July 2019
To: SRS Colleagues
The attached information statement on Intraoperative Neurophysiological Monitoring of Spinal Cord Function During Spinal Deformity Surgery was developed to provide updated information to the membership. The information expresses the opinion that intraoperative neurophysiological spinal cord monitoring is no longer investigational (as was stated in the previous 2009 SRS Neuromonitoring Information Statement), but is instead a standard modality that is a nearly universally used adjunct to improve safety of surgical deformity correction procedures when the spinal cord is at risk. It has been conclusively demonstrated that intraoperative spinal cord monitoring facilitates detection of impending spinal cord deficit and facilitates early interventions that are likely to preserve spinal cord function.
Innovation in surgical technique and spinal implants has allowed surgeons to correct increasingly complex spinal deformities. However, large corrective forces applied to spinal deformities risks potential neurological deficits including loss of motor and sensory function in the lower extremities, and bowel and bladder incontinence. Reports from the SRS Morbidity and Mortality Committee and independent investigators have documented this rare (0.14-0.79%) but potentially devastating risk. (Schmitt, 1981; MacEwen, 1975; Wilber, 1984 Diab, 2007; Burton, 2016; Bartley 2017). While all spine deformity correction surgery is inherently dangerous, patients with kyphosis, congenital scoliosis, and/or preexisting neurological abnormalities are at increased risk (1.3-3.6%). Furthermore, the use of pedicle subtraction osteotomy and three-column resection techniques independently increases this risk (adj. odds ratio 3.06 [1.14-8.19]) (Boachie-Adjei, 2015). Mechanisms of injury include direct stretching or compression of the spinal cord, direct injury to the cord from instruments/implants and/or interference with cord blood flow (Nuwer, 1988, Drummond, 2003).
Prior to the development of intraoperative neuromonitoring (IONM), the only available method to detect a perioperative neurologic disfunction was the Stagnara wake-up test (Vauzelle, 1973). The test entails waking the patient intra-operatively sufficiently to follow commands and then asking for movement in all four extremities. However, the wake-up test has obvious limitations with regard to the timely ability to detect neurological changes and to localize cord compromise. (Stephen, 1996; Schwartz, 1997; Padberg, 1999; Strike, 2017). Another limitation is an inability of some patients to cooperate with the wake-up test because of age or mental status. On the other hand, the Stagnara wake-up test is a useful adjunctive modality for those cases where IONM is not possible, including patients with severe preexisting myelopathy (Master, 2008). Finally, a properly performed wake-up test is still considered the gold-standard for the detection of neurologic deficits and can be used to confirm a deficit when the results of intraoperative neuromonitoring are under question (Ferguson, 2014).
Somatosensory evoked potentials
In the late 1970s, the monitoring of somatosensory evoked potentials (SSEPs) was developed to help identify injury to the spinal cord as it was occurring, so that early interventions could be performed (such as reducing the correction). This modality monitors the integrity of the dorsal columns of the spinal cord through which pass signals for vibration and proprioception; it does not evaluate the integrity of the anterior column (motor pathways). Subsequent research demonstrated that SSEPs have good specificity (99-100%) with respect to predicting spinal cord injury (Nuwer, 1988; Dinner, 1986; Bieber, 1988; Brown, 1984, Thirumala, 2014). Furthermore, it was demonstrated that responding with corrective procedures to a SSEP alert was protective of cord function and integrity (Jones, 1983; Bieber, 1988). Despite this success, further evaluation of the technique demonstrated that SSEPs alone have an unacceptably low level of sensitivity (25-43%) (Ginsberg, 1985; Bridwell , 1998; Schwartz, 2007), i.e., spinal cord injuries can be missed.
Motor-evoked potential monitoring
Improvements in IONM continued with the development of transcranial motor-evoked potential monitoring (TcMEP) which evaluates the spinal cord motor tracts located in the anterior spinal cord. The safety and efficacy of this modality has been demonstrated repeatedly by a number of investigators (DiCindio, 2003; Hillbrand, 2004 Schwartz, 2007; Langeloo, 2003; Acharya, 2017). The main benefit of including TcMEP is the very high sensitivity (100%) of the modality and the resultant impressive negative predictive value seen in most studies; there exist very few cases in the literature describing a false negative TcMEP (Neira, 2016). However, it may not be possible to appropriately monitor all patients with TcMEP due to a variety of reasons including seizure disorders, raised intracranial pressure, cortical lesions and skull-based metal implants, amongst other contraindications (MacDonald, 2002).
The combined monitoring of sensory evoked potentials and motor evoked potentials during spine surgery decreases the false-negative rates of reporting (Iwasaki, et al. 2003, Leppanen, et al. 2005; Hilibrand, et al. 2004, Schwartz, et al. 2007; Tsirikos, 2019; Thirumala, 2016). It has been conclusively demonstrated that intraoperative spinal cord monitoring facilitates detection of impending spinal cord deficit and facilitates early interventions that are likely to preserve spinal cord function (Lyon, et al. 2004; Schwartz, et al., 2007, Pastorelli, 2011). A survey recently completed by the SRS Safety & Value Committee found that approximately 80% of survey respondents use both SSEPs and TcMEPs during spinal deformity surgery. Furthermore, the use of both modalities assures some level of monitoring should circumstances preclude or suspend the use of one or the other modality.
It is typical to perform monitoring of the upper extremities, in addition to the lower extremities, during deformity surgery. This serves two main purposes: 1. Global changes in monitoring, affecting both the upper and lower extremities, may indicate a technical issue or anesthetic cause for lost signals rather than thoracic spinal cord injury. 2. Isolated signal losses in the upper extremity may indicate an upper extremity injury due to direct compression or inappropriate positioning of the arms during surgery. (Polly, 2016)
EMG options
Besides SSEP and MEP, free-run EMG response is a passive modality that provides immediate actionable information, especially in the lumbar spine, for specific muscles and nerve roots by monitoring spontaneous muscle electrical activity (Chung, 2011). It has a high negative predictive value (98%) but also has significant potential limitations including low specificity and sensitivity with muscle-relaxing anesthetics (Larrata, 2018). It is considered a useful adjunct, especially with respect surgical treatment of high-grade spondylolisthesis, to combined and active monitoring by SSEPs and TcMEPs, but should not be used as the only IONM modality.
Triggered EMG screw stimulation is predictive of the lack of nerve root injury or irritation. It is an active modality that helps to evaluate whether the pedicle screw breaches the cortex and impinges on the spinal canal and/or nerve root. This is performed by active electrical stimulation of the pedicle screw and measuring the threshold at which the adjacent nerve root and corresponding muscle group respond, mostly in the lower extremities, and therefore relevant for the lumbar spine; a lower threshold may indicate a cortical breach prompting repositioning of the screw. While there are no accepted standards with respect to threshold levels in the lumbar spine, there are some guidelines that suggest successful screw placement with a resistance greater than 10mA (Glassman, 1995; Lee, 2015). For thoracic screw placement, a lower threshold of 6mA has been proposed (Raynor, 2002), but interpretation of thoracic screw thresholds is challenging and perhaps falsely reassuring (Samdani, 2011); on the other hand, the use of intercostal EMGs may improve their utility (Rodriguez-Olaverri, 2008; Shi, 2003).
An abnormal EMG response to pedicle screw triggered EMG during a spine procedure may or may not be associated with a clinical deficit (Leppanen, 2005), while on the contrary, normal EMG responses do not insure against lateral breeches. If used, triggered EMG screw stimulation should be considered an adjunct to careful pedicle tract palpation and radiographic evaluation rather than as a standalone evaluation of screw position. 40% of the respondents to the 2019 SRS IONM survey typically use one or both of the above adjunct EMG modalities during deformity surgeries.
Team approach
Neuromonitoring services should be provided in a collaborative team-based intraoperative process that is centered on patient safety. Frequently, intraoperative data is acquired by technologists then relayed to the surgeon, anesthesiologist, neurologist, PhD neurophysiologist, and/or another professional for interpretation; in some settings a neurophysiologist, neurologist or the surgeon obtains and evaluates the IONM data. Technologists require a graded level of supervision depending on their level of education, experience, and credentials. Interpretation may be made in-person in the operating room or by remote consultation in a continuous or intermittent, yet timely, basis. At the interpretative level, there are a number of board certifications that directly apply to IONM. In the United States, The American Board of Neurophysiologic Monitoring requires the provider to have a doctoral degree and significant clinical experience, and the American Board of Clinical Neurophysiology is open to board-certified physicians in neurology, neurosurgery, or psychiatry who have done fellowship training in clinical neurophysiology. Outside the United States, relevant certification is recommended according to community norms for acceptable practice in each country. The development of specific IONM credentials, where none presently exist, may be a consideration, and the above described certification pathways may provide potentially useful models (MacDonald, 2013; Gertsch, 2019).
From a spinal deformity surgical team perspective, it is critical for IONM to be performed by practitioners skilled at both the technical and interpretative aspects of monitoring so that quick responses to changes can be made and conveyed in real time to the operating team. Efforts should be directed at creating the most direct and expeditious flow of information between all intraoperative team members to ensure quality of patient care and safety. Furthermore, the activities of the monitoring team should integrate well with those of the surgical and anesthesia teams, and should involve joint quality assurance and improvement activities (Skinner, 2019).
Anesthesia
Regarding anesthesia, different types of agents can have substantial effects on the utility of the various IONM modalities. Inhalational anesthetics can impact both SSEPs and TcMEPs (Diener, 2010). Total intravenous anesthesia (TIVA) techniques, avoiding volatile agents and nitrous oxide while relying more on intravenous agents, such as Propofol and remifentanil, have been developed to address these concerns such that both SSEPs and TcMEPs can be used successfully and concurrently. Muscle relaxants can also be used with TIVA, although only to a limited potency as they too have an adverse effect on TcMEP and EMG recordings (Owen, 1999). Once again, it is important to emphasize good communication and planning with the anesthesia team for each spinal deformity case in order to optimize the utility of IONM.
As in many other areas of medical intervention, checklists are an important decision aid in navigating the complex algorithm of potential responses when confronted with the loss of neuromonitoring signals and potential neurological injury. Several investigators have attempted to standardize the interpretation of IONM changes and the subsequent response. (Stecker, 2012; Kim SS, 2012). Most recently, in an effort jointly supported by the SRS and POSNA, Vitale et al developed a checklist for response to intraoperative neuromonitoring signal loss using a Delphi-driven consensus-based process which included environmental considerations, anesthetic/systemic factors, technical/neurophysiological variables and surgical details in the potential response. An important part of this checklist is the consideration of consultation with another spine surgeon if possible prior to continuing with the case when confronted with significant intraoperative neuromonitoring change (Vitale, 2014).
In conclusion:
In view of the accumulated research and clinical experience demonstrating the effectiveness of neurophysiologic monitoring, and based on the results of a 2019 member survey, the Scoliosis Research Society concludes that the use of intraoperative spinal cord neurophysiological monitoring during operative procedures that aim to correct spinal deformities is considered optimal care when the spinal cord is at risk, and is strongly recommended unless contra-indicated. The Scoliosis Research Society considers intraoperative real-time neurophysiological monitoring, specifically TcMEP and SSEP, with or without EMG modalities, the standard method for early detection of an evolving or impending spinal cord deficit during surgical deformity correction of the spine, that will allow timely intervention before permanent neurologic injury occurs. The SRS recommends that intra-operative neuromonitoring is adopted worldwide.
References
Acharya S, Palukuri N, Gupta P, Kohli M. Transcranial Motor Evoked Potentials during Spinal Deformity Corrections-Safety, Efficacy, Limitations, and the Role of a Checklist. Front Surg. 2017 Feb 13;4:8. Doi: 10.3389/fsurg.2017.00008. eCollection 2017.
Bartley CE, Yaszay B, Bastrom TP, et al. Perioperative and Delayed Major Complications Following Surgical Treatment of Adolescent Idiopathic Scoliosis. J Bone Joint Sug Am. 2017 Jul 19;99(14):1206-1212. Doi: 10.2106/JBJS.16.01331.
Bieber E, Tolo V, Uematsu S. Spinal cord monitoring during posterior spinal instrumentation and fusion. J Clinical Ortho and Rel Research 229:121-124, 1988.
Boachie-Adjei O, Yagi M, Nemani VM, et al. Incidence and Risk Factors for Major Surgical Complications in Patients With Complex Spinal Deformity: A Report From an SRS GOP Site. Spine Deform. 2015 Jan;3(1):57-64. Doi: 10.1016/j.jspd.2014.06.008. Epub 2014 Dec 18.
Bridwell KH, Lenke LG, Baldus C, Blanke K. Major intraoperative neurologic deficits in pediatric and adult spinal deformity patients: Incidence and etiology at one institution. Spine 23(3): 324-331, 1998.
Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring: A system for intraoperative monitoring of spinal cord function. J Spine 9(3):256-261, 1984.
Burton DC, Carlson BB, Place HM, et al. Results of the Scoliosis Research Society Morbidity and Mortality Database 2009-2012: A Report From the Morbidity and Mortality Committee. Spine Deform. 2016 Sep;4(5):338-343. Doi: 10.1016/j.jspd.2016.05.003. Epub 2016 Aug 21.
Chung I, Grigorian AA. EMG and evoked potentials in the operating room during spinal surgery. In: Schwartz M, ed. EMG Methods for Evaluating Muscle and Nerve Function. InTech; 2011:325-340.
Deiner S. Highlights of anesthetic considerations for intraoperative neuromonitoring. Semin Cardiothorac Vasc Anesth. 2010;14:51-53
Diab M, Smith AR, Kuklo TR. Neural Complications in the Surgical Treatment of Adolescent Idiopathic Scoliosis. Spine 32(24):2759-63, 2007
DiCindio S, Theroux M, Shah S, et al. Multimodality monitoring of transcranial electric motor and somatosensory evoked potentials during surgical correction of spinal deformity in patients with cerebral palsy and other neuromuscular disorders. Spine 28(16):1851-1856, 2003.
Dinner DS, Luders H, Lesser RP, et al. Intraoperative spinal somatosensory evoked potential monitoring. J Neurosurg 65:807-814, 1986.
Drummond D, Schwartz D, Johnston D, and Farmer J. Neurological Injury Complicating Surgery. In: Dewald, R. (ed.) Spinal Deformities: The Comprehensive Text. Thieme Medical Publishers, Inc., NY pp. 615-625, 2003.
Fehlings MG, Brodke DS, Norvell DC, Dettori JR. The evidence for intraoperative neurophysiological monitoring in spine surgery: does it make a difference? Spine (Phila Pa 1976). 2010 Apr 20;35(9 Suppl):S37-46.
Ferguson J, Hwang SW, Tataryn Z, Samdani AF. Neuromonitoring changes in pediatric spinal deformity surgery: A single-institution experience. J Neurosurg Pediatrics 13:247-254, 2014.
Gertsch, J.H., Moreira, J.J., Lee, G.R. et al. J Clin Monit Comput (2019) 33: 175. https://doi.org/10.1007/s10877-018-0201-9
Ginsburg HH, Shetter AG, Raudzens PA. Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. J Neurosurg 63:296-300, 1985
Glassman SD, Dimar JR, Puno RM, et al. A prospective analysis of intraoperative electromyographic monitoring of pedicle screw placement with computed tomographic scan confirmation. Spine 1995; 20: 1375-9.
Hilibrand A, Schwartz D, Sethuraman V, Vaccaro A, Albert T. Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery. J Bone Joint Surg Am. Vol. 86, 1248-1253, 2004
Iwasaki H, Tamaki T, Yoshida M, et al. Efficacy and limitations of current methods of intraoperative spinal cord monitoring. J. Orthop. Sci. 8:635-42, 2003
Jones SJ, Edgar MA, Ransford AO, Thomas NP. A system for the electrophysiological monitoring of the spinal cord during operations for scoliosis. J Bone Joint 65-B(2):134-139, 1983.
Kim SS, Cho BC, Kim JH, et al. Complications of perterior vertebral resection for spinal deformity. Asian Spine J. 2012;6:257-265.
Langeloo DD, Journee HL, Polak B, de Kleuver MA. A new application of TCE-MEP: spinal cord monitoring in patients with severe neuromuscular weakness undergoing corrective spine surgery. J Spinal Disord 2001, 14: 445-448.
Laratta JL, Ha A, Shillingford JN, et al. Neuromonitoring in Sinal Deformity Surgery: A Multimodality Approach. Global Spine J 8(1):68-77, 2018.
Lee CH, Kim HW, Kim HR, Lee CY, Kim JH, Sala F. Can triggered electromyography thresholds assure accurate pedicle screw placements? A systematic review and meta-analysis of diagnostic test accuracy. Clin Neurophysiol. 2015 Oct;126(10):2019-25. doi: 10.1016/j.clinph.2014.11.026. Epub 2014 Dec 15.
Leppanen RE, Abnm D. American Society of Neurophysiological Monitoring: J Clin Monit Cimput 19: 437-61, 2005.
Lyon R, Lieberman JA, Grabovac MT, Hu S. Strategies for managing decreased motor evoked potential signals while distracting the spine during correction of scoliosis. J. Neurosurg. Anesthesiol. 16:167-70, 2004
MacEwen GD, Bunnell WP, Sriram K. Acute neurological complications in the treatment of scoliosis: A report of the Scoliosis Research Society. J Bone Joint Surg 57A:404-408, 1975.
MacDonald DB. Safety of intraoperative transcranial electrical stimulation motor evoked potential monitoring. J Clin Neurophysiol. 2002 Oct;19(5):416-29.
Malhotra NR, Shaffrey CI. Intraoperative electrophysiological monitoring in spine surgery. Spine. 2010;35:2167-2179.
Master DL, Thompson GH, Poe-Kochert C, Biro C. Spinal cord monitoring for scoliosis surgery in Rett syndrome: can these patients be accurately monitored? J Pediatr Orthop. 2008 Apr-May;28(3):342-6. Doi: 10.1097/BPO.0b013e318168d194.
NeiraVM, Ghaffari K, Bulusu S, Moroz PJ, Jarvis JG, Barrowman N, Splinter W. Diagnostic accuracy of neuromonitoring for identification of new neurologic deficits in pediatric spinal fusion surgery. Anesth Analg 123(6):1556-66, 2016.
Nuwer M. Spinal Cord Monitoring, Chapter 3 in Evoked potential monitoring in the operating room. Raven Press, New York, 49-101, 1988.
Owen JH. The application of intraoperative monitoring during surgery for spinal deformity. Spine. 1999;24:2649-2662.
Padberg AM, Bridwell KH. Spinal cord monitoring: current state of the art. Orthop Clin North Am 30: 407-33, 1999.
Pastorelli F, Di Silvestre M, Plasmati R, et al. The prevention of neural complications in the surgical treatment of scoliosis: the role of the neurophysiological intraoperative monitoring. Eur Spine J. 2011 May;20 Suppl 1:S105-14. Doi: 10.1007/s00586-011-1756-z. Epub 2011 Mar 18.
Polly DW Jr, Rice K, Tamkus A. What Is the Frequency of Intraoperative Alerts During Pediatric Spinal Deformity Surgery Using Current Neuromonitoring Methodology? A Retrospective Study of 218 Surgical Procedures. Neurodiagn J. 2016 Mar;56(1):17-31.
Raynor BL, Lenke LG, Kim Y, Hanson DS, Wilson-Holden TJ, Bridwell KH, et al.: Can triggered electromyograph thresholds predict safe thoracic pedicle screw placement? Spine 27:2030-2035, 2002
Rodriguez-Olaverri JC, Mimick NC, Merola A, et al. Using triggered electromyographic threshold in the intercostal muscles to evaluate the accuracy of upper thoracic pedicle screw placement (T3-T6). Spine (Phila pa 1976). 2008;33:E194-197.
Samdani AF, Tantorski M, Cahill PJ, Ranade A, Koch S, Clements DH, Betz RR, Asghar J. Triggered electromyography for placement of thoracic pedicle screws: is it reliable? Eur Spine J. 2011 Jun;20(6):869-74.
Schmitt EW. Neurological complications in the treatment of scoliosis. A sequential report of the Scoliosis Research Society 1971-1979. Reported at the 17th annual meeting of the Scoliosis Research Society, Denver, CO, 1981.
Schwartz D, Drummond D, Schwartz J, et al. Neurophysiological monitoring during scoliosis surgery: A multimodality approach. Seminars in Spine Surgery Vol. 9, No. 2, 97-111, 1997.
Schwartz DM, Auerbach JD, Dormans JP, et al. Neurophysiological detection of impending spinal cord injury during scoliosis surgery. J Bone Joint Surg 89A:2440-9, 2007
Schwartz D, Dormans JP, Drummond DS, et al. Transcranial Electric Motor Evoked Potential Monitoring During Spine Surgery: Is It Safe? Presented at the 42nd Annual Meeting of the Scoliosis Research Society. Edinburg, Scotland, September 6, 2007.
Shi YB, Binette M, Marting WH, et al. Electrical stimulation for intraoperative evaluation of thoracic pedicle screw placement. Spine. 2003;28:595-601
Skinner SA, Aydinlar EI, Borges LF, et al. Is the new ASNM intraoperative neuromonitoring supervision “guideline” a trustworthy guideline? A commentary. J Clin Monit Comput. 2019; 33(2): 185–190. Published online 2019 Jan 5. doi: 10.1007/s10877-018-00242-3
Stecker MM. A review of intraoperative monitoring for spinal surgery. Surg Neurol Int. 2012;3(suppl 3):S174-S187. Doi:10.4103/2152-7806.98579
Stephen JP, Sullivan MR, Hicks RG, et al: Cotrel-Dubousset instrumentation in children using simultaneous motor and somatosensory evoked potential monitoring. Spine 21: 2450-7, 1996.
Strike SA, Hassanzadeh H, Jain A, et al. Intraoperative Neuromonitoring in Pediatric and Adult Spine Deformity Surgery. Clin Spine Surg 30(9):E1174-E1181, 2017.
Thirumala PD, Bodily L, Tint D, et al. Somatosensory-evoked potential monitoring during instrumented scoliosis corrective procedures: validity revisited. Spine J. 2014 Aug 1;14(8):1572-80. Doi: 10.1016/j.spinee.2013.09.035. Epub 2013 Oct 19.
Tsirikos A, Duckworth A, Henderson L, Michaelson C. Multi-Modal Intra-Operative Spinal Cord Monitoring (IOM) During Spinal Deformity Surgery: Efficacy, Diagnostic Characteristics and Algorithm Development Med Princ Pract. 2019 Jun 4. Doi: 10.1159/000501256. [Epub ahead of print]
Vauzelle C, Stagnara P, Jouvinroux P: Functional monitoring of spinal cord activity during spinal surgery. Clin Orthop 93:173-178, 1973.
Vitale MG, Skaggs DL, Pace GI, et al. Best practices in intraoperative neuromonitoring in spine deformity surgery: Development of an intraoperative checklist to optimize response. Spine Deformity. 2014;2:333-339.
Wilber RG, Thompson GH, Shaffer JW, et al: Post-operative neurological deficits in segmental spinal instrumentation. J Bone Joint Surg 66A:1178-1187, 1984.