Measurement of the stapedius reflex using chronically implanted EMG electrodes in sheep

Introduction The hearing experience of patients after receiving a cochlear implant (CI), depends on regular fitting of the CI during the first months. Since the stapedius reflex is elicited close to the uncomfortable level (UCL), it could be used for objective adjustment of CIs. The aim of the study was to examine the stability of recordings of electromyographic signals (EMG) of the stapedius muscle, using implanted electrodes, over 6 months.

Material and Methods The stapedius reflex was present in 10/11 merino sheep (4 years old). The muscle was exposed via the retrofacial approach without opening the ear canal or middle ear. This allowed the reflex to be elicited acoustically for measurements ipsilaterally using a tympanometer. Two electrodes (Ø 0.65 mm) were implanted in the muscle to record the EMG signal mono- as well as bipolar using PowerLab and LabChart after 1, 3 and 6 months. Following the final measurements, the muscles of both sides were collected and histologically processed.

Results Post mortem revealed that in 1/10 animals the electrodes were not correctly positioned in the stapedius. After 6 months EMG signals could be recorded in 5/9 animals. In 4/9 animals no final EMG measurement was possible, because intraoperatively no reflex could be elicited (3/9) or an electrode was broken (1/9). In contrast to the bipolar EMG signals, whose quality allowed a good correlation with the reflex, the monopolar recordings showed strong noise.

Discussion Our measurements showed that after 6 months EMG signals can be recorded bipolar by chronically implanted EMG electrodes, that is, over a period of time relevant for patients to gain significantly more benefit by regular adjustment of CIs.

Publication History

Article published online:
12 May 2023