Measuring the electrical stapedius reflex with stapedius muscle electromyogram recordings

Previous studies have demonstrated a correlation between cochlear implant recipients' comfort levels (C level, upper limit of dynamic range of stimulation) and the contralateral electrical stapedius reflex (ESR) threshold, detected by acoustic impedance change. However, the utility of the approach is limited because many recipients have no detectable impedance change. The goals of this study were to investigate the utility of the stapedial electromyogram (EMG) for estimating onset and strength of the ESR. Ketamine-anesthetized guinea pigs were implanted with Nucleus electrode arrays and stimulated with biphasic current pulse trains (250 pps) via a Cochlear Corporation CI24M stimulator. Typical EMG recordings (obtained with bipolar microwire electrodes) contained easily detectable unit potentials up to 300 microV in amplitude. Growth response curves (obtained from threshold-crossing counts or rms of the EMG signal) were typically monotonic with dynamic ranges spanning 700 microA or 8 dB. Based on adaptation and temporal properties, the stimulus protocol (500 ms duration with 4-5 s interstimulus intervals) was adequate for producing independent responses. The data presented are consistent with ESR characteristics (acoustic impedance technique) of cochlear implant recipients and with EMG properties of acoustically stimulated guinea pigs. Use of the EMG for characterizing the ESR may eventually be applied to human cochlear implant recipients as a guide in setting the upper limit of the dynamic range.