Intraoperative recordings of electromyogenic responses from the human stapedius muscle

Following surgical implantation of a cochlear implant, the external processor must be programmed individually, which is typically achieved by behavioral measurements, with the user indicating the perceived loudness between the threshold and the maximum comfort levels (MCLs). However, the stapedius reflex measurement could be used instead, as the basis for programming the fitting maps. The electrically evoked stapedius reflex threshold is known to have a high correlation with the MCLs and thus is used as an objective tool to determine the MCL, especially in children and non-cooperative patients. Previous studies demonstrated that the electromyogenic (EMG) activity of the stapedius muscle could be measured to determine stapedius reflex responses. The objective of this study was to demonstrate that intraoperative EMG recordings from the stapedius muscle could be performed with an EMG recording electrode designed for chronical implantation. Ten adult subjects with single sided deafness (SSD) participated in this study. The study was split up into a pre-, intra- and postoperative phase. In the preoperative phase the subjects were screened for the presence of the acoustic stapedius reflex by using a standard tympanometer in the ear to be implanted, while the contralateral ear was stimulated acoustically. During the intraoperative phase an EMG recording electrode was placed in the stapedius muscle and EMG responses were recorded during acoustical and electrical stimulation. The surgical procedure consisted of two interconnected procedures: A classical surgical approach for cochlear implantation with two additional surgical steps, and the temporary placement of an EMG recording electrode in the stapedius muscle made through an opening in the pyramidal eminence, allowing EMG recordings. The postoperative phase was used to evaluate the preservation of the stapedius reflex postoperatively at 1- and 6-months with contralateral acoustic and ipsilateral electrical stimulation. The EMG recording electrode could successfully be placed in the stapedius muscle in all subjects and was able to reliably record EMG signals from the stapedius muscle elicited by acoustic and electrical stimulation. Overall, EMG recordings were obtained intraoperatively in all subjects tested (8 out of 8 subjects). Contralateral acoustic stapedius reflexes were obtained in 6 out of 8 subjects and ipsilateral electrical stapedius reflex in 7 out of 8 subjects. In the postoperative phase, the preservation of the postoperative reflex was observed in 6 out of 10 subjects 1-month after surgery and in 8 out of 10 subjects 6-months after surgery. The study showed that intraoperative EMG recordings from the stapedius muscle can be performed with an EMG recording electrode designed for chronical implantation. The minimally invasive procedure is promising; in particular with regard to the future perspective of measurements in the long-term with an integrated closed-loop self-fitting system.