title: Experimental study of the course of threshold current, voltage and electrode impedance during stepwise stimulation from the skin surface to the human cortex.
abstract: 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.
  
  
  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 Ω; with the monopolar probe on skin 55 ± 28 mA, 100 ± 44 V, 1911 ± 683 Ω and scalp 59 ± 32 mA, 56 ± 28 V, 1010 ± 402 Ω; within the skull bone: outer compact layer 33 ± 23 mA, 91 ± 53 V, 3734 ± 2793 Ω; spongiform layer 33 ± 23 mA, 70 ± 44 V, 2347 ± 1327 Ω; inner compact layer (ICL) 28 ± 19 mA, 48 ± 23 V, 2103 ± 1498 Ω; on dura 25 ± 12 mA, 17 ± 12 V, 643 ± 244 Ω and cortex 14 ± 6 mA, 11 ± 5 V, 859 ± 300 Ω. 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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department for Neurosurgery, Johann Wolfgang Goethe-University Hospital, Frankfurt a. M., Germany. Andrea.Szelenyi@uni-duesseldorf.de
authors.forename: Andrea
authors.initials: A
authors.lastname: Szelényi
authors.forename: Henricus Louis
authors.initials: HL
authors.lastname: Journée
authors.forename: Simon
authors.initials: S
authors.lastname: Herrlich
authors.forename: Gianni M
authors.initials: GM
authors.lastname: Galistu
authors.forename: Joris
authors.initials: J
authors.lastname: van den Berg
authors.forename: J Marc C
authors.initials: JM
authors.lastname: van Dijk
mesh_terms: D000328:Adult; D000368:Aged; D003399:Craniotomy; D017097:Electric Impedance; D004558:Electric Stimulation; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D009044:Motor Cortex / Q000502:physiology*; D012867:Skin
keywords: Brain; Current stimulation; Direct cortical stimulation; Impedance; Motor evoked potential; Scalp; Skull; Transcranial stimulation; Voltage stimulation