WCTN: World Collaborative Textbook of Neurosurgery

Chapter IV.4: Neurophysiologic Monitoring During Spinal Cord Stimulation

Last modified by Max Gosey on 2010/06/18 15:52


Marina Abramova, MD - LSU Health Sciences Center, New Orleans, LA

About this chapter

EMG, or electromyography, and SSEP, or somatosensory evoked potentials, are two methods of neurophysiologic monitoring. This chapter describes how they can assist in ensuring correct placement of electrodes in spinal cord stimulation. Particular attention is paid to the mention of epidural paddle electrode systems.

Index of Sections

Section 4.1: General Uses of Neurological Monitoring in Spine Surgery

Section 4.2: Laminectomy Electrode Implantation, General Methods

Section 4.3: Technique of Midline Positioning of the Spinal Cord Stimulator – Tripolar paddle

Section 4.4: New frontiers of intraoperative EMG application

Section 4.5: References

Section IV.4.1: General Uses of Neurological Monitoring in Spine Surgery

Intraoperative neurophysiological monitoring has become a routine procedure in complex spine surgery.   Somatosensory-evoked potential (SSEP) recording has been advocated to monitor the functional integrity of the nervous system during surgical manipulation [22-24, 35].  When stimulated, sensory afferents give rise to signals, carried via the dorsal columns (DC), within the spinal cord to the medial lemniscus and spinocerebellar tracts, ending in the primary somatosensory cortex [4].  SSEP monitoring does not involve the motor pathways, which in some clinical situations can lead to false-negative results and postoperative neurological deficits undetected intraoperatively  [1-3, 5, 6, 7, 12].  Dermatomal SSEP testing allows for assessment of individual nerve roots during surgery and has been shown to be more sensitive [7, 8].  However, the sensitivity and specificity of this method varies and is less well-liked than electromyographic (EMG) monitoring [8, 9].  EMG has become the standard of practice in complex spine surgery, providing surgeons with accurate feedback about individual nerve root activity during surgical manipulation of neural structures [10-14].

Section IV.4.2: Laminectomy Electrode Implantation, General Methods

For a number of conditions, particularly the “postlaminectomy syndrome” (aka “failed back”) [25, 26, 31], and Complex Regional Pain Syndrome (CRPS I and II) [31, 32], spinal cord stimulation (SCS) has been found to be one of the more effective therapies [30, 31] cost effective [27-29].  Originally, it was based, conceptually, on the gate control theory of pain introduced by Melzack and Wall in 1965 [16], which proposed that the activity of large-diameter fibers (α/β fibers) in the DC plays an inhibitory role in transmission of pain signals to the brain.  SCS subsequently received FDA approval in 1979 as a class II device for chronic pain of the trunk and limbs.  Over 50,000 SCS systems are implanted for pain disorders around the world each year.  Certain principles have become clear through the use of SCS: 1) paresthesia coverage must precisely overlap the painful areas, the cathode is the “active” contact and must deliver its energy to the DC, and 2) accurate identification of the "physiological" midline (PM) is essential for treatment of bilateral pain.  Displacement of the cathode up to 1 mm from the affected  side works well for unilateral pain [18].  Placement of stimulating electrodes in the subdural space was initially described by Shealy [17].  This resulted in a high complication rate leading to placement of the electrodes in the epidural space. Percutaneous placement of spinal cord stimulator electrodes has several advantages including: minimal invasiveness, reduced cost, and increased flexibility in lead repositioning.  Percutaneous placement of electrodes is achieved with local anesthesia and fluoroscopic image guidance to ensure the proper placement of the leads [20, 34] and is routinely used for trial electrodes. However, percutaneous placement is still associated with a comparatively high rate of migration of the electrode (up to 13.5%)[19, 20, 34, 41].  Higher power requirements with omni-directional contacts result in increased battery drain and decreased device durability when used for permanent implants.  Accordingly, many centers prefer a laminectomy approach with placement of a “paddle lead” configuration.  This approach also allows the electrode to be directly anchored to the lamina at the site of entry to the epidural space [21, 33].  Due to its invasiveness and associated patient discomfort, this approach is often performed under general anesthesia with fluoroscopic guidance to determine the anatomic midline by bony landmarks. As mentioned previously, experience has shown that outcome is critically dependent on symmetric placement of the electrode on the PM , and that the physiologic and anatomic midlines are frequently not identical, differing by up to 2 mm in up to 40% of patients [40].  In the author’s experience, it is not at all uncommon for comparative midlines to deviate significantly over as little as a single spinal level .  The importance of proper positioning of electrodes during SCS is reinforced by the computer models developed by Holsheimer et al. [39], in which they show that thickness of the dorsal CSF layer and precise midline positioning of the electrodes are the most significant factors determining the perception threshold  of stimulation-induced paresthesias.

In order to address these concerns, some centers have employed a “wake up test” to demonstrate that the paresthesia maps to the painful areas before closing.  This has received very limited acceptance because of discomfort to the patient and the difficulty obtaining trustworthy feedback from an intubated patient.  Other centers have gone to laminectomy under only local anesthesia.  Difficulty obtaining adequate intraoperative pain control has limited the acceptance of this technique.  Some centers perform the procedure under a high (midthoracic) spinal or epidural anesthetic producing a regional anesthesia, which in experienced hands, does not interfere with the distal patterns of paresthesia coverage.  However, it is prone to difficulties in execution, which can lead to dense anesthesia or significant additional intravenous sedation inhibiting patient examination.

The remainer of this chapter is devoted to an alternative technique, which allows the comfort and safety of general anesthetic, while determining the PM by objective neurophysiological testing . The identification of PM by use of evoked potentials was introduced by Claudio Feler, who obtained a patent for a device to perform the mapping [US 6,027,456].  While the device did not gain widespread usage, a few centers adopted the methodology using standard intraoperative electrophysiologic monitoring equipment.  To obtain the optimal coverage over painful areas, two major criteria must be met: the applied stimulation should be positioned longitudinally along the DC and the PM must be identified.  When general anesthesia is used, intraoperative neurophysiological monitoring with evoked potentials becomes the only way to determine the PM.  Stimulation of various portions of the dorsal spinal cord produces paresthesia in a given distribution in the conscious patient, as well as reliable patterns of sensory (SSEP) and motor unit action potentials (MUAP’s) of the EMG in the patient under anesthesia. In addition, the output data may include interpolations between specific measured points for optimal assessment of applied stimulation between evaluated lateral positions [US 6,027,456].  

These fundamental findings have been implemented in practice by the senior author (KA), who began using this approach in 1999, noting a marked improvement in outcomes over the previous flouroscopically guided technique. In this technique, MUAP’s via EMG activation are used to determine the PM by examining the symmetry of the evoked potentials with presumed midline stimulation.  In addition, it became clear that objective MUAP’s via EMG activation of specific muscles corresponded with postoperative induced paresthesia in particular regions depending on laminectomy level.  For example, EMG activation of the external oblique muscle from a T9-10 thoracic paddle consistently correlates with low back paresthesia (Figure 4.1). 


Figure 4.1: Intraoperative view of thoracic paddle lead implantation. A.: The electrode is behind the body of T9 and T10. Stimulation is right-sided with the cathode at the second position and the anode at the third. B.: With right-sided stimulation, there is right-side gastrocnemius activation, which will correlate to an S1 dermatomal paresthesia.


These correlations are summarized in Table 1.  Additional correlations have been made for cervical SCS (Figure 2) and sacral nerve root stimulator (SNRS) (Figure 3) laminectomy placements (Tables 2 and 3).


Figure 4.2.  Intraoperative view of cervical paddle lead implantation. A. The electrode is behind the body of C3 and C4, entering at the C4/5 interspace.  B. With left sided stimulation, there is left side triceps activation, which will correlate to a C7 dermatomal paresthesia.



Figure 4.3.  Permanent retrograde implantation of sacral root paddle leads.  A.  Stimulation is left sided, with the cathode at the second position and the anode at the third.  B.  In this older tracing, the stimulation in the second left contact produces primarily adductor hallucis activation, solely on the left side.  This correlated with the postoperative paresthesia felt in the S3 perineal region.



EMG activation, muscle group


Induced paresthesia

















Table 4.2: Correlations between EMG activation of specific muscles with postoperative induced paresthesia - Cervical Paddle C3-4 (laminectomy C4/5) 


EMG activation, muscle group


Induced paresthesia







Adductor Hallucis







Table 4.3: Correlation between EMG activation of specific muscles with postoperative induced paresthesia - Sacral Paddle(s) S2-3 (laminectomy S1)


The general concept of using intraoperative EMG in the placement of the SCS on the PM of the spinal cord is similar with respect to the 2- and 3-column paddle configuration and differs in terms of whether the “expected” pattern should be symmetric (the middle column of a 3-column array) or “equally asymmetric” (a 2-column array).  The newest 5 column array (Penta, St. Jude Medical, Plano, Texas) has just begun PM evaluation with this technique.


Section IV.4.3: Technique of Midline Positioning of the Spinal Cord Stimulator – Tripolar paddle

Once the 3-column paddle is placed in the dorsal epidural space, the superior midline contact is stimulated at minimal settings and the EMG trace recording associated with the dermatomal level of stimulation is monitored.  The stimulus intensity is gradually increased until MUAP’s are seen on EMG.  The lowest stimulus intensity needed to elicit a motor response is referred to as the threshold stimulus.   MUAP’s will be seen bilaterally at the threshold stimulus if the midline contact of the SCS is in line with the PM of the spinal cord.  If MUAP’s are seen unilaterally, the threshold intensity for that side is recorded and the stimulus is further increased to elicit a response on the other side.  A difference in threshold stimulus intensity between the left and right sides indicates the SCS is lateral to the PM.  However, medial/lateral repositioning of the SCS is necessary only if the difference in threshold intensity between the two sides is greater than 2 mA.  In this case the paddle may not be perfectly flat on the lateral X-ray (Figure 4.4A), thus dissection of the lateral recesses or proximal/superior lamina is further performed until the electrode is perfectly aligned with vertebrae on a lateral view (Figure 4.4B).  This assures optimal electrode column symmetry and programmability of the PM.  Of course, as a last resort, the laminotomy can be extended to a full laminectomy to allow perfect alignment of the paddle on the PM.  In this case, tissue must typically be identified to suture the electrode in place to prevent migration.  These tenants hold true for all paddle (1, 2, 3 and 5 column) array configurations.


Figure 4.4: Lateral intraoperative views.  A.  Lead placed off the midline.  B. Perfect alignment on the midline.

Section IV.4.4: New frontiers of intraoperative EMG application

There appears to be a correlation between the muscles with objective EMG activation during intraoperative monitoring and the subjective paresthesia obtained postoperatively, as described in Tables 1-3.  Thus, this may be explored to generate a precise model of paresthesia coverage and create a functional dermatomal mapping of perceived stimulation threshold after the surgery.  Furthermore, the EMG activation threshold may be a reliable predictor of the patient’s perceived paresthesia threshold.  It has been the author’s experience that EMG activation correlated with pain control at amplitudes lower than the paresthesia threshold (i.e. subthreshold stimulation), and that occasionally persistent EMG activation intra- and postoperatively may be seen lasting as long as 15 minutes after the stimulation is discontinued.   These patients typically respond extremely well to the stimulation therapy.  It seems likely that these patients are the occasional patients who use their stimulation only intermittently, often having effective long term pain relief while using their systems for only a portion of each day.

Section IV.4.5: References

 Ben-David B, Haller G, Taylor P: Anterior spinal fusion complicated by paraplegia. A case report of a false-negative somatosensory-evoked potential. Spine (Phila Pa 1976) 12:536-539, 1987.

            Ben-David B, Taylor PD, Haller GS: Posterior spinal fusion complicated by posterior column injury. A case report of a false-negative wake-up test. Spine (Phila Pa 1976) 12:540-543, 1987.

  Wagner W, Peghini-Halbig L, Maurer JC, Perneczky A: Intraoperative SEP monitoring in neurosurgery around the brain stem and cervical spinal cord: differential recording of subcortical components. J Neurosurg 81:213-220, 1994.

Ryu H, Uemura K. Origins of the short latency somatosensory evoked potentials in cat--with special reference to the sensory relay nuclei. Exp Neurol. 1988 Nov;102(2):177-84.

Dunne JW, Silbert PL, Wren M. A prospective study of acute radiculopathy after scoliosis surgery. Clin Exp Neurol. 1991;28:180-90.

Meyer PR Jr, Cotler HB, Gireesan GT. Operative neurological complications resulting from thoracic and lumbar spine internal fixation.Clin Orthop Relat Res. 1988 Dec;(237):125-31.

Toleikis JR, Carlvin AO, Shapiro DE, Schafer MF. The use of dermatomal evoked responses during surgical procedures that use intrapedicular fixation of the lumbosacral spine. Spine (Phila Pa 1976). 1993 Dec;18(16):   2401-7.

Owen JH, Padberg AM, Spahr-Holland L, Bridwell KH, Keppler L, Steffee AD. Clinical correlation between degenerative spine disease and dermatomal somatosensory-evoked potentials in humans. Spine (Phila Pa 1976). 1991 Jun;16(6 Suppl):S201-5.

Rodriquez AA, Kanis L, Rodriquez AA, Lane D.  Somatosensory evoked potentials from dermatomal stimulation as an indicator of L5 and S1 radiculopathy. Arch Phys Med Rehabil. 1987 Jun;68(6):366-8.

Bose B, Wierzbowski LR, Sestokas AK. Neurophysiologic monitoring of spinal nerve root function during instrumented posterior lumbar spine surgery. Spine (Phila Pa 1976). 2002 Jul 1;27(13):1444-50.

             Weiss DS. Spinal cord and nerve root monitoring during surgical treatment of lumbar stenosis. Clin Orthop Relat Res. 2001 Mar;(384):82-100. Review.

              Holland NR, Kostuik JP.  Continuous electromyographic monitoring to detect nerve root injury during thoracolumbar scoliosis surgery. Spine (Phila Pa 1976). 1997 Nov 1;22(21):2547-50.

             Beatty RM, McGuire P, Moroney JM, Holladay FP. Continuous intraoperative  electromyographic recording during spinal surgery. J Neurosurg. 1995 Mar;82(3):401-5.

              Owen JH, Kostuik JP, Gornet M, Petr M, Skelly J, Smoes C, Szymanski J, Townes J, Wolfe F. The use of mechanically elicited electromyograms to protect nerve roots during surgery for spinal degeneration. Spine (Phila Pa 1976). 1994 Aug 1;19(15):1704-10.

              Kleissen RF, Buurke JH, Harlaar J, Zilvold G.  Electromyography in the biomechanical analysis of human movement and its clinical application. Gait Posture. 1998 Oct 1;8(2):143-158.

              Melzack R, Wall PD. Pain mechanisms: a new theory. Science. 1965 Nov 19;150(699):971-9. Review. No abstract available.

            Shealy CN, Mortimer JT, Reswick JB.  Electrical inhibition of pain by stimulation of the dorsal columns: preliminary clinical report. Anesth Analg. 1967 Jul-Aug;46(4):489-91. No abstract available.

             Aló KM, Holsheimer J.  New trends in neuromodulation for the management of neuropathic       pain. Neurosurgery. 2002 Apr;50(4):690-703; discussion 703-4. Review.

             Law J: Results of treatment for pain by percutaneous multicontact stimulation of the spinal   cord. Presented at the Annual Meeting of the American Pain Society, Chicago, Illinois, November 11–13, 1983.

              North RB, Fischell TA, Long DM: Chronic stimulation via percutaneously inserted epidural electrodes. Neurosurgery 1:215–218, 1977.

            Villavicencio AT, Leveque JC, Rubin L, Bulsara K, Gorecki JP.  Laminectomy versus percutaneous electrode placement for spinal cord stimulation. Neurosurgery. 2000 Feb;46(2):399-405; discussion 405-6.

            Mostegl A, Bauer R. The application of somatosensory-evoked potentials in orthopedic spine surgery. Arch Orthop Trauma Surg. 1984;103(3):179-84.

             Spielholz NI, Benjamin MV, Engler GL, Ransohoff J. Somatosensory evoked potentials during decompression and stabilization of the spine. Methods and findings. Spine (Phila Pa 1976). 1979 Nov-Dec;4(6):500-5.

             Ryan TP, Britt RH. Spinal and cortical somatosensory evoked potential monitoring during         corrective spinal surgery with 108 patients. Spine (Phila Pa 1976). 1986 May;11(4):352-61.

             Kumar K, Taylor RS, Jacques L, Eldabe S, Meglio M, Molet J, Thomson S, O'Callaghan J, Eisenberg E, Milbouw G, Buchser E, Fortini G, Richardson J, North RB. The effects of spinal cord stimulation in neuropathic pain are sustained: a 24-month follow-up of the prospective randomized controlled multicenter trial of the effectiveness of spinal cord stimulation. Neurosurgery. 2008 Oct;63(4):762-70; discussion 770.

Kumar K, Taylor RS, Jacques L, Eldabe S, Meglio M, Molet J, Thomson S, O'Callaghan J, Eisenberg E, Milbouw G, Buchser E, Fortini G, Richardson J, North RB. Pain. 2007 Nov;132(1-2):179-88. Epub 2007 Sep 12.

            North RB, Kidd D, Shipley J, Taylor RS. Spinal cord stimulation versus reoperation for failed back surgery syndrome: a cost effectiveness and cost utility analysis based on a randomized, controlled trial. Neurosurgery. 2007 Aug;61(2):361-8; discussion 368-9. Erratum in: Neurosurgery. 2009 Apr;64(4):601.

            Taylor RJ, Taylor RS. Spinal cord stimulation for failed back surgery syndrome: a decision-analytic model and cost-effectiveness analysis. Int J Technol Assess Health Care. 2005 Summer;21(3):351-8.

            Kumar K, Malik S, Demeria D. Treatment of chronic pain with spinal cord stimulation versus alternative therapies: cost-effectiveness analysis. Neurosurgery. 2002 Jul;51(1):106-15; discussion 115-6.

            Taylor RS. Spinal cord stimulation in complex regional pain syndrome and refractory neuropathic back and leg pain/failed back surgery syndrome: results of a systematic review and meta-analysis. J Pain Symptom Manage. 2006 Apr;31(4 Suppl):S13-9. Review.

            Kumar K, Hunter G, Demeria D. Spinal cord stimulation in treatment of chronic benign pain: challenges in treatment planning and present status, a 22-year experience. Neurosurgery. 2006 Mar;58(3):481-96; discussion 481-96.

            Taylor RS, Van Buyten JP, Buchser E. Spinal cord stimulation for complex regional pain syndrome: a systematic review of the clinical and cost-effectiveness literature and assessment of prognostic factors. Eur J Pain. 2006 Feb;10(2):91-101. Review.

            Spiegelmann R, Friedman WA. Spinal cord stimulation: a contemporary series. Neurosurgery. 1991 Jan;28(1):65-70; discussion 70-1. Review.

            Racz GB, McCarron RF, Talboys P. Percutaneous dorsal column stimulator for chronic pain control. Spine (Phila Pa 1976). 1989 Jan;14(1):1-4.

            Perlik SJ, VanEgeren R, Fisher MA. Somatosensory evoked potential surgical monitoring. Observations during combined isoflurane-nitrous oxide anesthesia. Spine (Phila Pa 1976). 1992 Mar;17(3):273-6.

            Richardson J. Facilitation of spinal cord stimulator implantation with epidural analgesia. Pain. 1996 May-Jun;65(2-3):277-8.

            Boswell MV, Iacono RP, Guthkelch AN. Sites of action of subarachnoid lidocaine and tetracaine: observations with evoked potential monitoring during spinal cord stimulator implantation. Reg Anesth. 1992 Jan-Feb;17(1):37-42.

            Lang E, Krainick JU, Gerbershagen HU. Spinal cord transmission of impulses during high spinal anesthesia as measured by cortical evoked potentials. Anesth Analg. 1989 Jul;69(1):15-20.

            Holsheimer J, Barolat G, Struijk JJ, He J. Significance of the spinal cord position in spinal cord stimulation. Acta Neurochir Suppl. 1995;64:119-24.

            Holsheimer J, den Boer JA, Struijk JJ, Rozeboom AR. MR assessment of the normal position of the spinal cord in the spinal canal. AJNR Am J Neuroradiol. 1994 May;15(5):951-9.

Washburn S, Monroe CD, Cameron T. A Review of Articles Published on Spinal Cord Stimulation Treatment for Chronic Pain: 1981 - 2008. NANS meeting, Las Vegas, December 4-7, 2008.

Created by Max Gosey on 2010/06/11 05:18

Chapter List


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-VOLUME V: SURGERY FOR EPILEPSY SPACE HAS BEEN CREATED (but don't try to create pages, as we don't have any content to put into them yet)\


-VOLUME VII: NEUROLOGICAL ONCOLOGY yes\ .\ .\ -\ -\ _.\ .\ .\ .\ .\ .\ .\ .\ .\ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


Chapter 9

-Chapter 10

-Chapter 11

-Chapter 12

-Chapter 13

-Chapter 14: Thalamic Ventral Intermediate Nucleus Stimulation for Essential Tremor


-Chapter 15: Histological Types of Tumors

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