title: Intra-operative subcortical electrical stimulation: a comparison of two methods.
abstract: OBJECTIVE
  For intra-operative subcortical electrical stimulation of the corticospinal tract, two techniques - originally described for cortical stimulation - have evolved: the 50-Hz-stimulation first described by Penfield in 1937 and the high-frequency multipulse train stimulation technique first described by Taniguchi in 1993. Motor thresholds of both methods in combination with a bipolar and monopolar stimulation technique and their reliability for eliciting motor evoked potentials (MEPs) were studied.
  
  
  METHODS
  Data were obtained in 20 patients (50±17 years; 10 females) undergoing tumour resection under general anaesthesia. Both 50-Hz-stimulation of 1-s duration and a multipulse stimulation (5 pulses interstimulus interval 4 ms, 0.5-Hz repetition rate) were applied with a bipolar probe (1.5-mm ball tip, 8-mm interelectrode distance) and a monopolar probe (1.5-mm-diameter tip). MEPs were recorded in muscles contralateral to the stimulated hemisphere. Comparison of different stimulation modalities was performed at the site where monopolar multipulse stimulation technique elicited MEPs with the lowest stimulation intensity (constant current monophasic cathodal stimulation, individual pulse width 0.5 ms, max. 25 mA).
  
  
  RESULTS
  MEPs were elicited by monopolar multipulse stimulation with an intensity of 8±3.9 mA (21/21 stimulation sites); monopolar 50-Hz stimulation with 12±5.4 mA (18/21 stimulation sites); bipolar multipulse stimulation with 14±8.1 mA (12/21 stimulation sites) and bipolar 50-Hz stimulation with 15±6.3 mA (11/21 stimulation sites).
  
  
  CONCLUSIONS
  Stimulation intensities for eliciting MEPs are significantly lowest for the monopolar multipulse stimulation (p<0.025). Monopolar compared to bipolar stimulation resulted in eliciting MEPs in a higher number of tested patients (Fisher's p<0.0001).
  
  
  CONCLUSIONS
  Subcortical stimulation with a monopolar probe and a multipulse stimulation is most efficient for the purpose of identifying the corticospinal tract. This is explained by the more radiant electric field properties of the monopolar probe compared to the bipolar probe.
authors.affiliation: Department of Neurosurgery, Johann-Wolfgang Goethe-University Hospital, Frankfurt/Main, Germany. A.Szelenyi@em.uni-frankfurt.de
authors.forename: A
authors.initials: A
authors.lastname: Szelényi
authors.forename: C
authors.initials: C
authors.lastname: Senft
authors.forename: M
authors.initials: M
authors.lastname: Jardan
authors.forename: M T
authors.initials: MT
authors.lastname: Forster
authors.forename: K
authors.initials: K
authors.lastname: Franz
authors.forename: V
authors.initials: V
authors.lastname: Seifert
authors.forename: H
authors.initials: H
authors.lastname: Vatter
mesh_terms: D000328:Adult; D000368:Aged; D000768:Anesthesia, General; D001932:Brain Neoplasms / Q000601:surgery; D002540:Cerebral Cortex / Q000502:physiology*; D003627:Data Interpretation, Statistical; D004558:Electric Stimulation / Q000379:methods*; D004566:Electrodes; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D019635:Neurosurgical Procedures; D011712:Pyramidal Tracts / Q000502:physiology*