SPINE Volume 35, Number 6, pp 714 –723 ©2010, Lippincott Williams & Wilkins

Letters

To the Editor: Re: False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009; 34:e896 –900. We read with concern the report by Modi et al1 in a recent issue of Spine detailing their unfortunate experience, with a patient awaking paraplegic after spine deformity correction surgery, with apparently no change in the evoked responses. This would appear to be, as they state, the first such case in the literature and as such is important. In this case, the surgeons used an automated neuromonitoring system that relied on MEPs only. This type of approach is new and controversial. Recently, Hsu et al proposed, in this journal, the MEP and EMG only approach for monitoring of spine deformity surgery.2 At the time, we expressed concern about this approach3 and the potential for sensory deficits to be missed. The relative ease, safety, and efficacy of SSEPs have been wellestablished and their use is, in our opinion, a required component of neuromonitoring in spine deformity surgery. In this case, we are provided with representative traces from 4 time points during the case, however, these are somewhat difficult to read given overlap between traces. It is apparent from these traces that there were some changes in the recordings during the surgery. It is of course, unknown whether these changes would have been detected using SSEPs, but that possibility must be considered. Also of grave concern is the possibility that this was a real change in MEPs that was not detected using the automated system. Reliance on automation in this field is a real concern among many professionals in the field. For many of us it is not clear that these systems are as safe as the neurophysiologist driven systems. Head-to-head trials of the 2 types of system are only just starting. We would support the call for more basic and applied research in this field. Jonathan Norton University of Alberta Hospital Alberta, Canada References 1. Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motorevoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 –900. 2. Hsu B, Cree AK, Lagopoulos J, et al. Transcranial motor-evoked potentials combined with response recording through compound muscle action poten-

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tial as the sole modality of spinal cord monitoring in spinal deformity surgery. Spine 2008;33:1100 – 6. 3. Norton JA. Re: Transcranial motor-evoked potentials combined with response recording through compound muscle action potentials as the sole modality of spinal cord monitoring in spinal deformity surgery. Spine 2008;33: 2576.

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

In Response: In their letters, the critics were convinced that due to a technical and interpretive error, the data obtained were actually exactly opposite, and that a true-positive was not recognized by the authors. They described that authors failed to recognize spinal cord injury, presumably due to the reversed channels. We have clarified that none of the leads were reversed. Both upper and lower extremity leads were placed correctly throughout the surgery. And therefore, there was no mistake while interpreting the data. Additionally, we always keep one wellexperienced neurotechnician for MEP application as well as timely reading. In our clinic, more than 100 cases of scoliosis surgery (idiopathic, neuromuscular, as well as congenital etiology) are getting operated each year, meaning that our surgical team including the neuromonitoring part is professional not an amateur. Hence, we decided to reply the given criticism. Reversed Electrode? Our arrangement of leads in upper and lower extremity with the connecting channel is such that the upper extremity lead is always close to head needle, and therefore, when the upper lead is placed wrongly in lower extremity, the length of electric wire will get tightened and will be identified. As long as technician is in conscious state, misplacement of leads is unlikely to happen. And as the connecting channel with the lead is the colored one, there are no chances of mismatching the color. Therefore, we cannot understand the “reversed leads” hypothesis. In addition, we do not have experiences regarding this point.

Letters 715

Figure 1. Tc-MEP signals should have theoretically shorter latency period in upper extremity than lower extremity; however, sometimes it show similar latency period (A–B) or even larger latency (C, bottom). (From starting to vertical dotted line is the stimulation period which is divided into eight stimulations and which stimulation evoke the potential cannot be known.)

Amplitude Is Always Larger in Upper Extremity? In our case, lower amplitude in upper extremity compared to lower extremity was probably due to the placement of the stimulating leads in the skull. We have several experiences in which the amplitude in the lower extremity is larger than the upper extremity, i.e., depending on the location of needle inserted near the motor cortex (C3, C4) medially or laterally, amplitudes in the upper and lower extremity can be changed (Figure 1A, B). And, if the MEP signal is not favorable to recognize, we can change the insertion point medial or lateral direction to obtain more precise and accurate MEP results (especially during neuromonitoring in neuromuscular scoliosis). Latency Is Always Shorter in Upper Extremity? Repetitive Stimulating System Yes, normally it acts like this. However, we checked few MEP records of other patients, which showed that latency of upper extremity is not always shorter than that of lower extremity. In many occasions, it also occurred simultaneously (Figure 1A–C). Our argument is that if it is due to only distance from the skull to extremity, upper extremity should always show shorter latency than lower extremity; and, in that case, similar latency also should not be observed. We believe there are some other factors which influence on the latent period. The first one is that we used neuromonitoring system (NIM Spine-Sofamore Danek, Medtronic, US) that stimulates the motor cortex several times (8 times in our case) per one seconds to elicit the motor-evoked potential. Of the 8 electrical stimulations, it is impossible to know which one evoked the potential. Also, depending on the position of needle in skull medially or laterally, threshold for evoked potential between upper and lower extremity could be different. If needle is placed medially closer to the center of motor cortex lower extremity, threshold for motor evoking in lower extremity will be lower. Therefore, low lower extremity can respond with first or second of 8 stimulations due to its lower threshold. For upper extremity, if needle is placed medially, i.e., distance from center of motor cortex of upper extremity is increased; it

will result in increased threshold for initiating evoked potentials and may take full period of 8 stimulations showing longer latency in upper extremity. For example, if needle is placed medially with lower extremity being closer with low threshold, the first of 8 stimulations would evoke a potential, and, on the other hand, upper extremity being farther due to medially placed needle, the last of eight stimulations would evoke a potential. Thus, there can be the difference of 1second in the latency period between upper and lower extremity. In monitoring system using single-time electrical stimulation, latency between upper and lower extremity can be measured as one might expect theoretically because latency between starting point for electrical stimulation and responding point can be measured clearly; however, in a system using repetitive stimulating system, we cannot measure it theoretically because it is impossible to know starting point which evoked the potentials in the upper and lower extremity i.e., which 1 of the 8 stimulations caused the evoked potential. If we take this characteristics and working mechanism of our repetitive stimulatory monitoring system, it is not impossible for the lower extremity to have shorter latency. Neurofibromatosis as an Underlying Pathology Discussing the second factor, in addition to mechanical view of velocity of wave and length of nerve fiber, there must be other biologic factors that affect the nerve conduction involved in the latency phase eliciting the MEP response. It means, nerve fiber is simply not an electric wire which is responding to electric stimulation mechanically. Neuronal fiber is a complicated structure responding to electrical stimulation differently with different mechanism of neurotransmission. This idea is supported by the fact that different latency of onset developed even between persons with similar height and limb length. We should remember that our patient has neurofibromatosis, and due to that there is possibility of different latency between upper and lower extremity. To prove our explanation, we called the same patient to outpatient clinic and performed MEP in the physical rehabilitation department. However, due to paralysis, lower extremity failed to show signal, therefore we could not compare

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Figure 2. A–E, Tc-MEP measurements in a patient with neuromuscular scoliosis. Figure shows that there are high fluctuations in the MEP amplitudes in the cervical spine even though there was no fixation in cervical or first two thoracic levels. Additionally we can observe reverse latency pattern in that showing shorter latency in L5 than C8 –T1 level. It is also possible to note that there is higher amplitudes in lower extremity than the upper extremity.

with upper extremity amplitudes and latency. Here, we should note that the patient has paralysis now, and therefore, the amplitude in lower extremity is absent. Criticism that upper extremity should have smaller, faster, or shorter latency period than lower extremity due to the distance difference length between upper and lower extremity might work only in a condition or assumption that human nerve fibers’ working mechanism is similar to electric wire; however, due to the reason that the neuronal fiber is biologic structure with complicated mechanism, science still could not fully understand it and can behave differently depending on individual difference or associated disease.1–3 We also want to clarify that inserting the pedicle screw in level T2-L3 can not affect the amplitudes in the cervical spine as asked by many other authors through correspondence. Here, we are attaching timely obtained MEP figures (Figure 2A–E) from a patient having scoliosis. The figure shows that there is a high fluctuation in the amplitudes in MEP for cervical spine in addition to observed difference in the latent period between different leads. Therefore, we further emphasize that amplitudes and the latency can be varied in a patient. We thoroughly considered all the concerns raised by many authors. We also thought that till now there was no false-negative report. In addition, one of other author’s papers reported that there was no false-negative

case in more than 1000 cases of monitoring in AIS surgery.4 We also support the reliability of MEP system, and we also use this system now. However, proven reliability in large cohort study cannot block the possibilities of the false-negative MEP monitoring results, even though it can be very difficult to believe. Our case report suggested that false-negative MEP can occur in scoliosis surgery. We again agree that it is not a common situation in AIS. We again would like to clarify that the data presented in our article is correct and we did not erroneously claim false-negative transcranial MEP. We also received the letter from another famous author threatening to retract the paper otherwise we would have to face public embarrassment. Why are we intended to erroneously claim this point? We have full faith in this system and we would continue to use it in future also. However, that does not mean that such false-negative case is not possible. On the other hand, we also thank to those who appreciated our manuscript via personal email and supported our conclusion. False-negative MEP could happen in surgery like scoliosis, especially when associated with neuromuscular disease. Therefore, we would like to propose further prospective animal studies using MEP. We also believe that simultaneously use of SSEP and MEP would further reduce such chances of false-negative results. Our article would be definitely

Letters 717

helpful for those who are closely related with the work of MEP and related research. Hitesh N. Modi, MS, PhD Seung Woo Suh, MD, PhD Jae-Hyuk Yang, MD Ji-Yoel Yoon, MD The Scoliosis Research Institute Department of Orthopedics Korea University Guro Hospital Seoul, South Korea References 1. Sailer A, Molnar GF, Paradiso G, et al. Short and long latency afferent inhibition in Parkinson’s disease. Brain 2003;126:1883–94. 2. Sartucci F, Tovani S, Murri L, et al. Motor and somatosensory evoked potentials in autosomal dominant and hereditary spastic paraparesis (ADHSP) linked to chromosome 2p, SPG4. Brain Res Bull 2009;74:243–9. 3. Tataroglu C, Genc A, Idiman E, et al. Cortical relay time for long latency reflexes in patients with definite multiple sclerosis. Can J Neurol Sci 2004;31: 229 –34. 4. Schwartz DM, Auerbach JD, Dormans JP, et al. Neurophysiological detection of impending spinal cord injury during scoliosis surgery. J Bone Joint Surg Am 2007;89:2440 –9.

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

To the Editor: Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 –900. We are writing in response to the case report published in Spine by Modi et al.1 This report claims that transcranial motor-evoked potential (tcMEP) monitoring failed to detect a spinal cord injury during correction of a severe scoliosis, i.e., a false-negative. We are convinced that, due to a technical and interpretive error, the data actually shows exactly the opposite, and that a truepositive was not recognized by the authors. The resulting misinterpretation not only led to a postoperative paralysis that might have been avoided, but to a case report that questions the utility of a technique that has become the standard of care for monitoring such cases. The essence of the problem can be simply stated: the traces labeled as lower extremity tcMEPs are clearly of a shorter latency than those labeled as upper extremity tcMEPs, which is physiologically impossible due to the greater length of the motor pathway to the lower extremities. This undoubtedly reflects an inadvertent and undetected reversal of the electrodes so that those from the lower extremities were plugged into the upper extremity channels, and vice versa. When the tcMEP responses are examined with this in mind, it is clear that at the time of the major scoliosis correction, the lower extremity responses (labeled upper) were virtually lost, with an amplitude decrement of

⬎95%. In contrast, the upper extremity responses (labeled lower) were essentially unchanged. This is consistent with a loss of spinal cord function at thoracic and/or lower levels, with preserved cervical cord function: exactly what was clinically observed after surgery. The authors failed to recognize this, presumably due to the reversed channels, (along with other technical problems). The authors of this report did not describe how the neuromonitoring was performed, nor by whom. However, it seems obvious to us that an experienced neurophysiologist was “not” involved, or else these technical errors would have been recognized and corrected at the outset, possibly avoiding the tragic outcome. It is also obvious to us that the manuscript was not reviewed by a similarly experienced individual, or these errors would have been immediately identified and the manuscript would not have been published. It is too late to change the outcome for the unfortunate patient in question. However, it is not too late to undo the potential damage to future patients, were this report to be allowed to stand and be used as evidence that the tcMEP technique lacks validity as an intraoperative indicator of spinal compromise. We feel that the only appropriate course of action is for this article to be formally withdrawn from publication, with an explanation of the error that occurred. We also strongly recommend that the journal protect its reputation for scientific integrity by ensuring that future submissions involving intraoperative neurophysiology be reviewed by at least one referee with documented expertise in this field. Jeremy A. Lieberman, MD Department of Anesthesia & Perioperative Medicine University of California San Francisco, CA Sigurd Berven, MD Department of Orthopedic Surgery University of California San Francisco, CA John Gardi, PhD, DABNM, FASNM, CCC-A Evoked Potential Associates, Inc. California Neuromonitoring Services (CNS) Serena Hu, MD Department of Orthopedic Surgery University of California San Francisco, CA Russ Lyon, MS, DABNM Division of Operating Rooms University Hospital, University of California San Francisco, CA David B. MacDonald, MD, FRCP(C), ABCN Section of Neurophysiology Department of Neurosciences King Faisal Specialist Hospital and Research Center Riyadh, Saudi Arabia

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Daniel Schwartz, PhD, DABNM, FASNM Surgical Monitoring Associates, Inc. Springfield, PA Anthony Sestokas, PhD, DABNM, FASNM Surgical Monitoring Associates, Inc. Springfield, PA Charles Yingling, PhD, DABNM, FASNM Department of Otolaryngology/Head and Neck Surgery Stanford School of Medicine California Neuromonitoring Services San Francisco, CA Reference 1. Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 – 900.

The device(s)/drug(s) is/are FDA-approved or approved by a corresponding national agency for this indication. No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

In Response: Dear author, thanks for your comment and for supporting our conclusion about the need for basic and applied research in this matter. This false-negative case using only transcranial MEP monitor in a case of scoliosis with neurofibromatosis was really an unfortunate event for us; and, the surgeon often feels disappointed when such complications happen despite of taking enough care and neuromonitoring. We agree with your concern about the previous report by Hsu et al, suggesting monitoring with only MEP and EMG.1 We feel that in cases where we cannot elicit a D wave while using MEP, SSEP should be combined for the safety. We have although provide 4 representative traces; however, the MEP recordings were taken periodically, which did not show any signs of possible injury. We agree with your previous article suggesting that using SSEP along with this case, complications might have been avoided or detected. Thanks again for supporting the need of further research on this issue. Hitesh N. Modi, MS, PhD Seung Woo Suh, MD, PhD Jae-Hyuk Yang, MD Ji-Yeol Yoon, MD The Scoliosis Research Institute Department of Orthopedics Korea University Guro Hospital Seoul, South Korea Reference 1. Hsu B, Cree AK, Lagopoulos J, et al. Transcranial motor-evoked potentials combined with response recording through compound muscle action poten-

tial as the sole modality of spinal cord monitoring in spinal deformity surgery. Spine 2008;33:1100 – 6.

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Rebuttal: The attempt by Suh et al to refute technical errors and clinical misinterpretations in their case report falls astonishingly short on neuroanatomical, neurophysiological, and technical grounds. While we feel that our initial “Letter to the Editor” and detailed appendix provide clear evidence that the case report was not a false-negative, but a true positive which was missed by the interpreting neuro-technician, it is important to address specific counter arguments made by the authors. Reversed Electrode? The authors state “our arrangement of leads . . . is such that the upper extremity lead is always close to head needle, and therefore, when the upper lead is placed wrongly in lower extremity, the length of electric wire get tightened and will be identified.” In further defense that the electrodes were not reversed the authors claim that this would not be possible because: (1) the electrodes are color coded and (2) experience in monitoring 100 scoliosis cases a year make such a basic technical error virtually inconceivable. The authors’ argument against the possibility of a basic technical error is unconvincing in light of the data presented in the case report and given our own collective experience in neuromonitoring of over 50,000 spine surgeries during the past 30 years. While it is true that one of the purposes of standardizing technique, including use of colored lead wires, is to help prevent plugging electrodes into the wrong amplifier input jacks, the fact is that such errors are far from uncommon. It is the responsibility of the neuromonitoring team in the operating room to identify these errors when they occur and to correct them. When there are inconsistencies in the baseline data, as in the present case, it is incumbent on the attending neurophysiologist or neuro-technician to look for explanations, starting with a re-examination of basic technical factors. Even if the authors are correct that they absolutely did not reverse the upper and lower extremity electrodes at the amplifier input box, it is still quite plausible that the reversal occurred within the settings of the NIM-Spine system. Our discussions both with Medtronic engineers and technical support personnel confirmed that the Medtronic system recording channels could have been relabeled incorrectly at the display level by the system operator, even if the electrodes were connected to the amplifier input box in standard and usual fashion.

Letters 719

The lack of attention to technical detail in the case report, inclusion of figures of marginal interpretability, disregard for anomalies in the baseline data and missed changes in the neurophysiological signals during surgery belie the authors’ claim of professional neuromonitoring in this procedure. This impression is reinforced by absence of somatosensory-evoked potential monitoring during the procedure, which would have been sensitive to the acute loss of sensory function below T6. Perhaps most disconcerting is that if the authors truly believed that the upper and lower extremity electrodes were not reversed, either at the amplifier input box or display levels, then why did they not question the fact that beginning immediately after upper thoracic pedicle screw placement (Figure 2B) the transcranial electric motor-evoked potential (tceMEP) amplitudes for left and right hand muscle responses decreased by 95% and 70%, respectively. At the very least this should have prompted an immediate alarm by the neuro-technician that something had caused an acute loss of response amplitude in the upper extremity recording sites, bilaterally. The dramatic signal change, whether due to pedicle screw impingement on the spinal cord or even emerging positional brachial plexopathy warranted prompt investigation by the neuro-technician and subsequent intervention, which did not occur. Moreover, it is difficult to justify continued lack of action when these responses disappeared completely, resulting in a “flat line”(Figures 2C, D) following deformity correction. To be sure, Dr. Suh’s presentation of data from another patient to argue that upper extremity response amplitudes and latencies can be variable does not address the issue at hand. Do the authors truly believe that an amplitude loss of 70% to 100% bilaterally falls within an acceptable range of normal variability as they state? If that were the case, then tceMEP monitoring would be effectively useless because virtually any change in signal amplitude could be explained by “normal” variability, resulting in zero test sensitivity. Who would use such a test and on what basis would a major medical instrument manufacturer like Medtronic sell such a device? Latency Is Always Shorter In The Upper Extremity? Dr. Suh maintains that normally this is true; however, “. . . we checked few MEP records of other patients who showed that latency of upper extremity is not always shorter than lower extremity. In many occasions it also occurred simultaneously.” Dr. Suh goes on to argue that because they used multipulse stimulation this may have been the cause of the unusual latency reversal for upper and lower extremity responses, or, perhaps it was due to the fact that this patient had neurofibromatosis. Here again, Dr. Suh’s explanation is completely unsubstantiated relative to both the world’s literature on evoked potential recordings, whether somatosensory or motor, as well as our combined experiences and data bases, including children with neurofibromatosis. Indeed, examination of the new Figure 1 submitted by the authors showing data

from another patient provides no evidence for latency reversal for upper and lower extremity responses. Dr. Suh’s claim that their stimulating electrodes were more medial and hence fired the feet first, defies basic principals of motor cortex stimulation. He tries to explain his case report data by suggesting that use of a NIM-Spine system, which incorporates multipulse tceMEP stimulation (N ⫽ 8 pulses), could reverse latency because the early pulses fire the feet and the later pulses in the series then trigger the hands. The NIM-Spine Neuromonitoring system is not unique in its use of multipulse stimulation for eliciting tceMEPs, a technique that has been used routinely to record myotomal tceMEPs since the early 1990s, more than a decade before introduction of the Medtronic NIM-Spine system. More pointedly, the argument that individual electrical pulses stimulate specific parts of motor cortex is completely unfounded. The multiple pulses used in this technique create a temporal series of D waves which are conducted down to the ventral horn of the spinal cord where they favorably summate to depolarize alpha motor neurons, facilitating larger compound muscle action potentials in both upper and lower extremities. The fact is that no one has ever observed a latency reversal secondary to multipulse stimulation, such as the one proposed by Dr. Suh and his associates. There also was no reason to believe that neurofibromatosis in this patient would produce reversal of upper and lower extremity response latencies. In fact, if anything, latencies of the lower extremity responses could be even more exaggerated relative to those of the upper extremities in the presence of neurofibromatosis owing to conduction delays from spinal cord mass effects in the thoracic region. Overall, the neuroanatomical, neurophysiological and technical arguments presented by the authors in response to criticisms of their case report are not convincing and suggest a fundamental misunderstanding of the principles and practice of neuromonitoring using transcranial electric motor-evoked potentials. Possibility of Delayed Onset? Even if the authors continue to reject outright the numerous criticisms that point to technical and interpretive neuromonitoring errors during the procedure, they still must consider the fact that the patient was taken to the ICU intubated, and that there was a long delay between the end of the operative procedure and the first neurologic examination. In light of this extended time delay between the end of surgery and verification of paralysis, how can the authors rule out the possibility that the onset of paralysis was not a delayed postoperative event that occurred after neurophysiological monitoring had ended? While we clearly do not believe this to be the case, given the compelling evidence for technical and interpretive error by whomever was responsible for the recording and interpretation of motor-evoked potentials, Dr. Suh and his associates still cannot claim without doubt that

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the spinal cord injury occurred intraoperatively while neurophysiological monitoring was being conducted. Finally, it is revealing that when neurophysiological testing subsequently was performed on the patient in the outpatient clinic, motor-evoked responses were indeed absent in the lower extremities, so the authors “could not compare with upper extremity amplitudes and latency.” If one accepts the arguments posited by Dr. Suh, this means that lower extremity tceMEPs persisted throughout the course of spinal cord injury during surgery and disappeared afterwards, while upper extremity tceMEPs disappeared in the absence of upper extremity injury during surgery and then reappeared. Not only does such a paradoxical finding defy logic, but it is contrary to our collective extensive experience and data on monitoring more than 30,000 spinal surgeries with transcranial electric motor-evoked potentials. We continue to request that Dr. Suh and his colleagues have this erroneous claim of a false-negative transcranial electric motor-evoked potential during scoliosis correction expunged from the published literature. To ignore basic tenets of clinical neurophysiology and neuroanatomy in general and intraoperative tceMEP stimulation and recording techniques and interpretation in particular, in favor of explanations that are tenuous at best, does an injustice to the uninformed reader, the field of intraoperative neuromonitoring, and most of all the 15-year-old female who sustained a paralyzing spinal cord injury. The authors’ refusal to acknowledge even the possibility of a mistake in the face of such strong evidence to the contrary is disconcerting and should not go unchallenged. Jeremy A. Lieberman, MD Department of Anesthesia & Perioperative Medicine University of California San Francisco, CA Sigurd Berven, MD Department of Orthopedic Surgery University of California San Francisco, CA John Gardi, PhD, DABNM, FASNM, CCC-A Evoked Potential Associates, Inc. California Neuromonitoring Services (CNS) Serena Hu, MD Department of Orthopedic Surgery University of California San Francisco, CA Russ Lyon, MS, DABNM Division of Operating Rooms University Hospital, University of California San Francisco, CA David B. MacDonald, MD, FRCP(C), ABCN Section of Neurophysiology Department of Neurosciences King Faisal Specialist Hospital and Research Center Riyadh, Saudi Arabia

Daniel Schwartz, PhD, DABNM, FASNM Surgical Monitoring Associates, Inc. Springfield, PA Anthony Sestokas, PhD, DABNM, FASNM Surgical Monitoring Associates, Inc. Springfield, PA Charles Yingling, PhD, DABNM, FASNM Department of Otolaryngology/Head and Neck Surgery Stanford School of Medicine California Neuromonitoring Services San Francisco, CA

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

To the Editor: Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 –900. A false-negative finding for transcranial motorevoked potentials (MEPs) despite dense motor deficits is extraordinary. However, analysis of the data provided (Figure 2 in Modi et al1) suggests that the false-negative result was due to misapplication of MEPs rather than an intrinsic gap in sensitivity. First, the “S1–S2” signals appear to the left of those labeled as “C8 –T1” and thus “S1–S2” signals have shorter latency than “C8 –T1”. (We can see that the xaxis time scale is identical across all traces because the stimulus artifact spacing is identical.) No typical muscles used for these myotomes would be expected to yield this latency relationship and standard muscles for the system used are abductor digiti minimi (C8 –T1) and abductor hallucis (S1–S2). Based on this we conclude that the muscles are mislabeled and that the muscles at risk in this procedure are actually those labeled as C8 –T1. This conclusion is further strengthened by the fact that “C8 –T1” signals were lost during the high-risk portion of the procedure and never returned, thus predicting the post operative paralysis. In addition, a number of suboptimal aspects of the neuromonitoring can be noted. Specifically, sensory evoked potentials were not monitored, there were overly restrictive MEP interpretative criteria used, the signals were displayed in a manner that made it impossible to critically analyze the waveforms and measurements, only a single target muscle below the level of surgery was used, and loss of “C8 –T1” signals was not scrutinized. These factors, coupled with use of a neuromonitoring system that may be marketed as “surgeon controlled”2 suggest that an experienced neuromonitoring team was not used. The benefits of neuromonitoring are lost with-

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out an experienced team3 to institute proper monitoring methods and critical analyze data obtained. The apparent absence of such a team here contributed to an erroneous false negative result. Robert Minahan, MD Allen S. Mandir, MD, PhD Department of Neurology Georgetown University Washington, DC References 1. Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 – 900. 2. Medtronic Sofamor Danek USA I. NIM-Spine™ System Neural Integrity Monitor Connection Procedure. Company marketing pamphlet. 2005. Ref Type: Pamphlet. 3. Nuwer MR, Dawson EG, Carlson LG, et al. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencephalogr Clin Neurophysiol 1995;96:6 –11.

tracted and expunged. The editorial board should draft a letter of apology to your readership. Almost certainly, you need to reconsider your editorial approach to specialty areas like neurophysiology to be sure such submissions are properly reviewed in the future. Please contact us if you would like to discuss this matter further or if you have any questions on the comments above. Stan Skinner, MD Medical Director, Department of Neurophysiology Abbott Northwestern Hospital Minneapolis, MN David Rippe, MD Department of Neurophysiology Abbott Northwestern Hospital Minneapolis, MN Reference

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

To the Editor: Re: False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009; 34:e896 –900. I write to alert you to a very serious editorial mishandling of a Case Report1 just published by your journal. The authors purport to detail “the first” false-negative TCE MEP during scoliosis correction. I regret to inform you that the case is, instead, an example of an appalling error in monitoring technique. Although the authors fail to describe any method to their intraoperative monitoring approach, it is obvious, by latency criteria, that they have mislabeled “C8-T1” (Hands) and “S1–S2” (Feet). The orange/brown traces (labeled “S1–S2”) occur at 25 milliseconds, typical latency for hand muscles. The blue traces (labeled “C8-T1”) occur at 40 milliseconds, typical of distal lower limbs. It is tragically ironic that the “C8-T1” (Hands) disappears after correction (Figure 2C). Let us be very clear, they mistakenly connected the electrodes from the hands into the S1–S2 port in the jack box and the feet mistakenly into the C8-T1 ports. The disappearing blue traces are thus lost TCE MEP potentials from the lower limbs at a time when this spinal cord most likely could still have been salvaged, and not a false-negative TCE MEP. Dr. Weinstein: you, your editorial board, and reviewers need to consider the damage such an illegitimate report does to potential patients who may be denied monitoring because of this report in an internationally acclaimed journal. This report must be immediately re-

1. Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 – 900.

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. One or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript: e.g., honoraria, gifts, consultancies, royalties, stocks, stock options, decision making position.

Rebuttal: We have reviewed the response from the authors. “Unfortunately, and incredulously, the authors continue in their failure to acknowledge that the patient’s electrophysiological potentials in 2 of 4 monitored limbs (those labeled C8 –T1) disappeared during deformity correction.” Despite the authors’ pronouncements, there is no attempt to explain how this loss of potentials could have occurred without a reversal of electrodes. Without being able to explain this critical loss of signals (those labeled C8 –T1), the evidence continues to overwhelmingly demonstrate an overlooked opportunity to intervene and prevent a neurologic catastrophe in their patient. The authors also failed to include the patient’s postoperative motor-evoked potentials recording performed in their physical rehabilitation department. Inclusion of this motor-evoked potentials in their response would have enabled a comparison of the postoperative response to the intraoperative response (in particular, the latency). If performed in the same manner, we believe that the C8 –T1 postoperative responses would show similar characteristics as the response labeled S1–S2 intraoperatively.

722 Spine • Volume 35 • Number 6 • 2010

All the evidence that we have reviewed indicates that the case report is, in fact, a missed true positive. The journal must take appropriate action. Stanley Skinner, MD Department of Neurophysiology Abbott Northwestern Hospital Minneapolis, MN David Rippe, MD Department of Neurophysiology

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. One or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript: e.g., honoraria, gifts, consultancies, royalties, stocks, stock options, decision making position.

ment regarding our answers. And the decision to retract the article, if we could not modify the amplitudes from the tracings of same patient, sounds somewhat strange to us. Because we used NIM Spine monitoring system, motor evoked potential (MEP) recordings always saved in jpg file format, and, so we are not able to modify or change it a little bit. We do not think that changing the amplitude is possible and that would make some differences. As we explained before, to check the MEP latency and amplitudes again, we called the same patient to out clinic; however, due to paralysis in both lower extremity, we could not check it to confirm our findings. We request to consider all our explanation in the latency and amplitude and reversal of leads in previously submitted answer, and considering it as a special case of neurofibromatosis, and would like to retain our stand that false-negative result might happen with MEP. Hitesh N. Modi, MS, PhD Seung Woo Suh, MD, PhD Jae-Hyuk Yang, MD Ji-Yeol Yoon, MD Scoliosis Research Institute Department of Orthopedics Korea University Guro Hospital Seoul, South Korea

To the Editor: Re: False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009; 34:e896 –900. I looked over the data and notations from Dr. Lieberman. I agree with Lieberman that the recording electrodes (leads) might have been inadvertently switched before surgery in which the lower extremity traces might have been lost just after correction which leads to a truepositive case. However, to make sure of this claim, I want the authors (Modi et al) retrieve these tracing from this particular patient and reduce the amplifications of all traces, so there would be no overlapping between the upper and lower limb traces at the baseline and afterwards. By doing that we can easily conclude if any traces were indeed lost during the surgery or remained intact. If authors can not access this traces for scaling down the amplification, I suggest retracting the article due to its clinical implications. Siavash S. Haghighi, DVM, MS, PhD Clinical Neurodiagnostic Department

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

In Response: We have gone through the reviewer’s (Siavash Haghighi) comments. As a normal case we agree with him that it appears that leads might have changed; however, we explained it in our response that they were not changed. We also explained the difference in latency and amplitudes in detail along with some other examples when submitting the response; however, we did not find any acknowledg-

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

To the Editor: Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 –900. We would like to comment on the recent paper by Modi et al.1 Does the case in question serve as the first definitive example of a “false negative” for MEP monitoring? We would argue the answer is “no.” Instead, a more likely explanation for the unfortunate outcome of this case can be found hidden within this paper’s Figure 2.1 The key point made by the authors was that intraoperative MEPs did not show significant change, yet the patient awoke with paraplegia. However, Figure 2B shows a ⬎50% decline or outright loss of MEP responses in 3 of the 4 monitored muscle groups. This change persists through Figures 2C and 2D for muscles innervated by the C8 –T1 levels (presumably hand muscles). But why would MEPs be lost in the hands bilaterally, without change from muscles in the legs (S1–S2 levels)? And why would this change not be mentioned by the authors? Individual MEP responses within this Figure are difficult to resolve, because the high sensitivity of the screen display causes the responses to obscure one another. Nevertheless, it’s clear that the apparent latency to MEPs in

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the leg muscles is considerably less than that for responses in the hands. Again, how can this be? The one explanation that can unify these anomalies is that electrodes from the hand muscles were mistakenly switched with those from leg muscles when making connections to the intraoperative monitoring amplifiers. This would explain the unusual response latencies, and more importantly, would show that changes in MEP responses did, in fact, predict this patient’s postoperative paraplegia. Other questions about pulse-train number, stimulus intensity, and changes in artifact can be raised from Figure 2. All of these points emphasize the intricacies involved in correctly implementing and interpreting transcranial MEP monitoring.

Miriam L. Donohue, BS Geoffrey Allott, BA, CNIM Blair Calancie, PhD Department of Neurosurgery Upstate Medical University Reference 1. Modi HN, Suh SW, Yang JH, et al. False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009;34:e896 – 900.

The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Re: False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis. Spine 2009; 34:e896–900.

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