A novel method for quantitative evaluation of motor evoked potential monitoring during cerebrovascular surgeries

•A strategy based on MEP onset latency is proposed to validate MEP results during cerebrovascular surgery.•MEP latencies at contralateral and ipsilateral (crossover) threshold define superficial and deeply penetrating stimuli.•Peri-cortical generated MEP are determined when onset latencies remain ≥ 2 ms longer than when crossover MEP are observed. Transcranial motor evoked potential (MEP) monitoring, intended to assess cerebral cortical ischemia, may produce false negative results when the stimulation inadvertently activates the deep, subcortical motor pathways. This study examined hand MEP onset latency as a potential means to differentiate superficial versus deep stimulus penetration in surgical patients monitored for cerebral ischemia. Intraoperative MEP data were prospectively collected from 40 patients treated for intracranial aneurysm or carotid stenosis. Onset latencies of hand MEP responses were measured over a range of stimulation intensities from both the contralateral and ipsilateral hand (crossover responses). At the threshold for superficial, cortical stimulation of the contralateral hand, the MEP latency was 26.9 ± 0.4 ms. MEP onset latencies measurements became shorter as stimulation intensities were increased. At the maximum intensity (when crossover response was usually generated), the contralateral hand MEP latency of 22.5 ± 0.3 ms was significantly shorter than at threshold stimulation (p