A comparison between threshold criterion and amplitude criterion in transcranial motor evoked potentials during surgery for supratentorial lesions

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OBJECTIVE

The aim of this study was to compare sensitivity and specificity between the novel threshold and amplitude criteria for motor evoked potentials (MEPs) monitoring after transcranial electrical stimulation (TES) during surgery for supratentorial lesions in the same patient cohort.

METHODS

One hundred twenty-six patients were included. All procedures were performed under general anesthesia. Craniotomies did not expose motor cortex, so that direct mapping was less suitable. After TES, MEPs were recorded bilaterally from abductor pollicis brevis (APB), from orbicularis oris (OO), and/or from tibialis anterior (TA). The percentage increase in the threshold level was assessed and considered significant if it exceeded by more than 20% on the affected side the percentage increase on the unaffected side. Amplitude on the affected side was measured with a stimulus intensity of 150% of the threshold level set for each muscle.

RESULTS

Eighteen of 126 patients showed a significant change in the threshold level as well as an amplitude reduction of more than 50% in MEPs recorded from APB, and 15 of the patients had postoperative deterioration of motor function of the arm (temporary in 8 cases and permanent in 7 [true-positive and false-negative results]). Recording from TA was performed in 66 patients; 4 developed postoperative deterioration of motor function of the leg (temporary in 3 cases and permanent in 1), and showed a significant change in the threshold level, and an amplitude reduction of more than 50% occurred in 1 patient. An amplitude reduction of more than 50% occurred in another 10 patients, without a significant change in the threshold level or postoperative deterioration. Recording from OO was performed in 61 patients; 3 developed postoperative deterioration of motor function of facial muscles (temporary in 2 cases and permanent in 1) and had a significant change in the threshold level, and 2 of the patients had an amplitude reduction of more than 50%. Another 6 patients had an amplitude reduction of more than 50% but no significant change in the threshold level or postoperative deterioration.

Sensitivity of the threshold criterion was 100% when MEPs were recorded from APB, OO, or TA, and its specificity was 97%, 100%, and 100%, respectively. Sensitivity of the amplitude criterion was 100%, 67%, and 25%, with a specificity of 97%, 90%, and 84%, respectively.

CONCLUSIONS

The threshold criterion was comparable to the amplitude criterion with a stimulus intensity set at 150% of the threshold level regarding sensitivity and specificity when recording MEPs from APB, and superior to it when recording from TA or OO.

ABBREVIATIONS APB = abductor pollicis brevis; GTR = gross-total resection; KPS = Karnofsky Performance Scale; MEP = motor evoked potential; MRC = Medical Research Council; NPV = negative predictive value; OO = orbicularis oris; PPV = positive predictive value; SMA = supplementary motor area; TA = tibialis anterior; TES = transcranial electrical stimulation.

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Article Information

Correspondence Tammam Abboud: University Medical Center Göttingen, Germany. tammamabboud@gmail.com.

INCLUDE WHEN CITING Published online September 7, 2018; DOI: 10.3171/2018.4.JNS172468.

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.

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Figures

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    Schematic drawing refers to the placement of the electrodes on the scalp according to the international 10–20 electroencephalography system. C4 to Cz stimulation was used to evoke MEPs on the left side and C3 to Cz stimulation was used to evoke MEPs on the right side. For a specific recording of MEPs from the leg muscles, C2 to C1 stimulation or C1 to C2 stimulation was performed.

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    Box plot showing the range of change in the threshold level and amplitude in patients with postoperative deterioration of motor function. Change in the threshold level always describes the increase on the affected side minus the increase on the unaffected side.

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