Variability in amplitude and stimulation threshold values in motor evoked potential (MEP) monitoring during the resection of brain lesions

Objective: Criteria for motor evoked potential (MEP) monitoring include the amplitude criterion and threshold criterion. The aim of our study for the amplitude criterion during removal of brain lesions was to determine a suitable stimulus intensity that can reduce the variability in amplitudes. We also assessed the usefulness of the threshold criterion and amplitude criterion by comparing the variability between the two methods as a preliminary study.

Methods: Seventeen patients including 12 cases with no neurological deterioration after surgery were enrolled in this study. The amplitudes in various stimulus intensities and the stimulation threshold (ST) were repeatedly measured during surgery. The stimulus intensities used in the amplitude criterion are described as 'the ST just after dura opening (ST(0))+additional intensity'. The variability was assessed with the coefficient of variation (CV).

Results: With direct cortical stimulation (DCS), the CVs of amplitudes were significantly lower at stimulus intensities of more than ST(0)+4 mA, with the smallest at ST(0)+10 mA. The CV of STs was significantly smaller than that of amplitudes.

Conclusions: The variability in the amplitude criterion was significantly reduced using stimulus intensities of more than ST(0)+4 mA. The variability in the threshold criterion was significantly smaller than that of the amplitude criterion, even when using a strong stimulus intensity.

Significance: The maximal stimulus intensity with no body motion should be used in the amplitude criterion for intraoperative MEP elicited by DCS.