Continuous electromyography monitoring of motor cranial nerves during cerebellopontine angle surgery

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Object. Electromyography (EMG) monitoring is expected to reduce the incidence of motor cranial nerve deficits in cerebellopontine angle surgery. The aim of this study was to provide a detailed analysis of intraoperative EMG phenomena 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 examined 30 patients undergoing surgery on acoustic neuroma (24 patients) or meningioma (six patients). Free-running EMG signals were recorded from muscles targeted by the facial, trigeminal, and lower cranial nerves, and were analyzed off-line with respect to waveform characteristics, frequencies, and amplitudes. Intraoperative measurements were correlated with typical surgical maneuvers and postoperative outcomes.

Characteristic EMG discharges were obtained: spikes and bursts were recorded 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. Three 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 train—a sinusoidal, symmetrical sequence of high-frequency and low-amplitude signals—was observed in 19 patients and could be well correlated with additional 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 degree of functional worsening is to be expected postoperatively, there is a good chance of avoiding major deficits by warning the surgeon early. Continuous EMG monitoring is superior to electrical nerve stimulation or acoustic loudspeaker monitoring alone. The detailed analysis of EMG-waveform characteristics is able to provide more accurate warning criteria during surgery.

Article Information

Address reprint requests to: Johann Romstöck, M.D., Department of Neurosurgery, University of Erlangen-Nuremberg, Schwabachanlage 6, 91054 Erlangen, Germany. email: johann.romstoeck@neurochir.med.uni-erlangen.de.

© AANS, except where prohibited by US copyright law.

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Figures

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    Electromyographic tracings. Spikes (upper) and bursts (lower) are the basic components of the intraoperative EMG signals. They are either elicited by direct nerve contact or arise spontaneously at any time during surgery. Expanding the horizontal time axis from 100 msec/division (center) to 20 msec/division (right) reveals typical triphasic and polyphasic waveform patterns.

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    Electromyographic train activity. Left: Examples of A trains of various duration and frequency. The upper tracings show the abrupt onset and termination (arrows) of this sinusoidal waveform pattern, which lasted 1600 msec. The fourth train from the top of the figure shows repeated short-term periods of activity, ranging in duration from 100 to 120 msec each. The lower A train, which was simultaneously recorded from two facial muscle groups, gives an impression of frequency variability between 120 Hz and 190 Hz. Right: Waveforms defined as B trains with spikes (BS) and B trains with bursts (BB) as predominant single components. The black and white arrows mark two individual B trains with spikes of higher and lower amplitudes recorded in the same channel. The lowest tracing represents irregular EMG activity, called a C train.

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    Clinical results. Facial nerve function in 30 patients was assessed according to the House—Brackmann grading system. For each patient the preoperative facial nerve status is represented by a square (black square = acoustic neuroma; white square = CPA meningioma). Long-term results 1 year after surgery are depicted by arrowheads. The tail of the arrows indicates early findings 1 week postoperatively. In 19 patients with early postoperative deterioration functional improvements were demonstrated within 1 year.

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    Electromyographic patterns correlated with additional facial nerve paresis in 30 patients 1 week after surgery. Of the 21 patients in whom postoperative facial nerve weakness (≥ one grade of the House—Brackmann scale) occurred, 18 (86%) A trains were identified in EMG recordings of the mimetic muscles postoperatively. One patient in whom an A train was recorded exhibited no additional morbidity. The B train was observed in almost all patients (28 patients [93%]) and the C train in only three patients (10%), none of whom exhibited clinical consequences. Large spikes (S) and bursts (B) (> 500 µV) demonstrated no significance, being equally distributed in the paresis and nonparesis groups.

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    Illustrative intraoperative events. I. The upper waveforms were recorded from a patient's orbicular muscle of the mouth, which produced B-train activity. During excision of a large acoustic neuroma, bleeding from the tumor capsule required electrocautery. Following the artifact produced by the bipolar coagulation, the recording remained flat until the end of surgery. The facial nerve had been well identified by electrical stimulation (black arrows) before but not after electrocautery (white arrows). The patient had a preoperative Grade III facial paresis that worsened to Grade VI 1 week after surgery but recovered to Grade IV after 1 year. II. The lower tracings were recorded from the nasal muscle during surgery on a medium-sized acoustic neuroma. The waveforms remained flat until electrical stimulation (0.2 mA) of the facial nerve (arrow), which resulted in A-train activity (triangle). Dissection was stopped and continued at a distant site. Postoperatively, the patient displayed mild additional paresis, bringing his preoperative Grade I paresis to Grade II.

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