Movement along the Spine Induced by Transcranial Electrical Stimulation (TES) Related to the Montage of Electrodes

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Introduction Transcranial Electrical Stimulates Motor Evoked Potentials (TES-MEP) is a safe technique for neuromonitoring during spinal surgery. However induced movement can cause injuries and delay surgical procedures. When TES movements are evoked by other than monitored muscles, one may expect that adjustments in stimulation paradigms and electrode montages may minimize movement while preserving quality of monitoring. In this study, TES evoked seismic evoked responses (SER) are studied at different electrodes montage during spinal deformity surgery. Materials and Methods In 12 patients undergoing corrective spine surgery, intra-operative TES-MEP was performed. Accelero transducers recorded SER in two directions at 4 different locations of the spine for TES-electrode montages groups Cz-Fz and C3-C4. A resulting acceleration value, SERr, was computed. A paired t-test was used to compare the means of SER and the relationship between movement and TES electrode montages. Results SER's are the strongest in the upper body. All mean SER's values of the Cz-Fz group are up to 5 times larger when compared with the C3–4 group. However, there are no differences between the C3-C4 and Cz-Fz groups in the lower body locations. Both electrode montage groups show gradual decreasing steps of all mean SER values along the spine from cranial to caudal. For the upper body locations there were no significant correlations of SERr between both montages, whereas in contrast, a significant correlation of SERr exists at the lumbar region. Conclusions At supramaximum levels, movements resulting from multipulse TES is likely caused by relative strong contractions from muscles in the neck resulting from extracranial direct stimulation. When interchanging electrode montages in individual cases, the movement in the neck may become reduced. At lumbar levels transcranial evoked muscle contractions dominate movement in the surgical exposed areas likely dominated by local muscles.