Experimental Study of the Course of Threshold Current, Voltage and Electrode Impedance During Stepwise Stimulation From the Skin Surface to the Human Cortex

Background: Transcranial electric stimulation as used during intraoperative neurostimulation is dependent on electrode and skull impedances. Objective: Threshold currents, voltages and electrode impedances were evaluated with electrical stimulation at 8 successive layers between the skin and the cerebral cortex. Patients and Methods: Data of 10 patients (6f, 53 +/- 11 years) were analyzed. Motor evoked potentials were elicited by constant current stimulation with corkscrew type electrodes (CS) at C3 and C4 in line with standard transcranial electric stimulation. A monopolar anodal ball tip shaped probe was used for all other measurements being performed at the level of the skin, dura and cortex, as well as within the skull by stepwise performed burr holes close to C3 resp. C4. Results: Average stimulation intensity, corresponding voltage and impedance for muscle MEPs at current motor threshold (CMT) were recorded: CS 54 +/- 23 mA (mean +/- SD), 38 +/- 21 V. 686 +/- 146 Omega; with the monopolar probe on skin 55 +/- 28 mA, 100 +/- 44 V. 1911 +/- 683 Omega and scalp 59 +/- 32 mA, 56 +/- 28 V. 1010 +/- 402 Omega; within the skull bone: outer compact layer 33 23 mA, 91 +/- 53 V. 3734 +/- 2793 Omega; spongiform layer 33 +/- 23 mA, 70 +/- 44 V.2347 +/- 1327 Omega; inner compact layer (ICL) 28 +/- 19 mA, 48 +/- 23 V. 2103 +/- 14980; on dura 25 +/- 12 mA, 17 +/- 12 V.643 +/- 244 Omega and cortex 14 +/- 6 mA, 11 +/- 5 V.859 +/- 300 Omega. CMTs were only significantly different for CS (P = 0.02) and for the monopolar probe between the cortex and ICL (P = 0.03), scalp (P = 0.01) or skin (P = 0.01) and between ICL and CS (P <= 0.01) or skin (P <= 0.01). Conclusion: The mean stimulation current of the CMT along the extracranial to intracranial anodal trajectory followed a stepwise reduction. VMT was strongly dependent on electrode impedance. CMT within the skull layers was noted to have relative strong shunting currents in scalp layers. (C) 2013 Elsevier Inc. All rights reserved.