations.

RESULTS

Clear and replicable EABRs and EMLRs were recorded for all durations. There was no significant change in EABR wave eV latency relative to the stimulus offset. There was no significant change in EMLR wave latencies relative to the stimulus onset. This was confirmed by a linear decrease in the interwave latency between the EABR wave eV and the EMLR wave eNa. No significant changes in wave amplitude were found as the pulse train duration increased for the EABR or the EMLR.

CONCLUSIONS

EABRs are dominated by the stimulus offset, whereas EMLRs appear to be dominated by stimulus onset. Clear, unchanging EABRs and EMLRs indicated that electrophysiologic measures were a valid tool for increasing durations.