Intra-operative measurement of the stapedius reflex via neurophysiological detection of stapedius muscle activity: A feasibility clinical study.

Objective: Objective fitting of cochlear implants (CI) still represents an important challenge in the audiological field. Reliable and reproducible fitting procedures might significantly contribute to improve the clinical benefits introduced by the CI's, most of all in non-compliant patients. Furthermore, potential development of next generation smart CI processors would greatly depend on a reliable and objective fitting methodology. Finding methodologies providing reliable monitoring of the stapedius reflex (SR) and detection of the SR threshold (SRT) -- given its acknowledged correlation with the most comfortable loudness levels (MCL) -- is therefore a crucial aspect. The retrofacial approach is a surgical procedure allowing a direct intraoperative access to the stapedius muscle (SM) unlocking the possibility of neurophysiological monitoring of the SM and the measurement of its electromyographic signal. Aim: The aim of the current study is to test the feasibility of intraoperative recording of SM electromyographic (EMG) signals related to acoustically and electrically evoked SR in single sided deaf CI candidates. Material and methods: A multicentred clinical study was set up in Tübingen and Jena, to assess SR-related EMG responses in 7 single-sided CI-patients. During standard CI implantation surgery to reach the belly of the stapedius muscle (SM), an EMG electrode was placed on the muscle belly. Access to SM was performed via either a retrofacial approach, or from drilling of the pyramidal eminence (PE), according to individual anatomy and pre-operative planning. During a pre-surgical decisional phase, 3D reconstructions produced after manual segmentation of Dyna Computed Tomography (Dyna-CT) images of the CI patient's temporal bone were evaluated. 3D renderings were available for consultation along all the duration of the surgery. Elicitation of the reflex was done via both contralateral acoustic stimulation (via tympanometer) and ipsilateral electrical stimulation (via the CI). Several frequencies and intensities were used for acoustic stimulation, as well as several CI channels were stimulated at different intensities during electrical stimulation. CI stimulation was performed at 250 pulses per second, in order to allow stimulation artefacts to be almost completely differentiated from EMG spikes, which enabled artefact removal and not only basic filtering during post-processing of the data. The contraction of the SM (movement of the stapedius tendon) was assessed visually via the microscope to determine whether a SR was present, and at the same time EMG recordings were performed to obtain the neurophysiological response. EMG measurements were analysed in post-processing, but for most supra-threshold stimulations, EMG spikes were clearly seen on the raw unfiltered signal during the trials. Results: The muscle was accessed via retrofacial approach in 71% of the cases (5/7 patients) and via anterior approach in 43% of the cases (3/7 patients). On 1/7 cases both approaches were performed following intraoperative decision of the surgeon since the anatomy allowed it. SR -- EMG signals were successfully recorded in 71% of the cases (5/7 patients). EMG-based thresholds were calculated based on the signal-to-noise ratio during the stimulation, data analysis showed a Pearson correlation of 0.95 between the thresholds detected via EMG and those detected visually by the surgeon -- movement at the level of the stapedius tendon. The EMG signal showed a high sensitivity, being detected even before the visual confirmation in 33% of the assessed cases. Conclusions: Our results confirmed that it is possible to reliably record SRrelated EMG signals intraoperatively. The methodology for accessing the SM and subsequent electrode placement was found safe and effective, following an imaging-based pre-operative evaluation. [ABSTRACT FROM AUTHOR]