Transcranial Electrocortical Stimulation to Monitor the Facial Nerve Motor Function During Cerebellopontine Angle Surgery

Abstract

OBJECTIVE This study was conducted to investigate the success rate of using the facial motor evoked potential (FMEP) of orbicularis oculi and oris muscles for facial nerve function monitoring with use of a stepwise protocol, and its usefulness in predicting facial nerve outcome during cerebellopontine angle (CPA) surgeries. METHODS FMEPs were recorded intraoperatively from 60 patients undergoing CPA surgeries. Transcranial electrocortical stimulation (TES) was performed using corkscrew electrodes positioned at hemispheric montage (C3/C4 and CZ). The contralateral abductor pollicis brevis muscle was used as the control response. Stimulation was always applied contralaterally to the affected side using 1, 3, or 5 rectangular pulses ranging from 200 to 600 V with 50 μs of pulse duration and an interstimulus interval of 2 ms. Facial potentials were recorded from needles placed in the orbicularis oculi and oris muscles. RESULTS FMEP from the orbicularis oris and oculi muscles could be reliably monitored in 86.7% and 85% of the patients, respectively. The immediate postoperative facial function correlated significantly with the FMEP ratio in the orbicularis oculi muscle at 80% amplitude ratio (P = .037) and orbicularis oris muscle at 35% ratio (P = .000). FMEP loss was always related to postoperative facial paresis, although in different degrees. CONCLUSION FMEPs can be obtained reliably by using TES with 3 to 5 train pulses. Stable intraoperative FMEPs can predict a good postoperative outcome of facial function. However, further refinements of this technique are necessary to minimize artifacts and to make this method more reliable.

Facial nerve motor evoked potentials during skull base surgery to monitor facial nerve function using the threshold-level method

“…13 More recently, the indication for MEP monitoring has been extended to also monitor the function of the corticobulbar tract, and, in particular, facial nerve function. 4,7 Most authors consider a reduction in FNMEP response amplitude of more than 50% as a warning criterion [1][2][3][4]7,8,12 as widely accepted for MEPs in supratentorial surgery. 15,19 The threshold-level method was first proposed for monitoring spinal cord function.…”

“…With this approach we obtained a sensitivity of 88% and a specificity of 82%, which is in the range of values reported by other authors. 2,4,7,8 With the warning criterion at 20 mA, an intraoperative warning was associated with a risk of PPV of 83% (CI 59%-96%) of postoperative deterioration of facial nerve function. A high NPV of 88% (CI 62%-98%) indicates that the absence of an intraoperative warning predicts good postoperative facial nerve function.…”

“…Most authors use the decrease in FNMEP response amplitude as a warning criterion. [1][2][3][4]8,12 However, FNMEP responses have inherent variability and are hard to quantify. Also, high FNMEP stimulation intensity is needed to achieve maximal FNMEP response already at baseline so that subsequent response deterioration can be assessed.…”

Intraoperative Transcranial Motor-Evoked Potential Monitoring of the Facial Nerve during Cerebellopontine Angle Tumor Resection

“…Although optimal stimulation parameters for TCMEP of the FN remain unknown, current data support previously published techniques using multipulse stimulation of 3 to 5 trains, duration 0.2 to 0.5 milliseconds, and interstimulus interval of 4 milliseconds. [20][21][22]24 These parameters successfully minimized peripheral FN activation yet were able to overcome anesthesia-induced suppression of MEP. In addition to stimulation parameters, many factors are known to affect TCMEP monitoring, including anesthesia, use of muscle relaxants, number and type of recording electrodes, and individual patient variation.…”

“…[20][21][22][23][24][25][26] Studies indicate that TCMEP of the facial nerve is feasible and may predict immediate postoperative outcome. Long-term functional data and the relationship between TCMEP and facial nerve function beyond the immediate postoperative period is lacking.…”