Intraoperative monitoring of facial corticobulbar motor evoked potentials: Methodological improvement and analysis of 100 patients

Objective: A) To describe an improved methodology for continuously monitoring the functional integrity of facial nerve by eliciting facial corticobulbar motor-evoked potentials (FCoMEP) and B) To establish the prognosis of facial nerve function based on changes in FCoMEP during skull base surgery.

Methods: Intraoperative monitoring of FCoMEP performed in 100 patients. Previously published methodology has been improved upon by a) doing preoperative mapping of the facial nerve, b) facilitating the corticobulbar tract (CBT) by continuous transcranial electrical stimulation (TES) at 2 Hz repetition rate, c) recording from multiple facial nerve innervated muscles, and d) eliciting blink reflex (BR). We analyzed changes in FCoMEP, comparing them with the clinical facial nerve outcome scored with the House-Brackman (HB) scale.

Results: The monitorability rate was 100%. Out of 100 patients, nine presented a new facial deficit after surgery. Eight of these showed significant changes in FCoMEP. In four patients FCoMEPs were lost; they presented a complete facial paralysis from which they did not recover. To discriminate the prognosis of patients, ROC analysis identified a cut-off at 65% for FCoMEPs amplitude decrease with a sensitivity of 89% and specificity of 99%. In four patients FCoMEP showed a decrease in amplitude greater than 65%, and they presented mild/moderate facial paresis that was transient. One patient did not present changes in FCoMEP but had a mild facial paresis from which the patient recovered.

Conclusions: The improved methodology allows the maximum rate of monitorability and minimizes false positive and false negative results. This study shows that prognosis of facial nerve may be reliably established based on FCoMEP parameters.

Significance: We improved the previously described methodology for continuously monitoring the functional integrity of the facial nerve by increasing the monitorability rate, and we describe the impact of FCoMEP intraoperative management of facial nerve. This method may permit establishing the short-term and long-term prognosis of facial nerve function in skull base surgery.