Letter to the Editor: Electric current application for motor tract mapping

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TO THE EDITOR: We read with interest the articles by Raabe et al.4 (Raabe A, Beck J, Schucht P, et al: Continuous dynamic mapping of the corticospinal tract during surgery of motor eloquent brain tumors: evaluation of a new method. J Neurosurg 120:1015–1024, May 2014) and Shiban et al.8 (Shiban E, Krieg SM, Haller B, et al: Intraoperative subcortical motor evoked potential stimulation: how close is the corticospinal tract? J Neurosurg 123:711–720, September 2015).

Stimulation of the corticospinal motor tract has been for a long time a valuable intraoperative technique of localization. Initially performed in awake surgeries with the so-called 60-Hz stimulation technique for cortical mapping,5 it is nowadays used under general anesthesia and even with the “trains of five” technique.2 The latter does not generate 60 pulses anymore, but 5 stimulating pulses per second, decreasing substantially the electric charge, and thereby reducing the risk of eliciting stimulation-induced seizures during mapping (approximately 720 μC with 1-msec pulse duration for the 60-Hz stimulation, vs 24 μC with 400-μsec pulse duration for trains of five at, for example, 12 mA).

Beyond localization, mapping techniques are becoming more and more a preventive tool for the anatomical delineation of a safe resection distance to the corticospinal tract.6,7 Recently it has been proposed that as an alternative to a dedicated stimulation probe, an aspirator could be used as the stimulation tool.4 This avoids the frequent change of tools in the hands of the surgeon that still hampers the process of surgery when using conventional stimulation probes. Stimulation through one of the surgical tools also offers the possibility for continuous mapping. In line with this original idea, stimulation through the resection tool itself (Söring handpiece, Söring GmbH; Cavitron ultrasonic surgical aspirator [CUSA], Integra) was initiated.

Stimulation through the resection tools per se is based on the knowledge of electrical principles: when applying an electrical current to a conducting path, free electrons flow in the wire as soon as the path is closed (i.e., once the tool touches the brain); when a current source stimulator is used, as offered by contemporary neuromonitoring systems, the same amount of electrical charges will flow through the conducting path, whatever its impedance.

As shown in Fig. 1, this path is made of the following: 1) the distal part of the tool (i.e., one part of the spherical or ring-shaped tip of the tool in contact with the brain); 2) the surface of the brain that is in contact with this tool; and 3) the return electrode in contact with the patient (e.g., subdermal electrodes placed contralaterally to the site of surgery). Figure 2 shows 4-mA currents flowing within the conducting path made with, from top to bottom, a stimulation probe (Inomed, stainless steel); an aspirator (Inomed, stainless steel); a CUSA tip (e.g., titanium nitride); and a Söring handpiece (titanium aluminum) applied on the biological tissue, measured across a 1-kOhm resistance, placed in series within the aforementioned current loop.

FIG. 1.
FIG. 1.

Scheme of the equivalent circuit of cortical or subcortical mapping.

FIG. 2.
FIG. 2.

Voltage generated by a 4-mA current, measured across a 1-kOhm resistance, placed in series within the loop made of the stimulator—either (from top to bottom) a stimulation probe (Inomed), an aspirator (Inomed), a CUSA tip, or a Söring handpiece—and the patient's brain. Irrespective of the probe, the same current flows through the loop. Note that the polarity of the pulses was alternated through the use of biphasic pulses for safety considerations (see Merrill et al.3 and Brummer and Turner1). Figure is available in color online only.

Again, whatever is used as a stimulation tool—a probe per se, an aspirator, or a resection tool—the current flowing through the brain will be the same, depending only on the amplitude requested to the current source. Measuring the muscle contraction thresholds with these different tools is neglecting these universal electrical rules. The measurements that do not respect the pure linear relation in Fig. 2 of Raabe et al.4 can only be attributed to measurement errors, like the reproducibility of the stimulation location. The variance described in Fig. 4 of Shiban et al.8 is also due to such measurement errors.

In the 21st century, while so-called smart and connected tools are becoming part of our lives, neurosurgical techniques benefit from integrating multidisciplinary teams, including engineers, in their daily work.

References

  • 1

    Brummer SBTurner MJ: Electrochemical considerations for safe electrical stimulation of the nervous system with platinum electrodes. IEEE Trans Biomed Eng 24:59631977

    • Search Google Scholar
    • Export Citation
  • 2

    Deletis V: Intraoperative monitoring of the functional integrity of the motor pathways. Adv Neurol 63:2012141993

  • 3

    Merrill DRBikson MJefferys JG: Electrical stimulation of excitable tissue: design of efficacious and safe protocols. J Neurosci Methods 141:1711982005

    • Search Google Scholar
    • Export Citation
  • 4

    Raabe ABeck JSchucht PSeidel K: Continuous dynamic mapping of the corticospinal tract during surgery of motor eloquent brain tumors: evaluation of a new method. J Neurosurg 120:101510242014

    • Search Google Scholar
    • Export Citation
  • 5

    Rasmussen TPenfield W: The human sensorimotor cortex as studied by electrical stimulation. Fed Proc 6:1841947

  • 6

    Sala FLanteri P: Brain surgery in motor areas: the invaluable assistance of intraoperative neurophysiological monitoring. J Neurosurg Sci 47:79882003

    • Search Google Scholar
    • Export Citation
  • 7

    Seidel KBeck JStieglitz LSchucht PRaabe A: The warning-sign hierarchy between quantitative subcortical motor mapping and continuous motor evoked potential monitoring during resection of supratentorial brain tumors. J Neurosurg 118:2872962013

    • Search Google Scholar
    • Export Citation
  • 8

    Shiban EKrieg SMHaller BBuchmann NObermueller TBoeckh-Behrens T: Intraoperative subcortical motor evoked potential stimulation: how close is the corticospinal tract?. J Neurosurg 123:7117202015

    • Search Google Scholar
    • Export Citation

Disclosures

Mrs. Boëx has given lectures during workshops for Medtronic.

Keywords:

Response

No response was received from Shiban et al.

Response

Improvement of mapping strategies is increasingly becoming a focus in supratentorial tumor surgery. Initially only performed on the cortex,2,10 mapping was soon also established at the subcortical level1,3 to preserve the white matter tracts. Recently, the spatial and temporal limitations of the conventional intermittent mapping technique have been overcome to create the possibility of continuous “dynamic” subcortical stimulation. This can only be achieved by integrating the stimulation probe in a surgical instrument that is used during tumor resection.14 We favored the integration of mapping into a standard suction device because continuous dynamic mapping should be available during all steps of tumor removal, including hemostasis or subpial dissection, which may be performed with a variety of instruments.

To obtain information about distance from a specific site of resection to the CST, many groups have correlated the stimulation intensity (in mA) needed to elicit motor evoked potentials with the distance (in mm) to the CST.4,5,7–9,11 Until now, no definitive statement on this relationship has been possible, but the rule of thumb “1 mA correlates to 1 mm” is increasingly being used. We know that this is only a rough estimate, but it has turned out to be a reliable and practical one. We have also tried to approach the dilemma from another perspective by correlating the lowest stimulation intensities during subcortical mapping to postoperative short-term and long-term motor outcome.6,13

Taking all these aspects into account, the importance of the statement made in the Letter's original title (“About the electric current applied for motor tract mapping”) becomes evident. Tissue impedance might influence motor thresholds during stimulation, and therefore constant current or current source stimulators are important.13 In the letter from Boëx et al., the experiment in Fig. 2 clearly demonstrates that the current will be the same regardless of the applied stimulation device (monopolar probe, suction device, or CUSA). Therefore the small differences in some measurements between suction device and monopolar probe (Fig. 2 of Raabe et al.) as well as between CUSA and monopolar probe (Fig. 4 of Shiban et al.14) might be explained by measurement errors arising because the stimulation site or depth used are not exactly the same, but they might also be attributable to other confounding factors.

It is particularly important to highlight once more that when discussing correlation of stimulation intensities and distance to the CST, more neurophysiological aspects should be considered, especially when comparing different studies.1,12,13,15

  • Stimulation paradigm: Penfield stimulation (50 or 60 Hz) versus short-train/high-frequency/train-of-five stimulationAnd, especially when discussing short-train stimulation studies, the following should be taken into account.12,13
  • Pulse duration
  • Pulse configuration (anodal vs cathodal)
  • Number of pulses in a train
  • Interstimulus interval between the pulses
  • Repetition rate of the individual trains

The choice of the stimulation probe configuration (monopolar vs bipolar) will also influence that correlation independently of the selected stimulation paradigm (Penfield vs short train).1,12,13,15

We believe that the integration of the stimulation probe into the surgical instrument could increase the reliability, acceptance, and clinical handling of subcortical mapping. We thank the reviewers for highlighting this topic once more.

References

  • 1

    Bello LRiva MFava EFerpozzi VCastellano ARaneri F: Tailoring neurophysiological strategies with clinical context enhances resection and safety and expands indications in gliomas involving motor pathways. Neuro Oncol 16:111011282014

    • Search Google Scholar
    • Export Citation
  • 2

    Berger MSRostomily RC: Low grade gliomas: functional mapping resection strategies, extent of resection, and outcome. J Neurooncol 34:851011997

    • Search Google Scholar
    • Export Citation
  • 3

    Duffau H: Contribution of cortical and subcortical electro-stimulation in brain glioma surgery: methodological and functional considerations. Neurophysiol Clin 37:3733822007

    • Search Google Scholar
    • Export Citation
  • 4

    Kamada KTodo TOta TIno KMasutani YAoki S: The motor-evoked potential threshold evaluated by tractography and electrical stimulation. J Neurosurg 111:7857952009

    • Search Google Scholar
    • Export Citation
  • 5

    Kombos TSüss OVajkoczy P: Subcortical mapping and monitoring during insular tumor surgery. Neurosurg Focus 27:4E52009

  • 6

    Landazuri PEccher M: Simultaneous direct cortical motor evoked potential monitoring and subcortical mapping for motor pathway preservation during brain tumor surgery: is it useful?. J Clin Neurophysiol 30:6236252013

    • Search Google Scholar
    • Export Citation
  • 7

    Maesawa SFujii MNakahara NWatanabe TWakabayashi TYoshida J: Intraoperative tractography and motor evoked potential (MEP) monitoring in surgery for gliomas around the corticospinal tract. World Neurosurg 74:1531612010

    • Search Google Scholar
    • Export Citation
  • 8

    Nossek EKorn AShahar TKanner AAYaffe HMarcovici D: Intraoperative mapping and monitoring of the corticospinal tracts with neurophysiological assessment and 3-dimensional ultrasonography-based navigation. Clinical article. J Neurosurg 114:7387462011

    • Search Google Scholar
    • Export Citation
  • 9

    Ohue SKohno SInoue AYamashita DHarada HKumon Y: Accuracy of diffusion tensor magnetic resonance imaging-based tractography for surgery of gliomas near the pyramidal tract: a significant correlation between subcortical electrical stimulation and postoperative tractography. Neurosurgery 70:2832942012

    • Search Google Scholar
    • Export Citation
  • 10

    Penfield W: Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60:3894431937

    • Search Google Scholar
    • Export Citation
  • 11

    Prabhu SSGasco JTummala SWeinberg JSRao G: Intraoperative magnetic resonance imaging-guided tractography with integrated monopolar subcortical functional mapping for resection of brain tumors. Clinical article. J Neurosurg 114:7197262011

    • Search Google Scholar
    • Export Citation
  • 12

    Seidel KBeck JStieglitz LSchucht PRaabe A: Low-threshold monopolar motor mapping for resection of primary motor cortex tumors. Neurosurgery 71:1 Suppl Operative1041152012

    • Search Google Scholar
    • Export Citation
  • 13

    Seidel KBeck JStieglitz LSchucht PRaabe A: The warning-sign hierarchy between quantitative subcortical motor mapping and continuous motor evoked potential monitoring during resection of supratentorial brain tumors. J Neurosurg 118:2872962013

    • Search Google Scholar
    • Export Citation
  • 14

    Shiban EKrieg SMObermueller TWostrack MMeyer BRingel F: Continuous subcortical motor evoked potential stimulation using the tip of an ultrasonic aspirator for the resection of motor eloquent lesions. J Neurosurg 123:3013062015

    • Search Google Scholar
    • Export Citation
  • 15

    Szelenyi ASenft CJardan MForster MTFranz KSeifert V: Intraoperative subcortical electrical stimulation: a comparison of two methods. Clin Neurophysiol 122:147014752011

    • Search Google Scholar
    • Export Citation

Disclosures

Dr. Raabe is a consultant for Inomed.

If the inline PDF is not rendering correctly, you can download the PDF file here.

Article Information

Contributor Notes

INCLUDE WHEN CITING Published online April 15, 2016; DOI: 10.3171/2015.12.JNS152830.
Headings
Figures
  • View in gallery

    Scheme of the equivalent circuit of cortical or subcortical mapping.

  • View in gallery

    Voltage generated by a 4-mA current, measured across a 1-kOhm resistance, placed in series within the loop made of the stimulator—either (from top to bottom) a stimulation probe (Inomed), an aspirator (Inomed), a CUSA tip, or a Söring handpiece—and the patient's brain. Irrespective of the probe, the same current flows through the loop. Note that the polarity of the pulses was alternated through the use of biphasic pulses for safety considerations (see Merrill et al.3 and Brummer and Turner1). Figure is available in color online only.

References
  • 1

    Brummer SBTurner MJ: Electrochemical considerations for safe electrical stimulation of the nervous system with platinum electrodes. IEEE Trans Biomed Eng 24:59631977

    • Search Google Scholar
    • Export Citation
  • 2

    Deletis V: Intraoperative monitoring of the functional integrity of the motor pathways. Adv Neurol 63:2012141993

  • 3

    Merrill DRBikson MJefferys JG: Electrical stimulation of excitable tissue: design of efficacious and safe protocols. J Neurosci Methods 141:1711982005

    • Search Google Scholar
    • Export Citation
  • 4

    Raabe ABeck JSchucht PSeidel K: Continuous dynamic mapping of the corticospinal tract during surgery of motor eloquent brain tumors: evaluation of a new method. J Neurosurg 120:101510242014

    • Search Google Scholar
    • Export Citation
  • 5

    Rasmussen TPenfield W: The human sensorimotor cortex as studied by electrical stimulation. Fed Proc 6:1841947

  • 6

    Sala FLanteri P: Brain surgery in motor areas: the invaluable assistance of intraoperative neurophysiological monitoring. J Neurosurg Sci 47:79882003

    • Search Google Scholar
    • Export Citation
  • 7

    Seidel KBeck JStieglitz LSchucht PRaabe A: The warning-sign hierarchy between quantitative subcortical motor mapping and continuous motor evoked potential monitoring during resection of supratentorial brain tumors. J Neurosurg 118:2872962013

    • Search Google Scholar
    • Export Citation
  • 8

    Shiban EKrieg SMHaller BBuchmann NObermueller TBoeckh-Behrens T: Intraoperative subcortical motor evoked potential stimulation: how close is the corticospinal tract?. J Neurosurg 123:7117202015

    • Search Google Scholar
    • Export Citation
  • 1

    Bello LRiva MFava EFerpozzi VCastellano ARaneri F: Tailoring neurophysiological strategies with clinical context enhances resection and safety and expands indications in gliomas involving motor pathways. Neuro Oncol 16:111011282014

    • Search Google Scholar
    • Export Citation
  • 2

    Berger MSRostomily RC: Low grade gliomas: functional mapping resection strategies, extent of resection, and outcome. J Neurooncol 34:851011997

    • Search Google Scholar
    • Export Citation
  • 3

    Duffau H: Contribution of cortical and subcortical electro-stimulation in brain glioma surgery: methodological and functional considerations. Neurophysiol Clin 37:3733822007

    • Search Google Scholar
    • Export Citation
  • 4

    Kamada KTodo TOta TIno KMasutani YAoki S: The motor-evoked potential threshold evaluated by tractography and electrical stimulation. J Neurosurg 111:7857952009

    • Search Google Scholar
    • Export Citation
  • 5

    Kombos TSüss OVajkoczy P: Subcortical mapping and monitoring during insular tumor surgery. Neurosurg Focus 27:4E52009

  • 6

    Landazuri PEccher M: Simultaneous direct cortical motor evoked potential monitoring and subcortical mapping for motor pathway preservation during brain tumor surgery: is it useful?. J Clin Neurophysiol 30:6236252013

    • Search Google Scholar
    • Export Citation
  • 7

    Maesawa SFujii MNakahara NWatanabe TWakabayashi TYoshida J: Intraoperative tractography and motor evoked potential (MEP) monitoring in surgery for gliomas around the corticospinal tract. World Neurosurg 74:1531612010

    • Search Google Scholar
    • Export Citation
  • 8

    Nossek EKorn AShahar TKanner AAYaffe HMarcovici D: Intraoperative mapping and monitoring of the corticospinal tracts with neurophysiological assessment and 3-dimensional ultrasonography-based navigation. Clinical article. J Neurosurg 114:7387462011

    • Search Google Scholar
    • Export Citation
  • 9

    Ohue SKohno SInoue AYamashita DHarada HKumon Y: Accuracy of diffusion tensor magnetic resonance imaging-based tractography for surgery of gliomas near the pyramidal tract: a significant correlation between subcortical electrical stimulation and postoperative tractography. Neurosurgery 70:2832942012

    • Search Google Scholar
    • Export Citation
  • 10

    Penfield W: Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60:3894431937

    • Search Google Scholar
    • Export Citation
  • 11

    Prabhu SSGasco JTummala SWeinberg JSRao G: Intraoperative magnetic resonance imaging-guided tractography with integrated monopolar subcortical functional mapping for resection of brain tumors. Clinical article. J Neurosurg 114:7197262011

    • Search Google Scholar
    • Export Citation
  • 12

    Seidel KBeck JStieglitz LSchucht PRaabe A: Low-threshold monopolar motor mapping for resection of primary motor cortex tumors. Neurosurgery 71:1 Suppl Operative1041152012

    • Search Google Scholar
    • Export Citation
  • 13

    Seidel KBeck JStieglitz LSchucht PRaabe A: The warning-sign hierarchy between quantitative subcortical motor mapping and continuous motor evoked potential monitoring during resection of supratentorial brain tumors. J Neurosurg 118:2872962013

    • Search Google Scholar
    • Export Citation
  • 14

    Shiban EKrieg SMObermueller TWostrack MMeyer BRingel F: Continuous subcortical motor evoked potential stimulation using the tip of an ultrasonic aspirator for the resection of motor eloquent lesions. J Neurosurg 123:3013062015

    • Search Google Scholar
    • Export Citation
  • 15

    Szelenyi ASenft CJardan MForster MTFranz KSeifert V: Intraoperative subcortical electrical stimulation: a comparison of two methods. Clin Neurophysiol 122:147014752011

    • Search Google Scholar
    • Export Citation
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