Stanley A. Skinner

Intraoperative motor evoked potential monitoring – A position statement by the American Society of Neurophysiological Monitoring

The following intraoperative MEP recommendations can be made on the basis of current evidence and expert opinion: (1) Acquisition and interpretation should be done by qualified personnel. (2) The methods are sufficiently safe using appropriate precautions. (3) MEPs are an established practice option for cortical and subcortical mapping and for monitoring during surgeries risking motor injury in the brain, brainstem, spinal cord or facial nerve. (4) Intravenous anesthesia usually consisting of propofol and opioid is optimal for muscle MEPs. (5) Interpretation should consider limitations and confounding factors. (6) D-wave warning criteria consider amplitude reduction having no confounding factor explanation: >50% for intramedullary spinal cord tumor surgery, and >30-40% for peri-Rolandic surgery. (7) Muscle MEP warning criteria are tailored to the type of surgery and based on deterioration clearly exceeding variability with no confounding factor explanation. Disappearance is always a major criterion. Marked amplitude reduction, acute threshold elevation or morphology simplification could be additional minor or moderate spinal cord monitoring criteria depending on the type of surgery and the programs technique and experience. Major criteria for supratentorial, brainstem or facial nerve monitoring include >50% amplitude reduction when warranted by sufficient preceding response stability. Future advances could modify these recommendations.

The Initial Use of Free-running Electromyography to Detect Early Motor Tract Injury during Resection of Intramedullary Spinal Cord Lesions

OBJECTIVE: The resection of intramedullary spinal cord lesions (ISCLs) can be complicated by neurological deficits. Neuromonitoring has been used to reduce intraoperative risk. We have used somatosensory evoked potentials (SEPs) and muscle-derived transcranial electrical motor evoked potentials (myogenic TCE-MEPs) to monitor ISCL removal. We report our retrospective experience with the addition of free-running electromyography (EMG). METHODS: Thirteen patients underwent 14 monitored ISCL excisions. Anesthesia was maintained with minimal inhalant to reduce motoneuron suppression and enhance the myogenic TCE-MEPs. Free-running EMG was examined in the four limbs for evidence of abnormal bursts, prolonged tonic discharge, or sudden electrical silence. Warning of an electromyographic abnormality or myogenic TCE-MEP loss prompted interventions, including blood pressure elevation, a pause in surgery, a wake-up test, or termination of surgery. Pre- and postoperative neurological examinations determined the incidence of new deficits. RESULTS: The combined use of free-running EMG and myogenic TCE-MEPs detected all eight patients with a new motor deficit after surgery; there was one false-positive report. In three of the eight true-positive cases, an electromyographic abnormality immediately anticipated loss of the myogenic TCE-MEPs. Two patients with abnormal EMGs but unchanged myogenic TCE-MEPs experienced mild postoperative worsening of motor deficits; myogenic TCE-MEPs alone would have generated false-negative reports in these cases. CONCLUSION: During resection of ISCLs, free-running EMG can supplement motor tract monitoring by TCE-MEPs. Segmental and suprasegmental elicitation of neurotonic discharges can be observed in four-limb EMG. Abnormal electromyographic bursts, tonic discharge, or abrupt electromyographic silence may anticipate myogenic TCE-MEP loss and predict a postoperative motor deficit.

Electromyography detects mechanically-induced suprasegmental spinal motor tract injury: Review of decompression at spinal cord level

OBJECTIVE Herein, we report use of electromyography (EMG) to anticipate corticospinal conduction block, as defined by muscle-derived transcranial electrical motor evoked potential (TCE MEP) loss, during extradural spinal cord decompression. METHODS One hundred and eighty-four patients underwent cervical (173) or thoracic (11) decompression. The same derivations were recorded for EMG and TCE MEP neuromonitoring. When highly repetitive, complex, and prolonged EMG discharges were identified in myotomes below the operated level (severe suprasegmentally-generated EMG discharges=severe SEDs), a report of possible spinal cord impact was made and a TCE MEP obtained. TCE MEP loss (with or without antecedent SEDs) was defined as >90% amplitude reduction compared to baseline recordings. RESULTS Severe SEDs, seen in 15 cases, anticipated TCE MEP loss in 7/15. In 13/15 severe SED cases, manipulations near dura were the proximate cause. Interventions after TCE MEP loss included changed instrumentation, re-positioning, increased blood pressure, wake-up test, and surgical pause. CONCLUSIONS SEDs can be identified during extradural spinal cord decompression. Severe SED occurrence is associated with a approximately 50% risk of subsequent corticospinal conduction block. SIGNIFICANCE Although SED occurrence does not provide specific information for lesions of the fast neurons of the corticospinal tract, SED surveillance during decompression at spinal cord level can supplement TCE MEP recording.

Practice guidelines for the supervising professional: intraoperative neurophysiological monitoring

The American Society of Neurophysiological Monitoring (ASNM) was founded in 1988 as the American Society of Evoked Potential Monitoring. From the beginning, the Society has been made up of physicians, doctoral degree holders, technologists, and all those interested in furthering the profession. The Society changed its name to the ASNM and held its first Annual Meeting in 1990. It remains the largest worldwide organization dedicated solely to the scientifically based advancement of intraoperative neurophysiology. The primary goal of the ASNM is to assure the quality of patient care during monitored procedures along the neuraxis. This goal is accomplished through programs in education, advocacy of basic and clinical research, and publication of guidelines. The ASNM is committed to the development of medically sound and clinically relevant guidelines for intraoperative neurophysiology. Guidelines are formulated based on exhaustive literature review, recruitment of expert opinion, and broad consensus among ASNM membership. Input is likewise sought from sister societies and related constituencies. Adherence to a literature-based, formalized process characterizes the construction of all ASNM guidelines. The guidelines covering the Professional Practice of intraoperative monitoring were established by a committee of nearly 30 total participants and ultimately endorsed by the Board of Directors of ASNM on January 24th 2013. That document follows.

An evaluation of motor evoked potential surrogate endpoints during intracranial vascular procedures.

OBJECTIVE MEPs are used as surrogate endpoints to predict the effectiveness of interventions, made in response to MEP deterioration, in avoiding new postoperative deficits. MEP performance in capturing intervention effects on these outcomes was investigated. METHODS A meta-analysis of studies using MEPs during intracranial vascular surgeries between 2003 and 2014 was performed. MEP diagnostic performance and relative risk of new postoperative deficits for reversible compared with irreversible MEP changes were determined. Intervention efficacy in reversing MEP deterioration and postoperative outcomes was compared across studies. RESULTS MEP diagnostic performance compared favorably with that of other tests used in medicine, with all likelihood ratios >10. The summary relative risk comparing reversible and irreversible changes was 0.40, indicating a 60% decrease in new deficits for reversible MEP changes. The proportion of MEP deteriorations which recovered was negatively correlated with the proportion of new postoperative deficits (r=-0.81, p<.005). CONCLUSIONS The effectiveness of interventions in recovering an MEP decline was predictive of preserved neurologic status. MEPs are provisionally qualified as surrogate endpoints given potentially major harms to the patient if they are not used, compared to the minimal harms and costs associated with their use. SIGNIFICANCE The performance of MEPs as substitute, or surrogate, endpoints during intracranial vascular surgeries for new deficits in motor strength in the immediate postoperative period was directly assessed for ten recent studies.

Journal of Clinical Monitoring and Computing 2015 end of year summary: anesthesia

Clinical monitoring is an essential part of the profession of anesthesiology. It would therefore be impossible to review all articles published in the Journal of Clinical Monitoring and Computing that are relevant to anesthesia. Because other reviews will address monitoring of the respiratory and cardiovascular system, the current review will limit itself to topics uniquely related to anesthesia. The topics are organized according to the chronological order in which an anesthetic proceeds: secure the airway; ventilate and deliver anesthetic gases; monitor vital organ function and anesthetic depth; and ensure analgesia during/after emergence from anesthesia (locoregional anesthesia and pain control).

IONM evidence: Putting the work and insight of Téllez et al. in context.

True positive (TP): Intraoperative testing meets alarm criteria through the end of surgery and the patient awakens with a new deficit relevant to the monitored neural system. True negative (TN): Intraoperative testing remains unchanged through the end of surgery and the patient awakens with no new deficit relevant to the monitored neural system. False negative (FN): Intraoperative testing remains unchanged through the end of surgery but the patient awakens with a new deficit relevant to the monitored neural system. False positive (FP): Intraoperative testing meets alarm criteria through the end of surgery but the patient awakens with no new deficit relevant to the monitored neural system. Reversible signal change: Intraoperative testing meets alarm criteria during surgery but reverses to an unchanged/baseline result (most often after an intervention). Intraoperative testing is unchanged/baseline) at the end of surgery. Note: This is a particular problem of IONM when the reference standard (clinical condition of the patient) cannot be assessed in real time. AB Hill’s cause/ effect criteria can be used to evaluate and score reversible signal change results (Skinner and Holdefer, 2014). Positive predictive value (PPV): The fraction of all positive results that are true is given by (TP/TP + FP). False positive discovery rate: The fraction of all positive results that are false is given by (FP/TP + FP). Harner reported better anatomical preservation of the seventh nerve in a monitored cohort versus a matched historical cohort of unmonitored large vestibular schwannoma resections. Among medium/large sized tumors, differences favoring the monitored group were seen at 3 months but not in the initial post-operative epoch (Harner et al., 1987). For three decades, cranial nerve intraoperative neurophysiological monitoring (IONM) has been predicated on EMG and in situ cranial nerve stimulation (CrN S). On anatomical and physiological grounds, there is reason to believe intraoperative transcranial electrical stimulation to effect a facial or other cranial nerve motor evoked potential (CrN MEP) should yield truer IONM results than CrN S. As with CrN S, the final/baseline peak amplitude ratio is the commonly reported metric (Dong et al., 2005; Akagami et al., 2005; Fukuda et al., 2008). Acioly introduced other intraoperative data points (tumor dissection/baseline amplitude, for example) and extended reports of follow up (3–26 months) in an attempt to flesh out the predictive power of CrN MEPs (Acioly et al., 2011). The theoretical ability of CrN MEP to assess corticobulbar, nuclear, and nerve conduction favorably contrasts to the frequent inability to stimulate the 7th nerve proximally in large vestibular schwannoma procedures (Dong et al., 2005). Among the limitations of CrN MEP is its predictive rather than preventative use. This was pointed out by Dong and colleagues early on. No reversible signal change was identified by them (Dong et al., 2005). Akagami et al. do allude to the benefit of ‘‘a temporary decrease in MEPs [which] can often alert the neurosurgeon to be careful and redirect work towards a different part of the tumor, and return to the original area of dissection once the MEP recovers.” However, this potentially preventative aspect of CrN MEP recording was not reported by these investigators or the Acioly group (Akagami et al., 2005; Acioly et al., 2011). Setting the alert criterion sensitive enough to avert FN reporting also limits CrN MEP. The commonly accepted >50% amplitude reduction alarm may contribute to FP prediction. The Dong group reported eighteen positive CrN MEPs (11 true, 7 false) at the 50% cut-off amplitude: a false discovery rate of 39% (FP/TP + FP = 1 PPV). Using similar scoring, the Akagami group’s false discovery rate is 42% (7 true, 5 false positive). This measure may be more meaningful to surgeons than the often reported FP rate (FP/FP + TN). The surgeon wants to know: what is the rate of true vs false reports after an alarm. Because many patients with new post-operative deficits tend to improve in later follow-up (Acioly et al., 2011), the immediate post-operative data may underestimate the ultimate FP discovery rate. The study by Téllez and colleagues in this issue of Clinical Neurophysiology raises a fresh concern regarding the accuracy of CrN MEPs (Téllez et al., 2016). Because peripheral subthreshold axonal stimulation generates a variable period of superexcitability, the extracranial leak of subthreshold pulse trains may generate a pseudo-CrN MEP and false negative reporting (Burke et al., 2001; Bostock, 2005). The already known exigencies of CrN MEP recording prevent all but the most experienced practitioners from attempting this modality. This preliminary report strongly suggests further inquiry into underlying CrN MEP physiology. The authors’ data is preliminary, but our hunch is that they have identified a critical flaw in the test. Fortunately, few FN CrN MEP cases are currently reported. That fact suggests that an imperfect understanding of CrN MEP physiology is not generally reflected in studies so far. Nevertheless, given this elegant test’s challenges, broader questions of diagnostic test accuracy and effectiveness should be asked. In the near term, the continued publication of case series (‘‘traditional focused studies”) and position papers (primarily

Practice patterns for intraoperative neurophysiologic monitoringAuthor Response

Many readers will applaud the defense by Nuwer et al.1 and Emerson and Husain2 of “local” intraoperative neuromonitoring (IONM) supervision. However, their nostalgic case for local supervision has already been put before the Current Procedural Terminology (CPT) panel, which made its findings very clear: there is no difference between remote and local IONM. Neither model mandates the normal predicates of patient care: a patient–physician relationship; interaction among …

Saturday, November, 2, 2001 3:29–4:12 pm Concurrent Session 7B: Monitoring

Abstract Purpose of study: Epidural myogenic MEP may enhance somatosensory evoked potentials (SSEP) spinal cord monitoring because of reported SSEP false negatives. This MEP method requires more stringent anesthesia and placement of epidural electrodes. We compared MEP to SSEP with regard to ease of acquisition, reproducibility and sensitivity to incipient and complete spinal cord conduction failure. Methods used: A total of 228 deformity corrections were included. The anesthetic regimen included propofol/narcotic infusion. A greater than 50% reduction in the monitored response, not explained by obvious technical failure or anesthetic change, indicated remedial increase blood pressure (BP) or change in instrumented position and, if no improvement, an elective Stagnara wake-up test. of findings: Monitorable potentials were acquired in 95.6% epidural myogenic MEP, 95.1% SSEP, 94.3% both and 2.6% neither. Epidural myogenic MEP demonstrated a better signal/noise ratio and faster acquisition times than SSEP. Thus, loss of signal was always identified first by the MEP. Combined MEP and SSEP loss occurred in seven cases: three were true positives (paraplegic on elective wake-up) with significant postoperative improvement, four recovered MEP-SSEP during the course of an elective negative wake-up with concomitant increased BP. MEP loss with preserved SSEP in two other patients recovered by increased BP in one case and change of instrumented position in the other. No patient woke up with a new myelopathic deficit when monitored potentials were normal. Relationship between findings and existing knowledge: Few studies are available comparing SSEP and spinal cord myogenic MEP in spinal cord monitoring. Furthermore, few studies are available describing evoked response changes in documented interoperative spinal cord injuries. Overall significance of findings: Epidural myogenic MEP detects complete spinal cord conduction block (paraplegic on elective wake-up) before SSEP. Incipient spinal cord conduction failure (EP loss and recovered by increase BP, instrumentation change or performance of a wake-up test) is recognized with greater sensitivity by MEP than SSEP. Disclosures: No disclosures. Conflict of interest: No conflicts.