Intraoperative facial motor evoked potential monitoring for pontine cavernous malformation resection

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OBJECTIVE

The aim of this study was to predict postoperative facial nerve function during pontine cavernous malformation surgery by monitoring facial motor evoked potentials (FMEPs).

METHODS

From 2008 to 2017, 10 patients with pontine cavernous malformations underwent total resection via the trans–fourth ventricle floor approach with FMEP monitoring. House-Brackmann grades and Karnofsky Performance Scale (KPS) scores were obtained pre- and postoperatively. The surgeries were performed using one of 2 safe entry zones into the brainstem: the suprafacial triangle and infrafacial triangle approaches. Six patients underwent the suprafacial triangle approach, and 4 patients underwent the infrafacial triangle approach. A cranial peg screw electrode was used to deliver electrical stimulation for FMEP by a train of 4 or 5 pulse anodal constant current stimulation. FMEP was recorded from needle electrodes on the ipsilateral facial muscles and monitored throughout surgery by using a threshold-level stimulation method.

RESULTS

FMEPs were recorded and analyzed in 8 patients; they were not recorded in 2 patients who had severe preoperative facial palsy and underwent an infrafacial triangle approach. Warning signs appeared in all patients who underwent the suprafacial triangle approach. However, after temporarily stopping the procedures, FMEP findings during surgery showed recovery of the thresholds. FMEPs in patients who underwent the infrafacial triangle approach were stable during the surgery. House-Brackmann grades were unchanged postoperatively in all patients. Postoperative KPS scores improved in 3 patients, decreased in 1, and remained the same in 6 patients.

CONCLUSIONS

FMEPs can be used to monitor facial nerve function during surgery for pontine cavernous malformations, especially when the suprafacial triangle approach is performed.

ABBREVIATIONS CN = cranial nerve; FMEP = facial MEP; KPS = Karnofsky Performance Scale; MEP = motor evoked potential.

Article Information

Correspondence Tetsuya Goto: Shinshu University School of Medicine, Matsumoto, Japan. tegotou@shinshu-u.ac.jp.

INCLUDE WHEN CITING Published online January 11, 2019; DOI: 10.3171/2018.8.JNS181199.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Schematic drawing of the suprafacial and infrafacial triangles in the floor of the fourth ventricle. MLF = medial longitudinal fascicle. Reprinted with permission from Kyoshima et al: J Neurosurg 78:987–993, 1993.

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    Lateral illustration of the brainstem for suprafacial (A) and infrafacial (B) triangle approaches. Arrows depict the direction of the surgical approach.

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    A: Illustration depicting the placement of stimulating scalp corkscrew electrode and cranial peg screw electrode. B: Photograph showing a peg screw electrode (arrow) connected to the electrical stimulator (asterisk) placed on the patient’s head.

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    Threshold pattern of the FMEP. A: Stable: FMEP threshold increased gradually but is regarded as stable because no sudden increase in threshold was observed throughout surgery. B: Warning sign: A sudden increase in the threshold during surgery associated with the surgical intervention. C: Recovered: Threshold recovered to the initial threshold and remained stable until the end of the surgery. D: Not recovered: The threshold was increased, but not recovered to the initial threshold throughout the surgery.

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    A: Preoperative T2-weighted axial MR image demonstrating a cavernous lesion with hematoma in the pons. B: Postoperative T2-weighted axial image showing the lesion has been removed. C: Intraoperative photograph during facial colliculus mapping in the fourth ventricle floor using a ball electrode (large arrow). Arrowheads: midline, small arrows: stria medullaris. D: FMEP reading during tumor resection by using anodal stimulation with current of 45–60 mA, 5 pulse stimulus train, duration of 0.3 msec. Above channel is the baseline recording, and below channel demonstrates warning sign (circle).

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