Continuous electromyography monitoring of motor cranial nerves during cerebellopontine angle surgery

Object. Electromyography (EMG) monitoring is expected to reduce the inciden ce of motor cranial nerve deficits in cerebellopontine angle surgery. The a im of this study was to provide a detailed analysis of intraoperative EMG p henomena with respect to their surgical significance. Methods. Using a system that continuously records facial and lower cranial nerve EMG signals during the entire operative procedure, the authors examin ed 30 patients undergoing surgery on acoustic neuroma (24 patients) or meni ngioma (six patients). Free-running EMG signals were recorded from muscles targeted by the facial, trigeminal, and lower cranial nerves, and were anal yzed off-line with respect to waveform characteristics, frequencies, and am plitudes. Intraoperative measurements were correlated with typical surgical maneuvers and postoperative outcomes. Characteristic EMG discharges were obtained: spikes and bursts were recorde d immediately following the direct manipulation of a dissecting instrument near the cranial nerve, but also during periods when the nerve had not yet been exposed. Bursts could be precisely attributed to contact activity. Thr ee distinct types of trains were identified: A, B, and C trains. Whereas B and C trains are irrelevant with respect to postoperative outcome, the A tr ain-a sinusoidal, symmetrical sequence of high-frequency and low-amplitude signals-was observed in 19 patients and could be well correlated with addit ional postoperative facial nerve paresis (in 18 patients). Conclusions. It could be demonstrated that the occurrence of A trains is a highly reliable predictor for postoperative facial palsy. Although some deg ree of functional worsening is to be expected postoperatively, there is a g ood chance of avoiding major deficits by warning the surgeon early. Continu ous EMG monitoring is superior to electrical nerve stimulation or acoustic loudspeaker monitoring alone. The detailed analysis of EMG-waveform charact eristics is able to provide more accurate warning criteria during surgery.