6. Transcranial voltage and current thresholds of muscle potentials in upper and lower extremities related to head impedance and electrode montage

Abstract

Previous studies show that, in contrast to constant voltage stimulation, constant current stimulation is least sensitive to electrode impedance but most sensitive to shunting effects from the scalp. This study evaluates the transcranial voltage (VMT) motor and current (CMT) motor thresholds to elicit muscle potentials (MEP) in all extremities for saggittal and coronal electrode montages. 50 Neurologically healthy patients diagnosed with idiopathic scoliosis were included. 2 Needle electrodes (length 35 mm, diameter: 0.4 mm) were placed at Cz (anode) and a cautery ground plate electrode was placed over the forehead (‘ ∼ F’; cathode). Additionally 2 electrodes were inserted at C3 and C4. Saggittal and coronal stimulation was respectively mono- and biphasic. Pulse width was 100 μ s. Electrode impedances were measured for both montages. The CMT was estimated by division of VMT by impedance (IMP). MTs were obtained of the abductor pollicis brevis (APB) and anterior tibial (AT) muscles bilaterally. The relation between VMT or CMT and IMP was statistically tested using the Pearson correlation coefficient ( r ). P < 0.05 (Bonferroni corrected) was considered significant. Mean IMP CzF = 178 ± 27.5 Ω , mean IMP C 3 C 4 = 323 ± 61.4 Ω . Correlation between VMT and IMP was absent. High significant weak negative correlations were found between CMT and IMP in the CzF groups and the C3C4 groups. Despite the fact that CMTs are likely biased estimates they show a highly significant and strong dependence on the electrode impedance, which is not the case for VMTs. This might be ascribed to the current shunting through the scalp layer. The weak correlation likely reflects wide interindividual variations of other impedance influencing parameters.