How many electromyography channels do we need for facial nerve monitoring?

PURPOSE: Continuous electromyography (EMG) monitoring is a standard method for intraoperative monitoring of facial nerve function. Typically, only two or three bipolar channels are recorded from needle electrodes to detect pathologic activity, which may provide an incomplete sample. Therefore, we evaluated the influence of channel number on monitoring quality.

METHODS

The EMG data of 30 patients undergoing surgery for vestibular schwannoma were recorded using 9 bipolar channels from subdermal needle electrodes located in the orbicular oculi, oris, and nasal muscle. Pathologic A-train activity was evaluated in regard to correlation to postoperative nerve function. Channel combinations with different channel numbers were compared.

RESULTS

A-train quantity showed high correlations to postoperative nerve function: Spearman rank correlation of 0.58 for 2, 0.61 for 3 channels. It increased further with every additional channel to 0.69 for all 9 channels (48% of variance accounted for). Single channels with more than one-third of total A-train activity ("hot spots") were observed in 17 patients, which did not show consistent spatial patterns and could only be completely detected with a high number of channels.

CONCLUSIONS

Few channels as used in conventional monitoring setups yield acceptable results. However, correlation between train time and postoperative functional results improves with every additional EMG channel.