Optimizing Spinal Cord Stimulator Placement with Neuromonitoring

Spinal cord stimulation (SCS) involves delivery of therapeutic doses of electrical current to the spinal cord for the management of neuropathic pain. The most common indications include post-laminectomy syndrome, complex regional pain syndrome, ischemic limb pain, and angina (Falowski, Celii & Sharan, 2008). There are numerous methods for placing a SCS system, depending on the needs of the patient and her physician. This article will discuss methods for optimizing electrode placement during surgical implantation of a permanent SCS system in the asleep patient.


Trial Stimulation

Prior to surgery, a short trial is performed in order to determine whether or not SCS will provide a patient with adequate pain relief. In the outpatient setting, under local anesthesia, a percutaneous electrode is placed in the epidural space over the spinal cord (Figure A). 

Figure A: Percutaneous placement of a SCS trial electrode in the outpatient setting.

Figure A: Percutaneous placement of a SCS trial electrode in the outpatient setting.

This trial electrode is attached to a small generator (stimulation device) that the patient will carry around, much like a cell phone. The generator will emit electrical pulses in order to target areas of pain. Placement of the trial stimulator generally takes less than 30 minutes, and the patient can go home soon after the leads have been inserted. The length of the trial depends on a number of factors, but usually lasts 1-2 weeks. If the patient enjoys significant relief from pain during the trial period, then a complete system with a generator can then be surgically implanted.


Surgery Overview

Surgery for placement of a SCS involves performing a cervical or thoracic laminotomy below the level where the electrode will be placed (Figure 1). Then, the surgeon slides a “paddle electrode” under the lamina above, positioning it either midline or off to one side, depending on the location/distribution of the patient’s symptoms.

Figure 1: Surgery involves laminotomy for positioning of a paddle electrode.

Figure 1: Surgery involves laminotomy for positioning of a paddle electrode.

Using an intraoperative C-arm, the surgeon takes an A-P x-ray to examine the location of the paddle electrode relative to anatomical structures, such as the spinous processes (Figure 2). In the example shown in Figure 2, the paddle electrode has 16 contacts – 5 on the left, 6 in the middle, and 5 on the right. There are many different configurations. This is a 5-6-5 configuration.

Figure 2: A-P film show electrode positioned in the thoracic spine.

Figure 2: A-P film show electrode positioned in the thoracic spine.

Once the positioning has been optimized through a combination of radiographic imaging and neurophysiological mapping (more on that in a moment), the surgeon will connect the electrode wires to a pulse generator that is implanted under the patient’s skin. As illustrated in Figure 3, the post-operative scan clearly shows the paddle electrode in the spinal column connected to the pulse generator.

Figure 3: A-P film show electrode positioned in the thoracic spine. This is a post-operative image, so it shows the whole system implanted. As you can see, the electrode is attached to a battery pack, which is implanted low in the patient's back new the buttocks.

Figure 3: A-P film show electrode positioned in the thoracic spine. This is a post-operative image, so it shows the whole system implanted. As you can see, the electrode is attached to a battery pack, which is implanted low in the patient’s back new the buttocks.


Monitoring Plan

Placement of a SCS paddle places the spinal cord at risk for iatrogenic injury, including paralysis and paresis (Barolat et al, 2005; Falowski et al, 2010; Levy et al, 2011; Meyer et al, 2007; Smith et al, 2011; Tamkus et al, 2014). Due to this risk alone, a comprehensive multimodality monitoring plan is recommended, including MEPs, SSEPs, EMG, EEG and Train-of-four (TOF). Each modality has a key role to play as indicated below:

  • SSEP
    • Monitor dorsal spinal cord function
    • Monitor brachial plexus (superman position)
    • Collision Testing/Mapping
  • MEP
    • Monitor ventral spinal cord function.
    • Monitor brachial plexus (superman position)
  • EMG
    • EMG Testing/Mapping
    • Approximate patient’s sensation of painful stimuli, guide anesthesia’s administration of narcotic agents (e.g., Remifentanil).
  • TOF
    • Confirm adequate conduction across the neuromuscular junction.
  • EEG
    • Approximate patient’s depth of hypnosis, guide anesthesia’s administration of hypnotic agents (e.g., Propofol).
    • Correlate changes in evoked potentials with changes in patient’s depth of hypnosis.

The muscles and nerves that you choose for MEP and EMG depend on the level at which the SCS is being placed. My approach to monitoring SCS placement is shown below:

Cervical SCS Placement

  • SSEP
    • Ulnar Nerve & Posterior Tibial Nerve.
  • MEP
    • Trapezius, Deltoid, Biceps, Wrist Extensor, First Dorsal Interosseous, Tibialis Anterior & Abductor hallucis.
  • EMG
    • Trapezius, Deltoid, Biceps, Wrist Extensor & First Dorsal Interosseous.
  • TOF
    • Stimulate bilateral ulnar nerve, Record bilateral first dorsal interosseous.
  • EEG
    • C3′-Fpz, C4′-Fpz

Thoracic SCS Placement

  • SSEP
    • Ulnar Nerve & Posterior Tibial Nerve
  • MEP
    • First Dorsal Interosseous, Rectus Abdominis, Iliopsoas, Quadriceps, Tibialis Anterior & Abductor hallucis.
  • EMG
    • Rectus Abdominis, Iliopsoas, Quadriceps, Tibialis Anterior & Abductor hallucis.
  • TOF
    • Stimulate: bilateral posterior tibial nerve, Record: bilateral abductor hallucis
  • EEG
    • C3′-Fpz, C4′-Fpz

Mapping Plan

The entire purpose of mapping during this procedure is to confirm the laterality of the paddle electrode placement. Note that the radiographic midline and the functional midline are only the same about 40% of the time! So, once imaging has confirmed the location of the electrode relative to the radiographic midline, neuromonitoring is used to confirm the location of the electrode relative to the functional midline.

The SCS sales representative will then connect the electrode to a hand-held pulse generator, which she operates by sending electrical signals through pairs of electrodes. These electrical signals activate the dorsal columns of the spinal cord, sending signals toward the head and toward the feet. We can take advantage of these signals as a means for mapping the laterality of the paddle.

Assuming that the surgeon wants the electrode placed on the functional midline of the spinal cord, we usually test the left side of the paddle to make sure it is to the left of midline, the right side of the paddle to make sure it is to the right of midline, and the middle of the paddle to make sure it is at midline (or close). There are multiple neuromonitoring methods that you can use to “map” the placement of the paddle. The reader should be informed that all of these methods are considered to be “off label”. Presently, the only on-label method for SCS placement/testing is with the patient awake.


EMG Mapping

As Figure 4 illustrates, the SCS activates the dorsal columns of the spinal cord, resulting in antidromic activation of the alpha-motor-neuron in the ventral gray matter. This, in turn, results in muscle contraction in the periphery.

Figure 4: Overview of EMG Mapping Technique

Figure 4: Overview of EMG Mapping Technique

Thus, activation of the left gracile fasciculus will activate muscles below the level of stimulation on the left side of the body, and activation of the right gracile fasciculus will activate muscles below the level of stimulation on the right side of the body. We take advantage of this basic physiologic principle in order to optimize placement of the SCS paddle electrode relative to the functional midline (Air et al, 2012; Falowski et al, 2010; Halter et al, 1983; Mammis & Mogilner, 2012; Shils & Arle, 2012; Tamkus et al, 2015; Yingling & Hosobuchi, 1986)

Paddle Electrode Stimulation Parameters:

  • Controlled by SCS Rep.
  • Activate pairs of contacts on the paddle electrode (left, right, middle, etc).
  • Common stimulation parameters:
    • Pulse Width: 100-600 µsec (I like 300)
    • Frequency: 3-5 Hz (some go up to 10, but this is not recommended).
    • Intensity: up to 12 mA.

EMG Recording Parameters:

  • Controlled by neurophysiologist.
  • Can use triggered- or free-running EMG window with all stimulation turned off.
  • Recording Parameters:
    • Filers: 10-3000 Hz
    • Time Base: 50-200 msec/div (strongly recommend 50).
    • Sensitivity: 50-200 µV/div
  • Recording Location: muscles are determined by location of paddle (i.e., cervical v. thoracic), as indicated in the monitoring plan (above).

Examples of EMG Mapping:

In each of the images to follow, the left column represents the left side of the body, and the right column represents the right side of the body. Free-running EMG recordings from top to bottom: rectus abdominis (RA), iliopsoas (IP), quadriceps (QD), tibialis anterior (TA) and abductor hallucis (AH) muscles. Importantly, Figures 5-7 use a 50 msec time base.

Figure 5: As stimulation of the paddle electrode begins, stimulus artifact is recorded from bilateral rectus abdominis muscles. The artifact appears larger on the right side.

SCS_tEMG_01

Figure 5: Free-running EMG recording with 50 msec time base.

Figure 6: As the intensity of the stimulus is increased, stimulus artifact remains present from bilateral rectus abdominis muscles; however, immediately following each stimulus, you can see a CMAP recorded from rectus abdominis and iliopsoas muscles. The CMAPs are much larger on the left side, indicating left dominance.

SCS_tEMG_02

Figure 6: Free-running EMG recording with 50 msec time base.

Figure 7: As the intensity of the stimulus is increased even further, CMAPs have increased in amplitude and are present in the left leg. Again, CMAPs are observed following stimulation artifact, with clear left side dominance.

Figure 7: Free-running EMG recording with 50 msec time base.

Figure 7: Free-running EMG recording with 50 msec time base.

NOTE: The reason that I advocate for using a **50 msec** time base is because the CMAPs are clearly visible and discernible from the stimulation artifact. Figures 8-9 use a **500 msec** time base. You can see what happens when the EMG recording depicts the activity too close together.

Figure 8: Stimulus artifact is recorded from bilateral rectus abdominis muscles. The artifact appears larger on the right side. With stimulus artifact so close together using a 500 msec time base, it can be challenging to observe CMAPs and differentiate them from the stimulus artifact.

Figure 8: Free-running EMG recording with 500 msec time base.

Figure 8: Free-running EMG recording with 500 msec time base.

Figure 9: As the stimulus intensity is increased, CMAPs appear on both sides, intermixed with the stimulus artifact. It isn’t really clear which side is dominant.

Figure 9: Free-running EMG recording with 500 msec time base.

Figure 9: Free-running EMG recording with 500 msec time base.

Troubleshooting EMG Mapping:

  • Recording (Neurophysiologist):
    • Are appropriate muscles selected?
    • Is train-of-four test 4/4 without fade?
    • Are impedances ≤ 5 kΩ?
    • Is sensitivity value set to appropriate range?
    • Is artifact reject turned off?
    • Confirm that software is not in “simulation mode”.
  • Stimulation (SCS Rep):
    • Check impedance.
    • Check delivery/return of current.
    • Ifresponsesare recorded in axis, but not getting to legs/arms, try:
      • Decreasing frequency (Hz).
      • Increasing pulse width (µsec)

EMG Mapping Pros and Cons:

  • Pros:
    • Can do with patient under general anesthesia.
    • Relatively non-invasive (requires placement of subdermal needle electrodes).
    • High rate of accuracy for laterality.
    • Mapping is very fast and rarely fails in the right hands.
  • Cons:
    • Patient cannot have neuromuscular blockade.
    • Little or no accuracy for distal extent of coverage.

SSEP Collision Mapping

As Figure 10 illustrates, stimulation of the posterior tibial nerve (PTN) results orthodromic in activation of the dorsal columns of the spinal cord, as the sensory signal travels toward the brain. Meanwhile, stimulation of the dorsal columns (SCS) results in antidromic activation of the dorsal columns of the spinal cord.

Figure 10: Overview of SSEP Collision Testing

Figure 10: Overview of SSEP Collision Testing

Because these two signals “collide”, the sensory signal resulting from PTN stimulation doesn’t make it to the brain. Thus, if collision is successful, the cortical SSEP (P37/N45) will be absent during testing. Stimulation of the left gracile fasciculus should obliterate cortical SSEPs to left PTN stimulation only, and stimulation of the right gracile fasciculus should obliterate cortical SSEPs to right PTN stimulation. Midline stimulation is likely to obliterate both left and right PTN SSEPs (Balzer et al, 2011; Halter et al, 1983; Shils & Arle, 2012; Tamkus et al, 2015).

Paddle Electrode Stimulation Parameters:

  • Controlled by SCS Rep.
  • Activate pairs of contacts on the paddle electrode (left, right, middle, etc).
  • Common stimulation parameters:
    • Pulse Width: 100-600 µsec (I like 300)
    • Frequency: 40-60 Hz.
    • Intensity: Titrated, steady state 1-3 mA. I like to use the intensity at which CMAPs first appear in EMG recordings.

Peripheral Nerve Stimulation Parameters:

  • Controlled by neurophysiologist.
  • Use standard SSEP stimulation (ulnar for cervical SCS, PTN for thoracic SCS).
  • Intensity: Supramaximal (30-50 mA; 300-400 µsec; 2-5 Hz).
  • Montage: Anode distal, cathode proximal on limb.

SSEP Recording Parameters:

  • Montage: Cpz-Fpz or CPi-CPc
  • Filers: 30-300 Hz
  • Time Base: standard for SSEP.
  • Sensitivity: standard for SSEP

Protocol:

  • Start paddle stimulation.
  • Then start SSEP stimulation.

Example SSEP Collision Mapping:

Figure 11: Collision testing suggests left-dominant placement as evidenced by the obliterated SSEP on the left side only, although a decrement is noted on the right.

Figure 11: Example of SSEP collision testing.

Figure 12: Collision testing suggests midline placement as evidenced by the obliterated bilateral PTN SSEP.

Figure 12: Collision testing obliterated the PTN SSEPs bilaterally (pink trace).

Figure 12: Collision testing obliterated the PTN SSEPs bilaterally (pink trace).

Troubleshooting SSEP Collision Mapping:

  • Problem:SSEP is always obliterated bilaterally.
    • Was it present at baseline?
      • Pre-existing pathology can preclude reliable recordings. Many patients have comorbidities that prevent recording PTN SSEPs at baseline.
      • Are we stimulating nerve (PTN or ulnar) with enough intensity?
    • Paddle stimulation is too high.
    • Possible spinal cord injury, recommend checking tceMEPs.
  • Problem:SSEP is never obliterated.
    • This happens sometimes. Collision is not the most accurate test.
    • Is correct extremity being stimulated?
    • This happens sometimes. Collision is not the most accurate test.

SSEP Collision Pros and Cons:

  • Pros:
    • Can be done under general anesthesia.
    • Relatively non-invasive.
    • Can be done with neuromuscular blockade (assuming no EMG/MEPs).
  • Cons:
    • Can be a slow, time-consuming test, as compared to EMG.
    • More complex test. Most neurophysiologists don’t have a standing protocol.
    • Many patients do not have PTN SSEPs at baseline.
    • Low rate of accuracy for laterality (only works on about 25-50% of patients).
    • No information about distal extent of coverage.

References:

  1. Air EL, Toczyl GR, Mandybur GT. Electrophysiologic monitoring for placement of laminectomy leads for spinal cord stimulation under general anesthesia. Neuromodulation. 2012 Nov-Dec;15(6):573-9; discussion 579-80.
  2. Balzer JR, Tomycz ND, Crammond DJ et al. Localization of cervical and cervicomedullary stimulation leads for pain treatment using median nerve somatosensory evoked potential collision testing. J Neurosurg 2011;114:200–205.
  3. Barolat G, Peacock WJ, Staudt LA. Pain and spasticity. In: Benzel EC, ed. Spine surgery: techniques, complication avoidance, and management, Vol. 2, 2nd ed. Philadelphia: Elsevier Churchill Livingstone, 2005.
  4. Falowski S, Celii A, Sharan A. Spinal cord stimulation: an update. Neurotherapeutics. 2008 Jan;5(1):86-99. Review.
  5. Hallstrom YT, Lindblom U, Meyerson BA. Distribution of lumbar spinal evoked potentials and their correlation with stimulation-induced paresthesiae. Electroencephalogr Clin Neurophysiol 1991;80:126–139.
  6. Halter J, Dolenc V, Dimitrijevic MR, Sharkey PC. Neurophysiological assessment of electrode placement in the spinal cord. Appl Neurophysiol. 1983;46(1-4):124-8.
  7. Levy R, Henderson J, Slavin K et al. Incidence and avoidance of neurologic complications with paddle type spinal cord stimulator leads. Neuromodulation 2011;14:412–422.
  8. Mammis A, Mogilner AY. The use of intraoperative electrophysiology for the placement of spinal cord stimulator paddle leads under general anesthesia. Neurosurgery. 2012 Jun;70(2 Suppl Operative):230-6.
  9. Meyer SC, Swartz K, Johnson JD. Quadriparesis and spinal cord stimulation: case report. Spine 2007;32:565–568.
  10. Shils JL, Arle JE. Intraoperative neurophysiologic methods for spinal cord stimulator placement under general anesthesia. Neuromodulation. 2012 Nov-Dec;15(6):560-71; discussion 571-2.
  11. Smith C, Lin J, Shokat M, Dosanjh S, Casthely D. A report of paraparesis following spinal cord stimulation trial, implantation and revision. Pain Physician 2010;13:357– 363.
  12. Tamkus AA, Scott AF, Khan FR. Neurophysiological Monitoring During Spinal Cord Stimulator Placement Surgery. Neuromodulation. 2015 Feb 10.
  13. Yingling CD, Hosobuchi Y. Use of antidromic evoked potentials in placement of dorsal cord disc electrodes. Appl Neurophysiol. 1986;49(1-2):36-41.

Dr. Rich Vogel is board-certified intraoperative neurophysiologist working for Safe Passage Neuromonitoring. He started the Neurologiclabs website and blog to connect with others in the field of neuromonitoring.

NeuroLogicLabs

12 thoughts on “Optimizing Spinal Cord Stimulator Placement with Neuromonitoring

  • Reply John August 19, 2015 at 11:39

    How are there no comments on this yet?! Everyone, this is pure gold, if you do not have any experience with Spinal Cord Stimulator Localization Mapping. Richard, Thank you for taking the time to create this and share with the IONM community! There is not enough transparency, when it comes to updated best practices and standardization of protocols, this is the only way for us to improve patient care

    • Reply Richard Vogel August 19, 2015 at 12:04

      Hi John, Thanks for the comment, and the feedback! Keep reading. I hope to publish many more posts like this in the coming months.

  • Reply Dr. Nishanth Sampath MD October 16, 2015 at 07:12

    Absolutely awesome. Thanks for this treasure, Rich.

  • Reply Aiman Mahfouz November 15, 2015 at 23:57

    Thanks for sharing, I hope to see this article posted as ASNM guidline for SCS.

  • Reply Happy Holidays from Neurologic Labs!NeurologicLabs December 2, 2015 at 11:41

    […] also published several didactic posts in 2015, including pieces about dorsal column mapping, spinal cord stimulators, speech & language mapping, and the difference between anodal & cathodal stimulation. All […]

  • Reply Case Report: Neuromonitoring Alert in Spinal Cord Stimulator Surgery - NeurologicLabsNeurologicLabs January 9, 2016 at 20:19

    […] a previous post, I described methods for optimizing spinal cord stimulator (SCS) placement under anesthesia with […]

  • Reply John-Paul January 20, 2016 at 20:45

    Have been using this the protocol for all my Lumbar SC Stim cases surgeon is VERY happy. Today we were going to do a Cervical but the rep started chiming in that because they have t increase the stim so high for Neuromonuitoring to get EMG, its a false positive compared to when the patient is woken up and they can feel the stim at much lower thresholds……the surgeon was skeptical but for safety chose not to monitor this case. Rep could not understand we are not able to test pain/temperature, agh

    • Reply Richard Vogel January 21, 2016 at 09:35

      That’s unfortunate. Why a surgeon would listen to a sales rep over a neurophysiologist is a mystery to me. Clearly the sales rep doesn’t understand what we do in these cases.

  • Reply C.Dodge June 12, 2016 at 07:01

    Hi Richard,

    Great write up. I see that you mention paddles only, what about percutaneous leads? Is the efficacy of using EMG only for cervical and lumbar, not thoracic? I have a surgeon that is interested in EMG monitoring, but places percutaneous leads regularly at T7-9 to alleviate leg pain. Any suggestions?

    • Reply Richard Vogel June 12, 2016 at 20:51

      You can use T-EMG to map percutaneous leads, too. Perc leads don’t usually require mapping because they are placed under light sedation and the patient can report their experience with pain relief. If your surgeon is placing perc electrodes under general anesthesia, then you can definitely guide their placement with T-EMG. These electrodes (paddle and perc) are usually placed in the cervical or thoracic spine, where they can stimulate the dorsal columns. For this reason, they don’t get placed in the lumbar spine.

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