Letter: Guidelines for the Use of Electrophysiological Monitoring for Surgery of the Human Spinal Column and Spinal Cord

To the Editor: We read with great interest the article recently published by Hadley et al,1 which purports to be a guideline on the use of intraoperative neurophysiological monitoring (IONM) in spine surgery, and would like to express our concerns regarding serious methodological flaws and systematic errors that substantially limit confidence in its recommendations. In this paper, the authors review and grade the literature on IONM in spine surgery and make recommendations regarding the diagnostic value, therapeutic value, and cost effectiveness of IONM according to modified North American Spine Society criteria. Broadly speaking, the authors conclude that IONM has diagnostic value in spine surgery, but the therapeutic value has not been shown; therefore, the expense of IONM does not justify its use in spine surgery. The work of Hadley et al1 claims to be a guideline yet lacks the critical components of a guideline. Clinical practice guidelines are statements that include recommendations intended to optimize patient care, and usually are created by multidisciplinary societal committees as opposed to single institutions.2 These statements are informed by a systematic review of evidence and an assessment of the benefits and costs of alternative care options.2,3 Additionally, a guideline considers the risks of each procedure, how patients value those risks, and the cost of not using IONM during high-risk surgery.2,3 Aside from increasing the cost of surgery, the harms associated with IONM are minor, and the only alternative to IONM for intraoperative assessment of neurological function is the wakeup test with its limitations.4 The potential benefits of IONM are specific to different surgical procedures that vary considerably both in baseline risks and available interventions that can potentially be used to avoid or limit iatrogenic injury.4 Unfortunately, due to a biased interpretation of the existing literature as we will describe below, the authors neglected to consider all of these points and the paper is more accurately classified as a systematic review. As a systematic review, the manuscript appears to suffer from bias in the inclusion and/or acknowledgement of available research. As just one representative example among many, consider the review of literature related to IONM for intramedullary spinal cord tumor (IMSCT) resection. Perhaps the strongest study ever published on the use of IONM during IMSCT surgery is that of Sala et al.5 Their historically controlled study, using a single, experienced surgeon, compared monitored and unmonitored groups using McCormick Scale scores relative to baseline, and found significant, statistically better outcomes with IONM. By comparison, a careful reading of Choi et al6 shows that they did not take into consideration the patients’ baseline neurological function, and simply compared McCormick Scale scores between the groups. While both studies used the same research design (retrospective comparative), Sala et al5 took the additional step of matching comparison groups for prognostic characteristics. It is thus unclear why Choi et al6 is classified as level II evidence and Sala et al5 is classified as level III evidence. Additionally, the Sala et al5 study is listed in a table but curiously missing from the results text for therapeutic studies where Choi et al6 is highlighted. Finally, the authors omitted 2 IMSCT studies evaluating IONM against unmonitored controls, both of which demonstrated significantly better outcomes with IONM.7,8 In their omission of controlled studies in this review, and in opting to highlight studies that support their position, the authors demonstrate bias that skews the reader's perception toward an unfavorable view of the utility and value of IONM. In summarizing the literature, the authors make the common mistake of repackaging no evidence for improvement with IONM as if it were equal to evidence against improvement with IONM. In making this mistake, the authors suggest that failure to reject the null hypothesis provides evidence equal and opposite to a rejection of the null hypothesis. For example, in their summary of Choi et al,6 the authors state, “The use of IOM did not result in improvement in rate of gross total resection or in neurological outcome. Not an effective therapeutic adjunct.”1 Statements such as this may actually misinform the reader. Absence of effect for a therapy or intervention can result from flawed methods or small samples that can render the study inadequately powered. Indeed, small sample sizes are common in the surgical literature. A perplexing action taken by Hadley and colleagues is their reclassification of the true positive data as false positive data in Nuwer et al.9 Ironically, the original, societal-approved classification strategy in Nuwer et al9 illustrates a fundamental concept regarding IONM in spine surgery: a significant loss of data followed by a timely intervention that returns the data to baseline lends just as favorable a prognosis as no change in data from baseline. Thus, when evoked potential changes are resolved and associated with preserved neurological status, they are most accurately classified as true positives in the literature.10,11 The reclassification by Hadley et al1 misses the main point: both methods for scoring diagnostic test results are susceptible to bias in the “flow and timing” domain because an intervention by the surgeon is often inserted between the IONM test result and postoperative neurological assessments.12 Throughout their review of the literature, the authors make incongruous statements regarding the diagnostic value of different IONM tests. For example, the authors correctly state that somatosensory evoked potentials (SSEPs) monitor somatosensory function (ie, vibration and proprioception), and motor evoked potentials (MEPs) monitor motor function (ie, voluntary muscle strength); then, they continuously report on the independent sensitivity and specificity of SSEPs in diagnosing motor dysfunction. This conflation of surrogate tests for specific clinical end points is a vexing residuum of an era in which SSEPs were the only test available to monitor “spinal cord function.” Contemporary techniques allow for specific functions to be monitored and therein lies the benefit of multimodality monitoring. This basic concept is common knowledge among neurophysiologists, but rarely discussed in the neurosurgical literature. It is rather unfortunate, thus, that Hadley and colleagues1 failed to clarify these issues. The authors repeatedly take issue with application of the phrase “gold standard” to describe IONM in spine surgery, but they fail to recognize that there is no other method available to monitor spinal cord function under general anesthesia. In this way, the authors appear to confuse “gold standard” with “standard of care.” In our view, IONM is the gold standard because there are no alternatives to IONM, but this does not translate to required use of IONM. Whether IONM is the standard of care in spine surgery is a different question altogether and beyond the scope of this “guideline” and our commentary. In summary, IONM is a niche profession with methods, techniques, and data that vary significantly with context and require extensive training and expertise to utilize and interpret. It is our perspective that guideline statements regarding IONM, and systematic reviews of the IONM literature, should contain at least 1 author who is an expert in the field of IONM. The real disappointment of this work lies in the authors’ failure to procure a meaningful guideline on IONM in spine surgery when one is needed. Guideline recommendations that engender confidence use best scholarship and evidence for the risks and benefits of treatment effects and how these are valued by patients. These standards, described by the Institute of Medicine,2 provide a foundation as we work together to improve patient care. Unfortunately, Hadley et al1 produced a rather biased review that failed to meet these standards. In doing so, their publication does irreparable harm because it provides erroneous information to patients and surgeons who may benefit from IONM. For all of the reasons indicated above, we recommend that the authors publish an erratum to both enhance objectivity and reclassify their paper as a systematic review. Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.