gms | German Medical Science

Objective: Continuous recording of EMG signals is an established method for intraoperative monitoring of facial nerve function in cerebello-pontine angle surgery. Although various electrode setups are in use, optimal placement of electrodes has not yet been investigated. The presented study was therefore aimed to develop an optimal setup based on neurophysiologic characteristics.

Methods: A group of 30 patients undergoing vestibular schwannoma surgery were investigated. EMG data were continuously acquired using four transdermal needle electrodes in each of the nasal, orbicular oris and oculi muscles. Pathological patterns (A-trains) were detected visually and marked in the respective channels when they showed a signal-to-noise ratio of at least two. Extent and frequency of occurrence were determined per muscle. Total A-train quantity (“train time”) in all channel combinations was compared to postoperative outcome.

Results: Most trains and highest train time were seen in the orbicular oris muscle (all comparisons p<0.05). Electrodes in the orbicular oculi muscle showed fewest trains and least train time. All channels showed significant correlations between train time and postoperative deterioration (0.37 to 0.77, p<0.05). Best values were found in proximal electrodes in the orbicular oris muscle (p<0.05), worst in electrodes close to the nasolabial fold. Including more channels into train time calculation resulted in increased correlations; however, largest improvement was found when 3-5 channels were considered. A-trains were seen in 2.6 (±1.2) channels on average. This corresponds to an average A-train extent of ~1.8cm in diameter.

Conclusions: A-train activity shows high correlation with postoperative deterioration of facial nerve function. To avoid overlooking relevant activity, optimal electrode placement should at least include 3 to 5 electrodes: in proximal parts of the orbicular oris muscle, approximately 1cm lateral of the nasolabial fold and close to the eyebrow.