title: Transcranial electric stimulation for intraoperative motor evoked potential monitoring: dependence of required stimulation current on interstimulus interval value.
abstract: OBJECTIVE
  To evaluate the relationship between stimulus intensity by constant current transcranial electric stimulation and interstimulus interval (ISI) for eliciting muscle motor evoked potentials (MEPs) in three different hand muscles and the tibialis anterior muscles.
  
  
  METHODS
  We tested intraoperatively different monophasic constant current pulses and ISIs in 22 patients with clinically normal motor function. Motor thresholds of contralateral muscle MEPs were determined at 0.5 milliseconds (ms) pulse duration and ISIs of 1, 2, 3, 4, 5, and 10 ms using a train of 2, 3, and 5 monophasic constant current pulses of 62 to 104 mA before craniotomy and after closure of the dura mater.
  
  
  RESULTS
  The lowest stimulation threshold to elicit MEPs in the examined muscles was achieved with a train of 5 pulses (ISI: 3 ms) before craniotomy, which was statistically significant compared with 2 pulses (ISI: 3 ms) as well as 3 pulses (ISIs: 3 and 10 ms). An ISI of 3 ms gave the lowest motor thresholds with statistical significance compared with the ISIs of 4 ms (2 pulses) and of 1 ms (3 pulses). All current intensity (mA) and ISI (ms) relationship graphs had a trend of the exponential function as y = a + bx + c ρ (x), where y is intensity (mA) and x is ISI (ms). The minimum of the function was determined for each patient and each muscle. The difference was statistically significant between 3 and 5 pulses before craniotomy and between 3 and 5 pulses and 2 and 5 pulses after closure of the dura mater.
  
  
  CONCLUSIONS
  In adult neurosurgical patients with a normal motor status, a train of 5 pulses and an ISI of 3 ms provide the lowest motor thresholds. We provided evidence of the dependence of required stimulation current on ISI.
authors.affiliation: Clinic for Neurosurgery, Clinical Centre of Serbia, Belgrade, Serbia.
authors.forename: Boban
authors.initials: B
authors.lastname: Joksimovic
authors.affiliation: Neurosurgical Clinic, University Clinic Dűsseldorf, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Szelenyi
authors.affiliation: Department for Neurosurgery, University Clinic Frankfurt, Germany.
authors.forename: Volker
authors.initials: V
authors.lastname: Seifert
authors.affiliation: Clinic for Psychiatry, Clinical Centre of Serbia, Belgrade, Serbia.
authors.forename: Aleksandar
authors.initials: A
authors.lastname: Damjanovic
authors.affiliation: Clinic for Psychiatry, Clinical Centre of Serbia, Belgrade, Serbia.
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Damjanovic
authors.affiliation: Clinic for Neurosurgery, Clinical Centre of Serbia, Belgrade, Serbia.
authors.forename: Lukas
authors.initials: L
authors.lastname: Rasulic
mesh_terms: D000328:Adult; D000368:Aged; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D018482:Muscle, Skeletal / Q000502:physiology*; D019635:Neurosurgical Procedures / Q000379:methods*; D015173:Supratentorial Neoplasms / Q000601:surgery*; D065908:Transcranial Direct Current Stimulation / Q000379:methods*