Spinal cord stimulation (SCS) is a well-established therapy used for more than 50 years in treatment for chronic, intractable pain conditions, including failed back surgery syndrome and complex regional pain syndromes.^[1] With traditional tonic stimulation and most newer waveforms, successful SCS treatment requires the superposition of SCS-induced paresthesias in regions of perceived pain, which confirms recruitment of the fibers serving the relevant spinal segment(s).^[2]

The accepted standard for confirmation of paresthesia overlap is via verbal feedback from a conscious patient, which also mitigates the risk of inadvertent injury to the neural axis. In the past several years, however, a new option called neuromonitoring has emerged that allows for both proper mapping of lead placement and safety monitoring of the neural axis in anesthetized patients.

Although neuromonitoring is relatively new in the application of SCS, surgeons have long accepted its use in various procedures, such as spinal surgery, to safely monitor the spinal cord in anesthetized patients. Additional benefits of neuromonitoring include a more predictable procedure, elimination of undesired patient movement, and less stress to the patient and the physician.

Anesthetic management can be difficult in interventional pain procedures,^[3] including awake lead implantation for SCS, because it is often challenging to balance appropriate pain control and the level of consciousness or responsiveness. In addition, patients can often become disinhibited or agitated with the use of anesthetics, making the procedure difficult to perform and placing the patient at increased risk of an untoward event. Also, delays can occur in mapping paresthesias because patients may be slow to arouse secondary to sedative medications, thus making the time required for surgery much less predictable. Lastly, although awake placement for spinal cord stimulators is commonly performed, no published data specifically investigate awake placement for either safety or confirmation of lead positioning. However, despite these factors, awake placement has been a preferred method for SCS implantation.

In some situations, placing patients under general anesthesia may be advantageous, such as a completely uncooperative patient who jeopardizes the safety and efficacy of the procedure, or perhaps during surgical lead placement because a laminotomy is required. When patients cannot give consistent conscious feedback because of general anesthesia, it is imperative to have a method to monitor cord protection. Neuromonitoring has been widely accepted in spinal surgery for this purpose.^[4] Asleep lead placements, however, do not allow for verbal feedback and could therefore contribute to suboptimal lead placement. Even with the advent of paddle electrodes, which are typically placed on the accepted anatomic midline by fluoroscopic imaging without paresthesia mapping, leads will produce inadequate coverage in one out of every six patients.^[5] Spinal mapping work has demonstrated the spinal cord’s physiologic midline does not match the anatomic midline in approximately 40% of patients.⁶ Therefore, protocols were needed and are now established to use intraoperative neuromonitoring to confirm proper positioning and placement of the leads.⁷

The recent Neurostimulation Appropriateness Consensus Committee (NACC) guidelines confirm the protocols of correct lead placement in either an awake patient or through the use of neuromonitoring in an asleep patient.^[8] Observation of compound motor action potentials (CMAPs)^[9] or somatosensory-evoked potentials¹⁰ in the painful dermatome(s) in response to intraoperative SCS can be used as a proxy for verbal confirmation of paresthesia coverage. CMAPs use myotomal coverage as a marker for dermatomal coverage. (See Figure 1.) In addition, neuromonitoring is used as a safety measure to monitor the spinal cord and neurologic function during an asleep procedure.

Initially, several retrospective studies demonstrated that the asleep procedure with neuromonitoring is at least as safe and efficacious as the awake procedure and may have fewer adverse events.^[11],[12] Interestingly, the studies demonstrated that the asleep method had reduced intraoperative time when compared to the awake surgical technique and had approximately half the incidence of device failure (defined as need for reoperation secondary to a device related issue).^[11] These studies led to the advent of published protocols⁷ and a prospective, multicenter (high-volume and experienced academic and private centers) study directly comparing asleep placement with the use of intraoperative neuromonitoring versus awake placement.¹³ Confounding factors were limited by including only paddle electrodes and performing an open-label study in which physicians implanted the SCS to the best of their ability in their preferred technique. The results demonstrated that asleep placement with neuromonitoring allows for significantly lower procedure and operating room times and more efficient and accurate positioning of the electrodes. It also showed that the asleep procedure provided superior paresthesia coverage of the painful areas and lower excess paresthesias (paresthesia in areas where patients did not have pain). Furthermore, a lower adverse event profile was noted in the asleep group. This has been the best data to date looking at SCS placement in any group, whether awake or asleep. This summary of the published data and protocols has been recognized throughout the field and was presented at the International Neuromodulation Society World Congress in 2017, North American Neuromodulation Society Annual Meeting in 2018, and World Congress on Regional Anesthesia and Pain Medicine in 2018.

Several different practice models for implementation exist, including using a large neuromonitoring company who charges a small nominal fee per hour or day for services but also maintains the ability to code for the procedure; having a sharing model with a neuromonitoring company for reimbursement; or employing your own staff and taking ownership of the billing and coding. Each scenario is a financially feasible platform in both hospital-based and acute surgical center practices. Lack of understanding and familiarity outside the surgeons’ space has led to inappropriate myths in regard to neuromonitoring’s utility, implementation, and finances, but dispelling those myths may have led to increased use.

Since the increased adoption of neuromonitoring during placement of SCS in asleep patients, several other applications and innovations have been studied. Perhaps the most impactful is that it is beginning to be used in the placement of dorsal root ganglion stimulators (DRG). A prospective series has shown that use of neuromonitoring during DRG stimulation placement is equal to awake placement in both number of adverse events and lead placement confirmation.^[14] Additional studies have used direct intraoperative recording to determine the mechanisms of action of each neuromodulation company’s different stimulation waveforms.^[15] Finally, it can be used in combination with the newer closed loop technology, in which objective measures of pain relief and paresthesia coverage can be determined.^[16]

In summary, use of intraoperative neuromonitoring has potentially shown superior benefit, but it’s important to realize that it will not eliminate the awake procedure, but it is a viable option for those who choose to implement it. Additional benefits include a streamlined and predictable procedure that decreases procedural and intraoperative time with more accurate placement of leads and fewer adverse events. Those benefits are balanced with a center’s ability to implement its use and the method in which they chose to apply it.

Figure 1: Placement of an electrode generating EMG responses using myotomal coverage as a marker of dermatomal coverage

1. Midline placement
   

    

   
2. Right-sided dominant placement
   

   

3. Left-sided dominant placement
   

    

References:

1. Kumar K, Taylor RS, Jacques L, et al. The effects of spinal cord stimulation in chronic pain are sustained: a 24-month follow-up of the prospective randomized controlled multicenter trial of the effectiveness of spinal cord stimulation. Neurosurgery. 2008;63(4):762–770. https://doi.org/10.1227/01.NEU.0000325731.46702.D9
2. North RB, Ewend MG, Lawton MT, Piantadosi S. Spinal cord stimulation for chronic, intractable pain: superiority of "multi-channel" devices. Pain. 1991;44:119–130.
3. Anson J, Bunty S. Anesthesia and sedation for interventional pain procedures. In: Goudra BG, Singh PM, eds. Out of operating room anesthesia. Switzerland: Springer International; 2016: 261–270.
4. Scibilia A, Raffa G, Rizzo V, et al. Intraoperative neurophysiological monitoring in spine surgery: a significant tool for neuronal protection and functional restoration. Acta Neurochir Suppl. 2017;124:263–270. https://doi.org/10.1007/978-3-319-39546-3_38
5. Shils JL, Arle JE. Intraoperative neurophysiologic methods for spinal cord stimulator placement under general anesthesia. Neuromodulation. 2012;15:560–571. https://doi.org/10.1111/j.1525-1403.2012.00460.x
6. Holsheimer J, den Boer JA, Struijk JJ, Rozeboom AR. MR assessment of the normal position of the spinal cord in the spinal canal. Am J Neuroradiol. 1994;15:951–95
7. Falowski S, Dianna A. Neuromonitoring protocol for spinal cord stimulator cases with case descriptions. Int J Acad Med. 2016;2(2):132–144. https://doi.org/10.4103/2455-5568.196863
8. Deer TR, Lamer TJ, Pope JE, et al. The Neurostimulation Appropriateness Consensus Committee (NACC) safety guidelines for the reduction of severe neurological injury. Neuromodulation. 2017;20:15–30. https://doi.org/10.1111/ner.12564
9. Air EL, Toczyll GR, Mandybur GT. Electrophysiologic monitoring for placement of laminectomy leads for spinal cord stimulation under general anesthesia. Neuromodulation. 2012;15:573–579. https://doi.org/10.1111/j.1525-1403.2012.00475.x
10. 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(1):200–205. https://doi.org/10.3171/2010.5.JNS091640
11. Falowski SM, Celii A, Sestokas AK, Schwarz DM, Matsumoto C, Sharan A. Awake vs. asleep placement of spinal cord stimulators: a cohort analysis of complications associated with placement. Neuromodulation. 2011;14:130–134. https://doi.org/10.1111/j.1525-1403.2010.00319.x
12. Mammis A, Mogilner AY. The use of intraoperative electrophysiology for the placement of spinal cord stimulator paddle leads under general anesthesia. 2012;70(2 Suppl Operative):230–236. https://doi.org/10.1227/NEU.0b013e318232ff29
13. Falowski SM, Sharan A, McInerney J, Jacobs D, Venkatesan L, Agnesi F. Nonawake vs awake placement of spinal cord stimulators: a prospective, multicenter study comparing safety and efficacy. Neurosurgery. 2019;84(1):198– https://doi.org/10.1093/neuros/nyy062
14. Falowski SM, Dianna A. A prospective analysis of neuromonitoring for confirmation of lead placement in dorsal root ganglion stimulation. Oper Neurosurg. 2018;14:654– https://doi.org/10.1093/ons/opx172
15. Falowski SM. An observational case series of spinal cord stimulation waveforms visualized on intraoperative neuromonitoring [published online ahead of print April 29, 2018]. https://doi.org/10.1111/ner.12781
16. Falowski SM, Parker J, Obradovic M, Karntonis D, Gorman R. Evoked compound action potentials to guide lead placement: a neuromonitoring technique. Presented at the North American Neuromodulation Society Annual Meeting; January 2018; Las Vegas, NV.