Transcranial electrical stimulation: significance of fast versus slow charge delivery for intra-operative monitoring

Objectives: Motor-evoked potentials (MEP) for intra-operative monitoring due to fast charge (fc: 1.0 Coulomb/s) and slow charge (sc: 0.1 Coulomb/s) delivery for multipulse transcranial electrical stimulation (TES) were compared. Methods: MEPs due to fc (n=162) and sc stimulation (n=182) were performed in parallel in a prospective study. The fc stimulation technique is characterized by an increased steepness of charge delivery with consequent reduction of stimulus duration of 50 μs compared to 500 μs in sc stimulation. Stimulation charges (C=Coulomb) and MEP parameters during spine surgery were analyzed. Results: MEPs were successfully recorded in 15/18 patients under total intravenous anesthesia. The mean charge to induce intra-operative MEPs (stimulation threshold) was significantly less in fc (0.195 mC) as compared to sc stimulation (0.298 mC). With both stimulation techniques, in all patients without impairment of motor function, MEPs could be recorded and no technique was superior with respect to successful stimulation. The mean MEP latencies, amplitudes and the extent of intra-individual variation of MEP parameters during surgery (shift of latency less than 10%, variability of amplitude less than 50%) were not different with both stimulation techniques. Conclusions: TES with either fc or sc stimulation can be used reliably for intraoperative monitoring. Fc and sc stimulation are comparable with respect to feasibility, intra-individual variability and mean parameters of MEP responses. However, fc stimulation provides a higher stimulation efficiency and requires about 35% less total charge for MEP monitoring