Pharyngeal motor evoked potentials elicited by transcranial electrical stimulation for intraoperative monitoring during skull base surgery

Clinical article

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The purpose of this study was to determine whether monitoring of pharyngeal motor evoked potentials (PhMEPs) elicited by transcranial electrical stimulation during skull base tumor surgery might be useful for predicting postoperative swallowing deterioration.


The authors analyzed PhMEPs in 21 patients during 22 surgical procedures for the treatment of skull base tumors. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli (220–550 V). Pharyngeal MEPs were recorded from the posterior wall of the pharynx through a modified endotracheal tube. The correlation between the final/baseline PhMEP ratio and postoperative swallowing function was examined.


Postoperative swallowing function was significantly (p < 0.05), although not strongly (r = −0.47), correlated with the final/baseline PhMEP ratio. A PhMEP ratio < 50% was recorded during 4 of 22 procedures; in all 4 of these cases, the patients experienced postoperative deterioration of swallowing function. After 18 procedures, the PhMEP ratios remained > 50%; nevertheless, after 4 (22.2%) of these 18 procedures, patients showed deterioration of swallowing function.


Intraoperative PhMEP monitoring can be useful for predicting swallowing deterioration following skull base surgery, especially in patients with swallowing disturbances that are mainly due to reduction in the motor functions of the pharyngeal muscles.

Abbreviations used in this paper: CMAP = compound muscle action potential; MEP = motor evoked potential; PhMEP = pharyngeal MEP.

Article Information

Address correspondence to: Masafumi Fukuda, M.D., Department of Neurosurgery, Brain Research Institute, University of Niigata, 1-757 Asahimachi-dori, Chuo-ku, Niigata City 951-8585, Japan. email:

Please include this information when citing this paper: published online December 9, 2011; DOI: 10.3171/2011.10.JNS111343.

© AANS, except where prohibited by US copyright law.



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    Modified endotracheal tube with adhesive surface electrodes (contacts 1, 2, 3, and 4 from the distal to the proximal sites) (upper). The tube was positioned with the surface electrodes attached to the pharyngeal musculature on the side ipsilateral to the lesion (lower).

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    Pharyngeal MEPs were recorded from 3 pairs of contacts (contacts 1 and 2, 2 and 3, and 3 and 4). The response from the most distal paired electrodes (contacts 1 and 2) had the longest onset latency and the response from most proximal electrodes (contacts 3 and 4) the shortest onset.

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    Correlation between postoperative swallowing deterioration (postoperative − preoperative grade) and the final/baseline PhMEP ratio. Postoperative deterioration of swallowing function was significantly correlated with the final/baseline PhMEP ratio (r = −0.47, p < 0.05).

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    Bar graphs showing the relationship between the final/baseline PhMEP ratios and postoperative swallowing deterioration. In 14 (100%), 2 (50%), and 2 (50%) of the procedures after which patients had swallowing function deterioration of 0, 1, and 2 grades, respectively, the intraoperative PhMEP ratios were > 50% (χ2 = 8.6, 2 df, p < 0.05). Swallowing deteriorated after all 4 procedures in which the intraoperative PhMEP ratios were < 50%.

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    Patient 2. Representative electromyograms obtained from the posterior wall of the pharynx before removal of a jugular foramen schwannoma (A), during removal of the intrajugular foramen tumor (B), and at the final recording (C). Note the marked reduction in the PhMEP responses during removal of the tumor located within the jugular foramen. The final PhMEP amplitude recovered almost entirely to the baseline value.

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    Patient 3. Representative electromyograms obtained from the posterior wall of the pharynx before removal of a jugular foramen schwannoma (upper) and at the final recording (lower). The final PhMEP ratio decreased to 44%.


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