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)
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.
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).
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 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.
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