P39. Fast versus slow charge cortical stimulation for motor evoked potential

Abstract

Fast charge (FC) and slow charge (SC) stimulation protocols are both available on modern neuromonitoring machines in order to induce motor evoked potentials (MEP). In order to define the best strategy for MEP recording, we have compared the efficiency and the feasibility of both stimulation protocols for the induction of motor evoked potential in neurosurgical patients that present without myelopathy. Fifteen consecutive adult patients (13 arthrodesis and 2 kyphoplasties) without clinical motor deficit undergoing spine surgery under neuromonitoring were included in this study. Fast (50 μs) and slow (500 ms) charge biphasic cortical stimulation was applied between C3 and C4 electrodes in trains of 5 at 500 Hz. Muscular response was quantified as the area under the curve between the MEP start and end time. We were able to deliver FC and SC in all patients. SC induced always-measurable MEP response up to 140 V. On the contrary, in some patients FC induced large recording artefacts that precluded measurement. Current delivery saturated between 100 (129 ± 32 mA) and 140 (117 ± 62 mA) V for SC while this was not the case for FC up to 450 V (660 ± 123 mA). In the same way MEP area under the curve value reached a plateau between 120 and 140 V for SC (96% of maximal response) while FC induced MEP area under the curve varied linearly with stimulation current (R2 = 0.998). When both stimulations were directly compared, FC was able to induce a maximal response that was approximately 1.3 times larger than SC for the same muscle in the same patient. In our experience, FC cortical stimulation is more efficient and more potent than SC stimulation. The only drawback of FC compared to SC is the large stimulation artefact when the ground electrode is placed on the scalp. The value of displacing the ground electrode to the sternum or the coccyx to minimize stimulation artefacts needs to be studied in the future. The results of FC stimulation also needs to be verified in myelopathic patients.