gms | German Medical Science

Objective: The quantity of A-train activity in continuous intraoperative electromyography (EMG) is known to strongly correlate with facial nerve function after vestibular schwannoma surgery. Objective of this study was to elucidate the distribution and relative prognostic value of A-train activity within the three main branches of the facial nerve, since from recent publications it is known that the intermediate nerve carries motor fibers to the orbicularis oris muscle.

Methods: A group of 16 consecutive patients undergoing vestibular schwannoma surgery were examined. Continuous EMG activity from the facial muscles (Mm. orb. oculi, nasalis, orb. oris) was recorded using intramuscular needle electrodes. A-train activity was quantified using a fully automated procedure. The distribution of A-trains within the facial nerve branches was assessed with and without the M. orb. oris and was compared to postoperative facial nerve function: no disfiguring palsy (group A, n=7) and disfiguring palsy (group B, n=9).

Results: The median percentage of total train time was 6% in the M. orb. oculi (interquartile distance (IQD) 21%), 34% in the M. nasalis (IQD 52%) and 48% in the M. orb. oris (IQD 60%).,The overall ratio of the median train time of group A to group B Including all branches was 1:13 (p~0.023, Mann-Whitney-U-test). The positive predictive value of increased train time for severe postoperative palsy was 70% and negative predictive value 100%. Excluding the M. orb. oris A-trains, the median train time ratio was 1:11 (p~0.005). The positive predictive value increased to 75%, whereas the negative predictive value was 88%.

Conclusions: Train time is a strong predictor of postoperative facial nerve function. Our results suggest that the increased amount of A-train activity in the M. orb. oris overrepresents the actual damage to the facial nerve. This may be due to the fact that the orbicularis oris muscle is coinnervated by the intermediate nerve which is usually damaged and sacrified during surgery. If maximum sensitivity is desired, monitoring should include the M. orb. oris, whereas its exclusion results in a more realistic estimation of postoperative facial nerve function.