The role of intra-operative motor evoked potentials in the optimization of chronic cortical stimulation for the treatment of neuropathic pain.
OBJECTIVE
To explore the significance of intra-operative motor evoked potentials (MEPs) obtained by monopolar and bipolar stimulation in determining the location of the electrode(s) giving most pain relief in chronic motor cortex stimulation (MCS).


METHODS
Eight patients with chronic refractory neuropathic pain were implanted epidurally with two parallel leads of four electrodes each and placed normal to the central sulcus (CS). We measured the peak-peak amplitude (V(p-p)) of the MEPs recorded intra-operatively at the contralateral hand with the same stimulus delivered by each single electrode used as an anode or a cathode. Those electrodes giving the largest MEPs in monopolar stimulation were also tested in bipolar stimulation with an adjacent electrode located on the same or the other lead. It was analyzed whether a relation was present between the electrode providing the largest V(p-p) in the monopolar condition and the bipolar combination selected for chronic stimulation.


RESULTS
In monopolar stimulation the median amplitude of MEPs evoked with an anode was 59% larger than with a cathode. The mean amplitude of the bipolarly evoked MEPs was only 21% and 37%, respectively, of the corresponding monopoles when the anode and cathode were separated by 6mm and by more than 8mm. A significant pain relief was obtained in 5 out of 8 patients post-operatively. In all these patients, one of the cathodes used in chronic stimulation was one of the anodes producing the largest MEP intra-operatively. Conversely, in the 3 patients who did not benefit from MCS, one of the cathodes used in chronic stimulation was one of the cathodes producing the largest MEPs intra-operatively.


CONCLUSIONS
Monopolar stimulation should be applied in intra-operative neurophysiological testing because, contrary to bipolar stimulation, the corresponding MEPs are unambiguously related to a single stimulating electrode and their amplitude is not affected by the anode-cathode distance. The anode providing the largest MEPs intra-operatively should be selected as the cathode in chronic stimulation. However, implantable pulse generators allowing monopolar (cathodal and anodal) stimulation for MCS should become available to compare the respective analgesic efficacy of monopolar and bipolar chronic cortical stimulation.


CONCLUSIONS
Intra-operative MEP recordings can predict which electrode should be used as the cathode to obtain the best analgesic effect with chronic MCS.
Holsheimer
Jan
J
Institute for Biomedical Technology, University of Twente, Enschede, The Netherlands. jan@holsheimer.nl
Lefaucheur
Jean-Pascal
JP
Buitenweg
Jan R
JR
Goujon
Colette
C
Nineb
Amine
A
Nguyen
Jean-Paul
JP
D000328:Adult; D000368:Aged; D003627:Data Interpretation, Statistical; D004599:Electric Stimulation Therapy; D004567:Electrodes, Implanted; D004824:Epidural Space; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005073:Evoked Potentials, Somatosensory / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D007432:Intraoperative Period; D008297:Male; D008475:Median Nerve / Q000502:physiology; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D019635:Neurosurgical Procedures; D010146:Pain / Q000209:etiology; D059408:Pain Management; D010147:Pain Measurement; D010523:Peripheral Nervous System Diseases / Q000150:complications*