Influence of electrode impedance on threshold voltage for transcranial electrical stimulation in motor evoked potential monitoring

Abstract

Motor potentials evoked by transcranial electrical stimulation (TES) are used for monitoring the motor pathways, with emphasis on the spinal cord and brainstem. The stimulus voltage threshold is the voltage below which no motor response can be elicited. It has frequently been used as a monitoring parameter. However, its value can be limited, because it is affected by the impedance of the stimulus electrode. For example, the voltage threshold can change owing to formation of oedema of the scalp. The relationship between the TES voltage threshold and the electrode impedance of different electrode types was studied and discussed in the context of neuromonitoring: 323 impedance and voltage threshold pairs were studied, and TES was performed with disc cup EEG electrodes (six), corkscrew electrodes (type I: seven, type II: eight), multiple EEG needle electrodes (16) and a large needle electrode Cz' (anode) together with a ground strip over the forehead (cathode) (286). The study found the voltage threshold to be strongly dependent on electrode impedance when the impedance was higher than 460 omega (correlation: R2=0.87; p < 0.001). Below 460 omega, which included 91% of the category with the largest electrode surfaces, 25% of the multiple EEG electrodes and 75% of type II corkscrew electrodes, no significant correlation (R2=0.0064; p=0.15) was found. It was concluded that the correlation between the TES voltage threshold and electrode impedance can be markedly reduced by using TES electrodes with large contact surfaces, resulting in limit values for these parameters. This also may improve the reliability of TES motor evoked potential monitoring.

Extramuscular Recording of Spontaneous EMG Activity and Transcranial Electrical Elicited Motor Potentials in Horses: Characteristics of Different Subcutaneous and Surface Electrode Types and Practical Guidelines

“…Between electrode impedances of 300 and 500 Ω, the relation between impedance and electrode contact surface transits from an uncorrelated independent to a linear relationship, in which the segmental impedance between the electrodes at 2.5 kHz becomes overruled by the local electrode impedance ( Journée et al, 2004 ; Berends and Journée, 2018 ). Only the latter is proportionally related with the electrode contact surface ( Journée et al, 2004 ; Berends and Journée, 2018 ). The finding of a further impedance decrease after 1 h in one horse confirms that the signal quality of surface electrodes further improves after a longer settling time.…”

“…Most of the noise is likely of electrochemical origin at the electrode contact interface and depends on the composition of the electrode gel (Piervirgili et al, 2014). In human for example, impedances of adhesive surface AG/AgCl electrodes can be 1-2 magnitudes higher than that reported for sc needle electrodes (Grimnes, 1983;Huigen et al, 2002;Journée et al, 2004). High electrode impedances also increase the sensitivity for power line noise (Piervirgili et al, 2014;Merletti et al, 2016).…”

Intraoperative Neurophysiology Monitoring for Spinal Dysraphism

“…Stimulation is delivered by transcranial electrical stimulation via subdermal needle electrodes in the scalp. Arrangement of electrodes (also called as "montage") is recommended at C3 and C4 in the scalp for monitoring upper or lower extremity MEP; Cz-Fz montage or C1-C2 montage can be also considered for lower extremity MEP stimulation but C3-C4 montage is more efficacious to yield lower extremity MEP than C1-C2 montage 6) and Cz-Fz electrodes are usually spared for sensory evoked potentials. Stimulation intensity should be maintained at the supra-maximal level.…”

Local tissue electrical resistances in transpedicular screw application in thoracolumbar region

“…Malnutrition may, independent of respiratory functions, decrease protein synthesis in chronic obstructive pulmonary disease (COPD) and lead to a reduction in body fat and muscle mass, a condition that can be quantified by calculating lean body mass with bioelectrical impedance measurement (23). Some studies have examined the effect of the difference of the electrode's impedance on the threshold value used for transcranial electrical stimulation during evoked motor potential monitorization (14).…”