11. Evoked movements in transcranial electrical stimulation on 4 locations of the body at different stimulation electrode montages and stimulation paradigms

Abstract

Introduction Muscle contraction from transcranial electrical stimulation (TES) induces seismic potentials: sMEP. These movements may be of discomfort to the surgeon or address safety issues. In this study sMEPs at the chin, occiput, at mid thoracic and thoracic-lumbar levels are studied at single and multipulse TES with 2 TES montage pairs. Methods Eight patients (5f/3 m, 22 ± 12 y) underwent TES-mMEP monitoring during spinal deformity surgery. sMEPs were recorded by 2-vector accelero transducers on 4 locations during the set-up procedure for TES-mMEP monitoring at the selected TES voltage for monitoring. The sMEPampl vector pairs were summed before analysis. Single and multi-pulse trains (1 or 5 ppt) TES was performed at Cz- Fz and C3-C4 montages. Results At CzFz (Cz anode), the latencies increase from chin and occiput to mid thoracic and thoraco-lumbar from 3.8 ± 0.6 ms to 12.4 ± 2.3 ms when 1 ppt is used. sMEPampl decrease from 1.7 ± 1.3 mV (chin) to 0.087 ± 0.008 mV (thoraco-lumbar). When using 5 ppt, latencies increase from 4.1 ± 0.6 ms (chin) to 12.9 ± 2.30 ms (thoraco-lumbar). sMEPampl decrease from 2.3 ± 1.0 mV (chin) to 0.05 ± 0.03 mV (thoraco-lumbar). At C3C4, latencies are similar to CzFz. sMEPampl at the chin is higher at C3C4 than at CzFz, where sMEPampl at the chin is 2.0 ± 1.6 mV and at thoraco-lumbar level 0.058 ± 0.021 mV for 1ppt, and 2.9 ± 2.1 (chin) to 0.42 ± 0.44(thoraco-lumbar) for 55pt. The higher sMEP amplitudes at the chin could result from stronger electrical fields from the C3/C4 electrodes close to the masseter muscle. Conclusion Because of the short latency times at 1 and 5 ppt, the movements of the jaw and head most likely result from extracranial stimulation, while sMEPampl at thoracic and lumbar locations apparently originate from the cortico-spinal route.