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In the Literature: Utility of Motor and Somatosensory Evoked Potentials for Neural Thermoprotection in Ablations of Musculoskeletal Tumors

Posted By Christopher Halford, Monday, May 18, 2020

Article Title: Utility of Motor and Somatosensory Evoked Potentials for Neural Thermoprotection in Ablations of Musculoskeletal Tumors

 

The Big Question:

 

First, let me apologize up front for the quote-heavy content of this write-up. Though I have been in the field of IONM for a while I have not had experience with ablations of musculoskeletal tumors so immediately this articles title intrigued me.

 

Essentially there are two techniques of ablation outlined in this article: cryoablation (where the tumor cells and adjacent tissue is frozen) and radio frequency ablation (which has the opposite effect by essentially cooking the tumor and adjacent tissue).

 

Though there is a history of studying the risks of these procedures, the published data shows a varying degree of risk and deficit percentages for each ablation modality. Although the authors (Yoon, et al.) cite many of these this publication seeks to further determine the utility of SSEPs and MEPs for these surgeries where the tumor ablation can potentially put a nearby neural structure at risk (thus the title).

 

Background:

 

As with all IONM, though it goes without saying, I liked the statement: “The inclusion of IONM was determined by the performing interventional radiologist based on qualitative risk assess-ment for nerve injury, ie, proximity to the spinal cord or spinal and/or peripheral nerves” (p. 2).

 

For the sake of the readers’ understanding and the fact that the results section included a lot of information I’ve included the specifics of both the methods and the results directly rather than paraphrase, with some mild revisions.

 

Method:

           

As reported in the study,

 

Warning criteria for abnormal SSEP changes were defined as a 60% reduction in baseline amplitude and/or 10% increase in latency per the institutional standards for spinal surgeries. Similarly, for TCeMEP monitoring, an abnormal change was defined as a 100-V increase above baseline threshold activation for a given myotome. When TCeMEP or SSEP warning criteria were met, the ablations were immediately terminated… (p.5)

 

Results:

 

As also reported in the study:

 

Warning criteria for TCeMEP and/or SSEP monitoring were met in 12 of 30 procedures (40%). Seven of 30 (23%) met warning criteria for TCeMEPs, 3 (10%) met warning criteria for SSEPs, and 2 (7%) met warning criteria for both. Eleven of these 12 procedures (92%) were cryoablations, and only 1 (8%) was an RF ablation. Nine of these 12 procedures (75%) targeted tumors involving the spine, and the remaining 2 (25%) involved the scapula.

 

[During the surgical period] five of the 12 abnormal TCeMEP/SSEP changes (42%) did not recover, with the remaining 7 (58%) being transient.

 

Three of 5 procedures with unrecovered abnormal changes (60%) and 2 of 7 procedures with transient abnormal changes (29%) had new charted motor (n = 1) and/or sensory (n = 4) symptoms.

 

As a whole, any abnormal TCeMEP or SSEP change was 100% sensitive… and 72% specific.,.. for neurologic sequelae, whereas any unrecovered change was 60% sensitive.. and 92% specific ….

 

Any abnormal TCeMEP change was 100% sensitive… and 72% specific… for new motor deficits; unrecovered TCeMEP changes had the same sensitivity, but a specificity of 93% …. Any abnormal SSEP activity change was 75% sensitive… and 92% specific… for new sensory deficits or radicular pain; unrecovered SSEP activity changes were 50% sensitive… and 100% specific. (p. 5)

 

Discussion:

 

In a nutshell, the authors’ acknowledge the sample size was small and the numbers related to risk in this study varied from other studies (though there are a number of contributing factors for this). In the end a total of 16% of patients done at this facility had reported IONM changes conveyed (based off the facility’s reporting criteria) that emerged from surgery with notable deficits. All of these patients with identified deficits were a result of cryoablation versus radio frequency ablation. Based off the sensitivity and specificity it seems that Neuromonitoring assisted in accurately identifying which patients could expect to have neurological deficits post-operatively. Unfortunately, as the authors also acknowledge, this is predictive versus preventative, the most important goal of IONM.

 

In conclusion, and on a brighter note, in the authors’ words:

 

Despite [the] limitations, the present study shows a correlation between neurologic sequelae and increased latency and/or decreased amplitude of SSEPs or an increase in TCeMEP threshold stimulation during percutaneous ablation procedures of musculoskeletal tumors. Monitoring of SSEPs and TCeMEPs should be considered in ablations in which there is concern for neural thermal injury

 

References:

 

  • J Vasc Interv Radiol. 2020 Apr 24. pii: S1051-0443(19)31079-6. doi: 10.1016/j.jvir.2019.12.015. Utility of Motor and Somatosensory Evoked Potentials for Neural Thermoprotection in Ablations of Musculoskeletal Tumors. Yoon JT, Nesbitt J, Raynor BL, Roth M, Zertan CC, Jennings JW.

 

Disclaimer:

 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

Tags:  In the Literature 

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In the Literature: Use of the train-of-five bipolar technique to provide reliable, spatially accurate motor cortex identification in asleep patients

Posted By Christopher Halford, Monday, May 18, 2020

Article Title: Use of the train-of-five bipolar technique to provide reliable, spatially accurate motor cortex identification in asleep patients

 

The Big Question:

 

The authors (Bander, et al.) point out that monopolar stimulation for direct cortical brain mapping is quickly become the standard when compared to the previous method of mapping: “low-frequency bipolar stimulation mapping” (also known as the Penfield method). However, the authors wanted to compare using what they refer to as “train-of-five” (also referred to as multipulse or pulse-train) stimulation to the low-frequency bipolar simulation mapping.

 

Background:

 

The idea behind this experiment is twofold. First is that the potential risk for tissue damage is possibly higher based off data recorded in animal studies (p. 1). And second, “monopolar [train-of-five] stimulation [causes] diffuse, radial spread of electrical stimulation that leads to spatially inaccurate motor cortex identification” (p.2).

 

Method:

            

Thirteen patients were used in this study. The two things this study wanted to compare was the reliability of locating the motor cortex through direct cortical stimulation when comparing low-frequency bipolar stimulation and train-of-five bipolar stimulation and the occurrence of intraoperative seizures (a known risk of any direct cortical stimulation, especially low-frequency bipolar stimulation). 

 

The authors’ used four steps for motor mapping and monitoring during these cases. First they would use SSEP phase reversal testing to identify the central sulcus. Next they would identify the regions of motor cortex at risk using the train-of-five bipolar stimulation technique while using the strip used for phase reversal to watch for after discharges through EEG monitoring. Then they would use low-frequency bipolar stimulation to see if they could re-identify those same areas of the motor cortex they had previously mapped using train-of-five stimulation. Finally they would run direct cortical MEPs using the strip for the duration of the resection at a rate of every “2–15 seconds” (p. 3).

 

Results:

 

When comparing methods the authors identified the motor cortex in all 13 patients using the train-of-five technique (max stim intensity = 53 V ± 17.7 V) compared to only 4 times with the low-frequency stimulation technique (max stim intensity = 8 mA ± 2.2 mA).

 

No seizures occurred when using the train-of-five technique while two seizures occurred during the low-frequency stimulation technique along with two instances of after discharges that did not progress to seizures. These number line up very closely with other studies testing the seizure frequency when using the low-frequency (or Penfield) technique.

 

Discussion:

 

The authors acknowledge a “lack of comparison with a monopolar [train-of-five] stimulation” (along with “small sample size”) as limitations to this study however I would say neither of these should have a big impact on the whether this information is useful and the technique should be further tested and verified. 

 

Comparison to monopolar direct cortical stimulation would likely be of little use considering this technique (monopolar multipulse stimulation) is already becoming the mainstream method for cortical mapping. However, if it could be demonstrated that the direct risk of tissue damage is a serious factor linked to monopolar stimulation, the bipolar pulse train technique presented by the authors could be relevant very quickly. Also, though the sample size is small successful recording in 100% of patients indicates a high potential for reliability (in my opinion).

 

The technique of bipolar/monopolar, Penfield/Multipulse techniques have been compared in subcortical mapping by Szelenyi, et al. in 2011. They found that multipulse stimulation, whether with a monopolar or bipolar probe, was superior for stimulation for subcortical mapping versus the low-frequency (50 Hz) stimulation technique.

 

This article appears to offer a promising, potentially reliable stimulation alternative in an area of IONM that has received a lot of attention in recent years.

 

References:

  • Neurosurg Focus. 2020 Feb 1; 48(2):E4. doi: 10.3171/2019.11.FOCUS19776. Use of the train-of-five bipolar technique to provide reliable, spatially accurate motor cortex identification in asleep patients. Bander ED, Shelkov E, Modik O, Kandula P, Karceski SC, Ramakrishna R1
  • Clin Neurophysiol. 2011 Jul; 122(7):1470-5. doi: 10.1016/j.clinph.2010.12.055. Intra-operative subcortical electrical stimulation: a comparison of two methods. Szelényi A1, Senft C, Jardan M, Forster MT, Franz K, Seifert V, Vatter H.

 

Disclaimer: 

The views, thoughts, and opinions expressed in this blog post are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

Tags:  In the Literature 

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In the Literature: Solutions to the technical challenges embedded in the current methods for intraoperative peripheral nerve action potential recordings

Posted By Christopher Halford, Wednesday, March 4, 2020

In the Literature:  Solutions to the technical challenges embedded in the current methods for intraoperative peripheral nerve action potential recordings

 

 The Big Question/Background:

This paper begins by giving an overview of the challenges of recording intraoperative nerve action potentials (NAPs). The objectives of the authors, in a nutshell, was to test the current method for stimulating and recording NAPs and see if a more effective way to obtain reliable responses could be found. In their own words, “The authors’ goal was to improve intraoperative NAP recording techniques by revisiting the methods in an experimental setting” (p. 1).

 

Method:          

Animal testing on non-human primates was used initially to attempt to remove the typically confounding stimulus artifact caused by recording potentials so close to the stimulation source. They used the standard method of lifting the nerve from the surrounding tissue but faced the same stimulus artifact problems that negatively affects NAP recording. They then tried a novel technique where they used a saline-soaked gauze under or around the portion of the nerve between the stimulation and recording sites. The end result was that the stimulus artifact was removed and the NAPs were recorded at significantly larger amplitudes at lower stimulus thresholds and with very little stimulus induced interference. 

 

The authors hypothesized that the saline gauze created a salt bridge between the outside of the nerve and the surrounding tissue thus preventing the stimulus current from looping back on, around, or through the nerve and confounding the equipment’s average/amplifier (termed “the loop effect” (p.6)). Next the authors, based off the information they had obtained through the gauze salt bridging, successfully recorded NAPs with the same conclusions by simply using insulated stimulation and recording electrodes and not lifting (“nonlifting technique” p. 6) the nerve from the surrounding tissue. The authors suggest that by isolating both the stimulation and recording mediums in the electrodes the current loop that the gauze prevented was prevented in the same fashion.

 

Finally they verified their results through a “stimulus polarity switch test and by the intensity-response function test” (p. 3). This is done by reversing polarity of the stimulation delivered to the nerve. Only the deflection of the stimulus artifact should change direction thus verifying your NAP. However, they also noted that when polarity was switched the stimulus threshold needed to generate the same NAP approximately doubled when stimulating anodally versus cathodally. Something to be aware of if the reader plans to attempt this method.

 

Finally they tested the “nonlifting technique” in the OR setting on patients and similar results occurred and the results were again verified with the intensity-response deflection test (p. 8).

 

Results:

Briefly explained, and again in the authors’ own words, “We identified exaggerated stimulus artifacts being a major problem and found bridge grounding to be a simple and effective solution. Ultimately, we brought our new methodology forward into clinical practice, where clinical rather than research equipment was used. The outcome was the same, validating the principal concept shared by recordings in these different settings” (p. 9).

 

Discussion:

The authors were able to consistently record action potentials in both the experimental and clinical settings by removing “the loop effect” with either “bridge grounding” with a saline-soaked gauze or by insulated stimulating and recording electrodes and not lifting nerve from the surrounding tissue (the grounding source) thus allowing that tissue to shunt the stimulus before looping back through the nerve.

 

The authors acknowledge the biggest limitation of this study was that, in the clinical setting, this technique (or more specifically the “bridge-grounding” version of this technique) was only tested on four patients intraoperatively. They encourage the IONM community to verify this technique through “systematic and quantitative evaluations of these methods, additional investigations in healthy and, more importantly, chronically injured nerves” (p. 9).

 

This method minimizes the major confounding factor in recording NAPs and could improve the confidence of technologists, neurophysiologists, and surgeons in the testing being done and the results displayed. If further testing found it to consistently work intraoperatively this research could have a major impact on the reliability and use of NAP recording.

 

References:

Wu G, Belzberg A, Nance J, Gutierrez-Hernandez S, Ritzl EK, Ringkamp M. Solutions to the technical challenges embedded in the current methods for intraoperative peripheral nerve action potential recordings. J Neurosurg. 2019 Aug 16:1-10.

 

Disclaimer: 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

Tags:  In the Literature 

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In the Literature: Intraoperative Neuromonitoring during Spinal Cord Tumor Resections

Posted By Scott Mohr, BS, CNIM, MBA, Tuesday, February 25, 2020

Intraoperative Neuromonitoring (IONM) Is There a Role in Metastatic Spine Tumor Surgery?

 

The Big Question:

Can Intraoperative Neuromonitoring (IONM) make an impact on surgeons’ efforts to preserve patient quality of life during Metastatic Spinal Tumor Surgery (MSTS)?  The team of Kumar et al. published a retrospective study over the past year taking a closer look at the correlation between neuromonitoring utilization and the impact on the surgical outcome.  Did the data they uncovered suggest that IONM utilization during MSTS procedures makes an impact on patient outcomes?

Background:

Cancer is a nasty business, often causing loss of motor and sensory function in the course of its progression.  Our business in neuromonitoring is to provide the surgical team with the information they need to optimally preserve motor and sensory function within the constraints of the surgical procedure.  When surgery becomes part of the treatment plan for a patient with spinal metastases, surgeons often need to weigh risk versus reward.  Can a sufficient level of pain relief and preservation of function be achieved in light of possible further post-surgical deficits?  IONM becomes useful to a surgeon in these cases, and we as the neuromonitoring community stand to make a crucial difference in the patient’s quality of life.

The team of Kumar, et al. – consisting of specialists in ortho, trauma and spinal surgery – astutely noted a lack of literature quantifying the relationship between IONM and MSTS patient outcomes.  They engineered a retrospective study of 135 patients over the course of a seven -year time frame to match relationships between significant event alerts and patient outcome for MSTS patients.

Method:     

The study spans a seven-year period and includes surgical procedures on 135 patients, all with spinal cord metastases from various sources (the article provides a breakdown in table format, with lung and breast carcinomas being primary instigators).  The 135 surgical procedures were all performed by one of five surgeons and were monitored by one of two certified neuromonitoring technologists.

Monitoring was performed under and anesthetic regimen suited for TcMEP, EMG and somatosensory modalities.  Total Intravenous Anesthesia (TIVA) using fentanyl and propofol were administered to maintain an anesthetized state throughout the procedure.  Short-acting paralytics were administered during the intubation phase only, and Train of Four (TOF) was monitored during the surgical procedure to confirm absence of paralytic effect on the patient.

From a neuromonitoring perspective, patients were monitored on two different platforms the Nuvasive NVM5 and the Cadwell Elite system, employing from 20-32 channels depending on monitoring package and surgical approach.  All MEP data was elicited from transcranial stimulation from electrode sites C1 and C2.  Recording muscle groups ranged from deltoid, biceps brachii, brachioradialis, abductor digiti minimi, vastus medialis, tibialis anterior, extensor hallucis longus, abductor hallucis longus, and gastrocnemius, while somatosensory data was elicited from stimulation of the ulnar, median, and posterior tibial (PTN) nerves.  Somatosensory alert criteria included the ’50-10’ rule, or greater than 50 percent loss from baseline amplitude and greater than 10 percent increase in latency from baseline.  TcMEP responses were reported as either present or absent, and EMG alerts were reported upon observing “irregular, aperiodic bursts repeatedly elicited by surgical maneuvers greater than a 3 second period” (Kumar, et al. 2019).

A note about the patient population.  Of the 135 patients included in the study, seven were eliminated from data inclusion in the retrospective due to a lack of baseline data (i.e., no followable baselines to report to the surgeon, either sensory or motor). This narrows the field to 128 patients whose monitoring experiences contributed to the final report.  The population consisted of 61 males and 67 females who were on average 61 years of age.

Patients were scored pre-operatively based on the impact the metastases had on their quality of life using the ASIA score, so a note here about this format may be useful.  ASIA stands for American Spinal Injury Association, and the ASIA pre-operative score assesses motor and sensory function of a patient with a spinal injury.  The exact nature of the testing and scoring is perhaps a bit too complex for the scope of this literature review, but it is worth knowing some basics.  Ten muscle groups are assessed for motor function, five upper extremity and five lower extremity muscles, using range of motion (ROM), ability to move a limb against the force of gravity, active resistance, etc.  Sensation is assessed with pin prick applications along a series of dermatomes. A final letter grade is assigned to a patient; A, B, C, D or E.  Patients with a grade of E will have what is considered normal sensory and motor function, while letter grades B, C, and D cover patients with incomplete spinal cord injuries, demonstrating some deficits in motor or sensory function.  A patient with a grade of A has what is deemed a ‘complete’ injury and exhibits no motor or sensory function for purposes of ASIA scoring (SCIRE Project, 2016).

From a surgical perspective, of the 128 patients, 54 patients had surgery to address a neurological deficit, 66 underwent a procedure for instability pain and 8 were listed as going under the knife for intractable pain.  As mentioned earlier, seven patients were excluded from the study due to their ASIA scores; 5 patients had an ASIA score of A (total injury) and 2 had an ASIA score of C – all seven patients failed to present baseline data sufficient for monitoring and reporting to the surgeon during the procedure.

Results:

Of 128 with spinal cord metastases who underwent surgical procedures with neuromonitoring, 13 patients had significant alerts. That amounts to 10.2% of the patient population, or 1 in 10 patients.  Five patients had TcMEP alerts, 5 patients had TcMEP and somatosensory alerts, 2 patients experienced MEP and EMG alerts, while one patient had alerts in all three modalities.

Of the 128 patients included in the study, there were 114 true negatives, 13 true positives, and 1 false negative.  No false positive was reported.  Of the 114 procedures resulted in true negatives – no significant alerts were reported during the procedure and the patient woke up without additional deficits.  Of the 13 patients with true positives – patients where an alert was reported and either corrections resulted in a return to baseline, or the patient awoke with a deficit - occurred in 9 open procedures and 4 minimally invasive Surgical (MIS) procedures.

The paper further breaks down the true positives into three groups – Group A, Group B and Group C.  Group A included one patient (8.3% of the true positive patient population) who exhibited a decrease of signals during a lateral psoas approach.  The patient’s responses returned to baseline after the surgeon changed the plane through which the muscle dissection approach occurred.  Group B incorporated 5 patients (38.46% of the patient total) who experienced alerts during instrumentation.  Four of these patients’ data returned to baseline after either pedicle screw placement adjustment or decreasing the size of the interbody cage placed.  One of the patients in Group B did not experience data recovery to baseline after all available interventions were exhausted, and this patient did wake up with complete paraplegia. 

Finally, Group C included 7 patients (53.84% of true positives reported) where a significant alert was communicated during the decompression phase of the operating, and these patients all returned to baseline after the decompression was complete.

The final patient was reported as a false negative; the patient awoke with a C5 palsy post-operative after undergoing a cervical laminectomy with hardware placement.  Specifically, the right deltoid and biceps function degraded from a grade 5 to 2 immediately upon wakeup assessment.  The patient did recover to normal status at the 9-month post-operative mark.  This completes the total of procedures with a post-operative deficit; one true positive and one true negative, or 1.6% of the patient population.         

Discussion:

The authors conclude that IONM exhibited a high degree of sensitivity and specificity for detecting changes to neurologic status intraoperatively during MSTS procedures.  Of particular note is that a number of patients experienced changes in Somatosensory and motor data during the procedure that resolved after intervention by the surgical team.  These interventions included changing the surgical approach, trying a different placement or size of hardware, elevating patient’s mean arterial pressure or administering steroids depending on the scenario.  

Many of these patients’ IONM data changes resolved at that time, lending support to the concept that effective use of IONM can allow a surgeon to make informed course corrections during a procedure, mitigating the potential for post-operative deficits.  The goal in MSTS procedures is often to improve quality of life, either by reducing pain or restoring sensation and function when possible.  Surgical actions and conditions that put the patient at risk of incurring further post-operative deficits can be countermanded with use of IONM, making neuromonitoring a powerful tool in the surgeon’s kit when the procedural goal is to improve the patient’s quality of life.  The highest degree of accuracy reported in the study was with multimodal IONM, including TcMEP, somatosensory and EMG recording, both passively and with triggered mapping of nervous structures. This was followed by SSEPs in combination with TcMEP, EMG with SSEPs, and finally the lowest diagnostic sensitivity was produced when each of these modalities were used individual during a procedure, and not in concert with other IONM approaches.  This finding reinforces the principle that neuromonitoring is at its best when we as professionals can use the full range of our monitoring tools to produce the best results.

The article dives down in the one instance of a false negative included in the study, concluding it was likely a technical error, but providing no further description.  This paper provides an encouraging report on the impact of IONM on patient quality of life post-operatively when facing the challenges of spinal cord tumor metastases.  While the retrospective study is limited by the constraints of providing data from only two monitoring professionals in the service of five surgeons at one facility, the authors begin to fill in a literature gap that is much needed.  More information from other facilities reported in this manner on the impact of IONM on MSTS procedures will be of great benefit to the neuromonitoring community.

References:

  1. Kumar N, G V, Ravikumar N, Ding Y, Yin ML, Patel RS, Naresh N, Hey HWD, Lau LL, Liu G. Intraoperative Neuromonitoring (IONM): Is There a Role in Metastatic Spine Tumor Surgery? Spine (Phila Pa 1976). 2019 Feb 15;44(4):E219-E224.
  2. Noona, V., Mak, J., Zhu, J., Diab, K., Queree, M. (2016). American Spinal Injury Association Impairment Scale (AIS): International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI). Retrieved from https://scireproject.com/ (2020).

Disclaimer: 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

Tags:  In the Literature 

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In the Literature: Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie

Posted By Jay L. Shils, PhD, DABNM, FASNM, Monday, February 10, 2020
Updated: Wednesday, January 29, 2020

Article Title: Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie 

 

Article Summary:

 

The choice of stimulation parameters for direct cortical stimulation is based primarily on observational data. This data has suited us well over time but there were no specific true basic neurophysiologic studies that could better help define the optimal stimulation parameters. 

 

An important property of a nerve and more specifically neural tissue is the strength duration (S-D) curve. This curve relates how much energy is needed to activate a nerve based on the stimulation intensity and the pulse width of a square shaped pulse, which is what we in the neurophysiology community use to stimulate the nerve or neural tissue. 

 

When evaluating the strength duration curve there are two key properties of this curve, the Rheobase (Rb) and the Chronaxie (Cx). The Rb is the smallest stimulation amplitude that can cause a nerve (axon) to generate an action potential (AP). It is defined for a infinitely long pulse width (DC current pulse), all pulse width’s smaller than this value will require a greater stimulation amplitude to generate an action potential. 

 

The Cx is the pulse width value on the strength duration curve that crosses the stimulation amplitude value that is two times the Rb current. The Cx is a excitability time constant. The Cx point on the curve also defines the minimal energy point for AP generation. 

 

In the paper by Abalkhail et. al. they investigate the Rb and Cx to help define the optimal interstimulus interval (ISI) and the pulse width (D) for direct cortical stimulation. Common parameters used in the operating room for DCS are an ISI of either 2 or 4 mSec and a pulse width of 500 uSec.

 

Key points from this paper

  1. Standard S-D curves are based on single pulse stimulation. This study evaluated the S-D using a pulse train. They were not able to determine if the S-D curve was based on the complete pulse train or a set of individual pulses. Additionally, the S-D curve for this study is a composite of the axon in the CST and the alpha-motor neuron (AM). First, a AP needs to be generated in the CST and second that AP, or set of APs needs to cause the AM to fire. If neither of those occurs there will be no MEP. Yet, the data is still valuable since we are activating this network and the network still has a minimal energy point even if individual elements of that network are not being activated at their minimal energy points.
  2. Evaluation of basic neurophysiological parameters of neural tissue can come from standard tools that we are already using in the operating room. By just varying pulse width and amplitude of the stimulus it is possible to optimize the stimulation parameters to each patient. But given the data in this paper the values are relatively constant. In Abalkhail the values of Cx varied between 160 uSec and 200 uSec which is more than half of the common 500 uSec pulse that is most commonly used for DCS.
  3. The strength duration curve does not take ISI into consideration thus the authors evaluated these values for multiple ISI values. This is important because they used the Rb to define the lowest stimulation current needed to generate an action potential. This value was for an ISI of 4 mSec which is a common value used in the OR presently.
  4. It is critical to note that using these changes in parameters the actual values that indicate safety distances may vary and this was not part of their study.

 

Reference:

  1. Abalkhail TM, et. al. Clinical Neurophysiology. 2017;128:2300-2308

 

Disclaimer: 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

Tags:  In the Literature 

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In the Literature: Factors that Modify the Risk of Intraoperative Seizures Triggered by Electrical Stimulation During Supratentoral Functional Mapping

Posted By Christopher D. Halford, Wednesday, February 5, 2020
Updated: Wednesday, January 29, 2020

Article Title: Factors that Modify the Risk of Intraoperative Seizures Triggered by Electrical Stimulation During Supratentoral Functional Mapping

Overview:

This paper presents information from a very large retrospective sample (544 cases) and attempts to demonstrate whether intraoperative seizures can be reduced during functional mapping (as the title says). The article also discusses some facts and information about intraoperative seizures caused by direct cortical electrical stimulation previously published by other authors. They begin by discussing the specific risks of intraoperative seizures: 1) “during awake craniotomies… there is no airway,” 2) the state of the neurons in “the post-ictal state… can impede, at least temporarily, the continuation of reliable mapping,” 3) “postictal cortical depression usually results in an increase in the mapping threshold that is hard to predict; this further hinders reliable mapping,” 4) “seizures can spread to eloquent cortical regions distinct from those stimulated resulting in the false localization of eloquent cortex” (pp. 1058-1059).

The goal of this study is to demonstrate “a better way to avoid the complications associated with intraoperative stimulation triggered seizures is to have a means of preventing them that can be broadly applied to all patients undergoing a mapping procedure and which will be effective regardless of the magnitude of the individual risk” (p. 1059).

 

Methods:

 

The authors reviewed 544 cortical mapping cases, “both awake [TIVA] and asleep,” done with either the Penfield method (“repetitive biphasic pulses at 60 Hz, pulse duration of 1 ms, intensities 1–15 mA applied using a bipolar handheld stimulator”) or what has been called the pulse-train method, “multi-pulse train technique,” or “high frequency anodal stimulation”(repetitive trains at 2 Hz, 6 pulses/train, train frequency 250 Hz, pulse duration 0.5 ms, intensity 1–22 mA applied using a monopolar handheld stimulator, with the active electrode connected to the anode (+ positive) and a subdermal needle electrode placed at the margin of the surgical field, connected to the cathode (- negative)) (p. 1059). 

 

The variables that were analyzed included data like gender, age, history of anti-epileptic drug (AEDs) use, etc. to see if particular variables would likely affect the likelihood that a patient would have an intraoperative seizure (defined as rhythmic runs of self-propagated stimulation triggered AD with a duration of 10 s or more) (p. 1059). The more pertinent correlational variables were thought to be variable 4 of 12, “pre-operative maintenance treatment with AED” (defined as, “daily oral administration of any AED regimen for at least 3 days prior to surgery”) and variable 5 of 12, “loading with AED (intravenous administration at the beginning of the surgery of: 1–500 mg or more of either levetiracetam, fosphenytoin or valproic acid; or 2–200 mg or more of lacosamide)” (p.1059)

 

Results:

 

Of the 544 surgical patients reviewed 330 (≈ 61%) had seizures before their surgery. Of the total reviewed 204 (≈ 38%) “were already receiving a maintenance daily AED dose at the time of the surgery.” Also, “356 patients (65.4%) received intravenous loading doses of AED.” Intraoperative seizures occurred 135 (≈ 25%) of patients. (p. 1060)

 

Of the 12 factors the authors analyzed the ‘factors were found to significantly increase the risk of triggering intraoperative seizures’ were: 

 

  1. Penfield method (OR = 2.16, p = 0.0002) (which in their final analysis increased the likelihood of causing an intraoperative seizure by 2x*)
  2.  awake state (OR = 1.61, p = 0.01)
  3. diffuse pathology (OR = 2.37, p = 0.002) (in which case the patient was 2.4x* more likely to have an intraoperative seizure)
  4. stimulation in the temporal lobe (OR = 1.72, p = 0.01)

 

However, of those four, the authors point out that ‘mapping during awake state was found to be collinear with the use of Penfield paradigm and thus the former was excluded from the final model. Also, “the effect of stimulation in the temporal lobe was positively confounded by the use of Penfield paradigm.”

 

Moreover, they found that “intravenous administration of loading doses of AED decreased the odds of triggering seizures by 45%” while no other factors (including “maintenance AED treatment” and “history of seizures”) were found to statistically affect the likelihood of intraoperative seizures. (p. 1061).

 

*kitchen sink multivariate logistic regression

 

Conclusion:

 

Of the patients that received a loading dose of AEDs, “about two thirds (73.3%) of the patients who received intra-venous loading with AED at the beginning of the surgery, had not been previously on maintenance AED” demonstrating that these patients were not receiving the potentially positive effects of maintenance AEDs but still saw a reduction in their intraoperative seizure risk. However, “about a fourth (26.7%)” of the AED loaded patients were on maintenance AEDs but pre-procedure “loading was performed in this… group because of lack of information regarding the effectiveness of the AED maintenance” (p. 1062) which the authors believe may have a “protective effect” for those persons with a ‘positive history of pre-operative seizures’ (p. 1064). In a nutshell the authors can claim, based off of a size case study, “our results show that AED can efficiently protect against electrical stimulation triggered seizures in humans and that such protective effect is independent of other risk factors” (pp. 1061-1062). The authors also caution readers that, ‘special attention should be given to cases where map

ping is performed via Penfield method of stimulation and in the presence of diffuse pathology’ (p. 1064).

 

Limitations:

 

The authors are comprehensive in the in their presentation of statistical methods and even an additional overview of many of the risk factors they included as they analyzed the information they collected (in the discussion section which I covered very briefly). However, and as the authors acknowledge and promise to address ‘in future prospective studies’ there are certain areas when dealing with seizure history of patients and their use of AEDs prior to surgery that could (and apparently will be) useful to technologists and neurophysiologists when previewing patient information while preparing for the potential risks our soon-to-be-monitored patient might face.

 

The IONM Big Picture Perspective:

 

If the information from this study were to be shared with, and implemented by, anesthesiologists and surgeons and the results reliably replicated then surgical teams involved in cortical mapping could potentially cut intraoperative seizures in half. Also, the benefits of avoiding the high costs risks of intraoperative seizures (briefly discussed in the first paragraph) would increase the usefulness and accuracy of intraoperative cortical mapping and thus increase overall safety for these patients. Finally, with the information presented in this article we can be better prepared as technologists and neurophysiologists to anticipate patients that are at the greatest risk for intraoperative seizures and be ready to act regardless of whether other members of the surgical team have converted this information into practice.

References:

  1. Dineen J, Maus DC, Muzyka I, See RB, Cahill DP, Carter BS, Curry WT, Jones PS, Nahed BV, Peterfreund RA, Simon MV. Factors that modify the risk of intraoperative seizures triggered by electrical stimulation during supratentorial functional mapping. Clin Neurophysiol. 2019 Jun;130(6):1058-1065.

 

Disclaimer: 

 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

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In the Literature: MEPs in Infants

Posted By W. Bryan Wilent, PhD, DABNM, FASNM, Friday, January 31, 2020
Updated: Wednesday, January 29, 2020

In the Literature: MEPs in Infants

 

IONM plays a critical role in pediatric spinal procedures. Preserving motor function is of paramount importance in any surgical procedure, but it is especially precious when the patient is an infant. There are, however, unique challenges and sensitivities associated with the IONM of infants; it is not uncommon for surgeons and surgical neurophysiologists to have questions regarding optimal MEP technique and the overall feasibility of MEPs.

 

There are three papers from this year that demonstrate the utility of MEPs in infants of less than 3 months age. Here are FOUR Key Insights from the papers and from Alier Franco, PhD, an author on the Flanders et al study and the manager of the IONM service at the Children’s Hospital of Philadelphia. 

 

1. MEPs can be safely performed and obtained at any age

  • Yi et al obtained MEPs in at least one extremity in 24 of 25 infants from 1.5 months to 3 months of age.
  • Aydinlar obtained MEPs in 15 infants from 1.5 months to under 12 months (average age 5.8 months) with at least one monitorable muscle from both upper and lower extremities in all infants.
  • Flanders et al set the bar illustrating robust MEPs in an infant at 15 days of life!

 

Thus, it has been shown that MEPs can safely and effectively be performed in infants at essentially any age.  There is no evidence to contraindicate MEPs from a safety perspective or data showing that MEPs cannot be obtained because of early age. 

 

2. The display window may need to be widened (200-300 ms) because of delayed responses

 

As illustrated in Flanders et al, the latency for lower extremity MEPs may be >100 ms, which is beyond the typical display window used for MEPs in adolescents or adults. This occurs because of the combination of pathology and a still maturing nervous system with slower conduction velocities. If responses from the lower extremities are absent at baseline using a 100 ms window, the window should be expanded to check for responses that are delayed.

 

3. TIVA and anesthesia management are critical

 

Continuous communication with anesthesia and titrating TIVA in concert with hemodynamic management ensure optimal conditions for reliable neonatal evoked potentials. The presence of any residual inhalational agents can significantly impede the reliability of MEPs throughout the procedure. High doses of propofol are often needed at induction with neonates and infants, but concentrations should be titrated to minimal safe levels throughout the course of the procedure. Awareness of decreases in core body temperature, to which infants are susceptible, is also important, as this cooling can significantly affect morphology, latency, and (in extreme cases) the monitorability of evoked potentials. 

 

 

4. Will often have to vary the stimulation parameters and use higher intensities

 

When using constant voltage technique for MEPs, intensities of >500V are often needed, and that is assuming a high train count, i.e. > 7 pulses and a pulse width up to 75 microsec. Regarding the optimal ISI, 2 ms (500 Hz) is effective and lower ISIs such as 1 ms (1000 Hz) are typically less effective, but note however, that neonates and early infants can sometimes require longer ISIs such as 5 ms (200 Hz) train and/or double train stimulation. Overall, baseline monitorability can be highly dependent on stimulus parameters and thus varying stimulus parameters is sometimes critical. 

 

References: 

 

  • Flanders TM, Franco AJ, Hines SJ, Taylor JA, Heuer GG, “Neonatal intraoperative neuromonitoring in thoracic myelocystocele: a case report.”, Child Nerv Syst, 2019, Nov 10
  • Aydinlar EI, Dikmen PY, Kocak M, Baykan N, Seymen N, Ozek MM, “Intraoperative Neuromonitoring of Motor-Evoked Potentials in Infants Undergoing Surgery of the Spine and Spinal Cord”, J Clinic Neurophys, 2019, 36 (1): 60-66
  • Yi YG, Kim K, Shin HI, Bang MS, Kim HS, Choi J, Wang KC, Kim SK, Lee JY, Phi JH, Seo HG, “Feasibility of intraoperative monitoring of motor evoked potentials obtained through transcranial electrical stimulation in infants younger than 3 months”, J Neurosurg Pediatr. 2019 Mar 15:1-9. 

 

Disclaimer: 

 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

 

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In the Literature: Technical Tips: A Checklist for Responding to Intraoperative Neuromonitoring Changes

Posted By Scott Mohr, BS, CNIM, MBA, Wednesday, January 29, 2020

Article Title: Technical Tips: A Checklist for Responding to Intraoperative Neuromonitoring Changes. 

 

The Big Question:

 

How can checklists enhance a neuromonitoring team’s response to changes in patient neurophysiologic data? The Duke Health System neurodiagnostic department recently published an article detailing their experiences coordinating the entire surgical team with an action-oriented checklist. The checklist - based off of the Vitale format - establishes defined roles and responses for each member of the surgical team, which are initiated once the neuromonitoring technologist reports a change in patient data (Vitale, et al. 2014). The Duke experiment provides the neuromonitoring community with a promising example of a neuromonitoring program effectively integrated with the surgical team.  This paper provides some evidence that working from a shared script can allow rapid and focused response to significant surgical events.

 

Background:

 

The positive impact of checklist practices in healthcare has been well documented since Dr. Atul Gawande published his landmark book The Checklist Manifesto.  When a health care team can develop uniform, standardized protocols for repeatable tasks, best practices develop which leave less room for error, omission, and confusion.  In essence, employing a checklist format can help prevent failure, which was a main tenet of Dr. Gawandes’ publication (Gawande, 2010).

 

Opportunities for failure abound when the neuromonitoring team identifies a change in data during a surgical procedure.  The oversight physician and the neuromonitoring personnel need to jointly identify the event, communicate the change to the surgical team, and work to troubleshoot possible technical causes, while maintaining timely documentation of the events.  Without education and awareness about the value of neuromonitoring data in critical situations, confusion and disregard can delay rapid interventions by the surgeon or the anesthesia team, which could negatively impact patient outcome.

 

The Duke team was looking to establish a uniform format that assigned everyone in the operating room a role once a neuromonitoring data change was announced.  Once the checklist was initiated by the neuromonitoring technologist in the room, each person would know their role in addressing the change.  For example, if the technologist reported a loss of motor responses, the anesthesiologist would take steps to prepare for a possible wakeup test.  The surgeon could reverse any surgical manipulations (if relevant), all while the technologist would work through the listed steps to troubleshoot technical components and optimize data collection.

 

Method:

            

The authors employed the Vitale checklist, published as a best practice for neuromonitoring reporting during spinal surgery.  The Vitale checklist was published in 2014 after a research team headed up by Dr. Michael Vitale sought to establish a consensus-based set of guidelines for reporting neuromonitoring changes in a format that coordinated the response of the surgical team as as whole.  The checklist resulted from extensive literature review and surveying of over 20 neurosurgeons.  The format was initially applied to standard, low-risk spinal procedures.

 

The Vitale checklist functions by breaking the OR team up into four categories based on their role.  These are Control of Room (head circulating nurse), Anesthetic/Systemic (head anesthesiologist and primary anesthesia provider), Technical/Neurophysiologic (the neurotechnologist and their oversight physician, and finally, Surgical (the surgeon and scrubbed personnel).  The checklist provides a defined response role for each category if the technologist reports a data change.

 

The first step for the Duke program was education and awareness for the surgical teams that would participate - the checklist format was only effective with full compliance from all personnel in the operating room.  In addition to training meetings, each member of the surgical team was given a copy of the Vitale checklist.  Each neuromonitoring platform had a copy of the checklist, and a laminated copy was posted to each operating room involved in the study.  The cases involved were entirely composed of spinal procedures.

 

Once a change was announced during a procedure by the technologist, the circulator (Control of Room) would read the steps out loud to the room.  Having a central coordinator providing verbal cues reduces confusion and enhances teamwork during the tense moments of a patient status change. 

 

Most all of the steps were standardized to a point where each team member could readily anticipate the actions of others.  The technologist, for example, would know that in response to a loss of cortical amplitude, the anesthesia provider would work to treat blood pressure and raise the mean pressure.  

 

The sample size for the study was 9 surgical participants (though the study does not make clear if this number refers to surgeons, surgical teams, or neuromonitoring personnel).  After an undisclosed duration for the study, participants were surveyed for their impressions and feedback.

 

Results:

 

A post-implementation survey of the 9 participants resulted in 100% of the sample population reporting the Vitale checklist clarified their role during a neuromonitoring data change, and respondents were more confident that the practice would lead to improved safety and efficiency.  Survey reports reveal the surgical staff feel the checklist streamlined their response to critical events when every moment counts.  The Duke neuromonitoring program reported at the time of this writing that they continue to revise and customize the checklist implementation in response to the continued feedback they have received.

 

Discussion:

 

What are some key takeaways from this report for the neuromonitoring community?  Dr. Gawande wrote that checklist practices enhance consistency of care and reduces errors born from omission and confusion (Gawande, 2010).  The ability to be confident in the next practical step you take - and predict your teammates’ actions - leads to a higher rate of success when time is critical and each decision carries greater weight on the patient’s outcome.

The Duke neuromonitoring department harnessed the mechanics of the Vitale checklist with these goals in mind.  They discovered the benefits of implementing these practices; faster response times, enhanced coordination of the surgical team, and reduced technical error.  An added benefit was the checklist itself as a documentation tool; the recording of actions and data collection provide an accurate and concise report for the surgeon after the surgery.  

 

Neuromonitoring organizations must continue to drive awareness and engagement in operating room culture and surgical workflow.  What the Duke team is doing here is an excellent example for our field; they are involving their team in upstream staff education and downstream surgeon follow-up while becoming actively involved in surgical event management during a patient status change.  This level of play is neuromonitoring at its best - the Duke team is part of the patient care experience, not a backup system sitting behind the anesthesia cart with a laptop.

            

There are some questions left to be answered after this report.  A sample size of 9 is a good start, but too small to draw confident conclusions about the impact of the Vitale Checklist.  Furthermore, the paper reports that “100% [of participants] agreed that the checklist positively impacted patient safety and case efficiency” (Rendahl and Hey, 2019).  Reports of 100% success in unquantifiable reporting often reflects an element of group think  - everyone involved in the practice felt like the checklist format was a good idea and was helpful to their daily practices in the operating room.  While this sentiment is encouraging, it doesn’t give the neuromonitoring community much to learn from.  In healthcare, we learn from systems and practices when they break, where they fail.  It would be helpful to read more about where this system went wrong, and whether or not a dissenting opinion could shed light on components of the protocol that could change for the better.

 

Dr. Gawande’s book mentions a study at Johns Hopkins that ties into current Surgical Time Out practices.  The study notes that when, at the beginning of the procedure, each nurse in the room was given the chance to state their name and any concerns they had before the procedure began, the participants were more likely to note potential problems and offer up viable solutions, leading to better outcomes.  Researchers dubbed this an ‘activation phenomenon’, and the empowerment experienced by the nurses in the Johns Hopkins study is reflected in the Duke neuromonitoring department’s checklist experiment (Gawande, 2009).  The neuromonitoring community will continue to benefit from checklist best practices such as this encouraging report.

 

References:

  1. Rendahl R, Hey LA. Technical Tips: A Checklist for Responding to Intraoperative Neuromonitoring Changes. The Neurodiagnostic Journal, 2019; 59:2, 77-81.
  2.  Gawande, A. (2010). The checklist manifesto: How to get things right. New York: Metropolitan Books.
  3.  Rebecca Rendahl & Lloyd A. Hey (2019) Technical Tips: A Checklist for Responding to Intraoperative Neuromonitoring Changes, The Neurodiagnostic Journal, 59:2, 77-81,
  4.  Vitale MG, Skaggs DL, Pace GI, Wright ML, Matsumoto H, Anderson RCE, Brockmeyer DL, Domans JP, Emans JB, Erickson MA, et al. 2014. Best practices in intraoperative neuromonitoring in spinedeformity surgery: development of an intraoperative checklist to optimize response. Spine Deform.2(5):333–339.

 

Disclaimer: 

 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

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In the Literature: IONM of RLN During Thyroidectomy with Adhesive Skin Electrodes

Posted By Scott Mohr, CNIM, MBA, Friday, December 20, 2019

Article Review/Summary: Intraoperative Neuromonitoring of Recurrent Laryngeal Nerve During Thyroidectomy with Adhesive Skin Electrodes

The Big Question:

Can gel-based adhesive dermal electrodes serve as a suitable alternative to Nerve Integrity Monitor (NIM) endo-trachial (ET) tubes during Recurrent Laryngeal (RLN) monitoring for thyroidectomy procedures?  A research team based out of multiple institutes in the Republic of Korea joined forces to determine just that.  The strengths and limitations they discovered during their exploration invites further investigation into what could be a cost-effective and less-invasive alternative to standard NIM ET tube usage.

 

Background:

The IONM community can play a strong supporting role in ENT procedures such as thyroidectomies.  As cited in this paper, a former meta analysis support the position that RLN monitoring during thyroidectomies adds value to the surgical procedure; rapid and accurate identification of nerve tissue, reduction of vocal cord paralysis and overall improved surgical technique.  Monitoring of the RLN - a branch of cranial nerve X which innervates the intrinsic muscles of the larynx - is generally accomplished with electrodes either built into or adhesed to an ET tube and placed by intubation.  

However, this method offers certain complications.  Some anesthesiology programs have concerns about the size and potential esophageal damage NIM ET tubes can cause, and this concern extends to the brands of adhesive pads which are attached to the exterior of an ET tube prior to patient intubation.  Furthermore, NIM ET tubes can move during surgical manipulations or due to anesthesia team activity, leading to poor contact between the vocal folds and NIM ET tube, resulting in loss of signal (LOS).  

Finally, certain patient populations - such as difficult airway patients and certain pediatric cases - do not readily lend themselves to accurate NIM ET tube placement.  Having an alternate means of recording RLN activity during thyroid and other ENT procedures would be ideal.  Clearly, the subdermal adhesive pad recording technique warrants a closer look in this research paper. 

 

Objective:     

The research team recorded both free-run and handheld probe-triggered EMG activity from patients undergoing thyroidectomy procedures using both a NIMT ET tube with embedded recording electrodes and also dermal adhesive electrodes (referred to as ‘skin electrodes’ hence forth) to measure and compare the quality and quantity of data obtained from both techniques.  The research team wanted to ask the question, ‘could skin electrodes produce the same quality and reliability of data as a NIM ET tube?’

 

Methods:

Study participants - 39 patients in total - were intubated with a Medtronic Xomed NIM ET tube (6mm for women and 7mm for men).  Additionally, Medtronic Xomed adhesive gel pads skin electrodes were applied in the montage V1-R1 - R2 -V2  to lateralize the vocalis muscles.  During the course of the surgery, free-run EMG activity was monitored and the superior branch of the RLN was identified with a handheld stimulation probe.  Amplitude of these triggered responses were recording (in microvolts) as well as the responses’ latency (mSec). 

 

Results:

Fortunately, all 39 patients in the study awoke with no new deficits and experienced favorable outcomes.  After assessing the data collected, the researchers noted the following.  First, data was successfully collected in all 39 surgical instances from the skin electrodes.  This is in contrast to four episodes of Loss of Signal (LOS) from the NIM ET tube recording electrodes.  In essence, even when the ET tube failed to record data, the skin electrodes were able to record a triggered response during nerve stimulation.

Second, the recorded amplitude of responses was lower for the skin electrodes when compared to the NIM ET tube recording electrodes’ response amplitude - by a magnitude of four times.  Therefore, when comparing NIM ET tube recording versus skin electrode recording during nerve stimulation on the same patient, the amplitude of response was on average 4 times larger from NIM ET tube recording electrodes versus skin electrodes.  There was no difference in latency noted in any of the cases.

 

Discussion:

What does the data mean for the IONM community?  The researchers concluded that this study was an encouraging step toward establishing skin electrodes as an acceptable alternative to NIM ET tube and in situ needle recording for RLN monitoring during thyroidectomies.  With a study population of 39 patients, more data in future studies will be helpful in reinforcing the team’s conclusions.

For the neuromonitoring community, NIIM ET tube recording offers hurdles to our involvement in ENT procedures.  Many anesthesia programs and surgeons are wary of the bulk and potential for damage to the esophagus and trachea perceived as a risk of such recording devices.  Patients with difficult airways, tracheostomy patients and certain pediatric patients are often not good candidates for NIM ET tube recording.  

Most significantly, NIM ET tubes require accurate placement with a glidescope for good visualization of the vocal folds and recording electrode contact points.  Skin electrode placement offers easy visualization and accurate placement prior to prepping and draping the surgical site.  Anesthesia activity and surgical manipulations can displace NIM ET tube recording electrode contact, resulting in LOS.  This study suggests that skin electrodes do not experience the same rate of LOS as electrodes on an ET tube, a definite advantage of skin electrodes.

There are still questions that need to be answered.  Can the adhesive electrodes reliably retain good contact during prolonged procedures?  What if a patient has oily skin or other conditions precluding adhesive pad placement?  Larger incisions and retraction can displace the leads away from ideal recording locations, reducing their efficiency.  Above all, the consistent loss of recorded amplitude is a significant tradeoff when replacing NIM ET electrodes with skin electrodes.  Future inquires could provide a cost-effective and minimally invasive option for thyroidectomy patients, but until then, NIM ET tube electrodes recording of the RLN remains a best practice for the neuromonitoring community.

 

References:

Hyoung Shin Lee, Jungho Oh, Sung Won Kim, Yeong Wook Jeong, Che-Wei Wu, Feng-Yu Chiang, Kang Dae Lee. Intraoperative Neuromonitoring of Recurrent Laryngeal Nerve During Thyroidectomy with Adhesive Skin Electrodes.World J Surg. https://doi.org/10.1007/s00268-019-05208-3

 

Disclaimer: 

The views, thoughts, and opinions expressed in this blog post  are solely those of the author(s). Blog posts do not represent the thoughts, intentions, strategies or policies of the author’s employer or any organization, committee or other group or individual, including the ASNM. The ASNM, along with the author(s) of this post, makes no representations as to the completeness, accuracy, suitability, validity, usefulness or timeliness of any information in this blog and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. Any action you may take based upon the information on this website is strictly at your own risk.

Tags:  In the Literature 

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In the Literature: Utilization of MEPs During Posterior Lumbar Procedures to Diagnose and Avoid ‘Foot Drop’ Dorsiflexion Injuries

Posted By W. Bryan Wilent, Thursday, November 7, 2019

Foot drop is a condition resulting from nerve or nerve root injury in which patients cannot properly dorsiflex the foot. While it may be a focal deficit isolated to a couple muscles, it can have a devastating impact on a patients' quality of life.  Patients may not be able to walk without assistance, are prone to further injury due to falls, and are forced to live with the distress of being unable to perform previously normal activities. 

This injury can occur during posterior lumbar fusions, but unfortunately the IONM modalities typically used during these procedures (spontaneous EMG and posterior tibial nerve SSEPs) have been historically very poor in diagnosing the injury. But, there is good news from the literature! There are two papers from ‘The Spine Journal’ this year (Wilent et al, Lieberman et al) and one from ‘Spine’ (Tamkus et al) last year that focused on the ability of MEPs to accurately diagnose foot drop dorsiflexion injuries.  

FIVE Key Points from the Papers

1. The MEP alert criterion is critical

Diagnostic accuracy is dependent on using a 50-60% amplitude attenuation as the alert criterion for MEPs when diagnosing nerve root dysfunction.

In Lieberman et al, the average change in amplitude was 65% in the Tibialis Anterior (TA) muscle and 60% in Extensor Hallucis Longus (EHL) muscle. Tamkus et al found an average decrease of 59.5% (they linked TA and EHL in the recording channel). Wilent et al emphasized that it was typically a greater than 50% decrease in amplitude of TA MEPs that prompted an alert. 

It should be noted that while amplitude is most common MEP response characteristic assessed intraoperatively, Tamkus et al found that the area under the curve (AUC) of the response was slightly more reliable in diagnosing nerve root dysfunction. 

2. MEPs > sEMG in diagnosing nerve root dysfunction

HISTORICAL BELIEF: During spine procedures, sEMG monitors nerve root function and MEPs monitor cord function.

DATA SAYS: During spine procedures, sEMG (with subdermal needles) provides information about proximity to nerve roots or if mechanically manipulated but this modality does NOT reliably diagnose dysfunction; in contrast, MEPs do reliably diagnose spinal cord motor dysfunction & motor nerve root dysfunction.

From Lieberman et al, “Our study further challenges the fidelity of EMG monitoring for detecting a nerve root injury. Out of 25 injured patients, only 10 (40%) had an episode of tonic EMG that occurred concurrently with acute changes in the MEPs. Moreover, no patients had any significant EMG activity that suggested motor nerve injury without also having MEP amplitude changes.”

Tamkus et al found that 40% of the patients with foot drop also had free-run EMG alerts that were reported. However, free-run EMG alerts were also reported in 56.9% of the procedures in which the patients had NO deficit. Thus, 56.9% of the time, sEMG did not portend dysfunction 100% of the time.

In Wilent et al, 100% of patients with nerve root injuries had unresolved MEPs, but only 14% of those procedures had an EMG alert called.

3. Contrary to what is commonly thought, MEPs do NOT have many false positives

Of the 4,382 procedures in Wilent et in which patients had no new deficit, only 15 had a false positive unresolved TA MEP alert. That’s it. Just 0.3% of procedure had false positive isolated TA MEP alerts.  The overall specificity of MEPs was 97.9%, which was higher than the specificity of sEMG.  

Lieberman et al reported, “For detecting any injury, a 50% threshold represents a desirable balance between sensitivity (96%) and specificity (97%)”.

Using an alert criterion of a >50% decrease in amplitude, Tamkus et al found that the sensitivity was 100% and the specificity was 87.9%. This specificity was lower than the other two studies; however, as Tamkus et al notes in their conclusion, a total intravenous regimen (TIVA) should be considered to reduce the number of false positives. In their study, a balanced anesthesia regimen with inhalational agents at 0.5 MAC was employed; in contrast, in Lieberman et al, a propofol and opioid TIVA regimen was primarily used and inhalational agents were used only occasionally and if so always limited to 0.3 MAC and were always removed if signals were initially weak or fading.

4. The precipitating event is most likely related to stretch after vertebral displacement and NOT pedicle screw insertion

Neuromonitoring during posterior lumbar fusion is often focused on the safe insertion of pedicle screws, but that surgical maneuver does not typically correlate with the intraoperative diagnosis of foot drop dysfunction. 

Tamkus et al stated “No pedicle bone violation was reported in any of the patients with the foot drop.”

Liberman et al stated, “Injury rates were highest among patients who underwent reduction of high-grade L5-S1 spondylolisthesis or had a PSO. All seven injured high-grade spondylolisthesis patients had MEP amplitude changes between 14 and 55 minutes after reduction of the spondylolisthesis. Seven of the nine injuries in the PSO group occurred after closure of the osteotomy (five after L5 PSO; two after L4 PSO).”

Wilent et al provides an example where the MEPs were reduced in amplitude after L4-5 distraction. 

Thus, the precipitating event is typically vertebral distraction/displacement likely resulting in a stretch of the neve root.

5. If MEPs are resolved, deficits are avoided.   #Therapeutic impact

Lieberman et al stated, “Many of our subjects sustained large reductions in MEP amplitude (e.g., >50%) during their surgical procedures. These transient changes resolved and these subjects did not develop new weakness. We were not able to measure the frequency of these events nor correlate them to the patient’s risk of developing an injury.”

 Wilent et al reported 100% of the patients which TA MEPs were resolved by closure had no new deficits postoperatively. Most procedures in which TA MEPs were resolved involved a clear surgical intervention, as shown in the aforementioned example on which a prompt intervention to release distraction resulted in the resolution of the MEPs and the patient had no postoperative dysfunction.  

CAVEAT: For IONM to have a therapeutic impact, you not only need an accurate diagnosis (correct MEP alert criterion), you need a timely diagnosis and a proper intervention. This is only accomplished via relatively continuous MEP acquisition and immediate communication so an alert has context within the sequence of surgical events. This facilitates clinical decision making and impacts the therapeutic benefit of interventions.

References:

  • Wilent, WB, Tesdahl, EA, Harrop JS, Welch WC, Cannestra AF, Poelstra KA, Epplin-Zapf T, Stivali T, Cohen J, Sestokas AK, “Utility of motor evoked potentials to diagnose and reduce lower extremity motor nerve root injuries during 4,386 extradural posterior lumbosacral spine procedures”, The Spine Journal, 2019
  • Lieberman JA, Lyon R, Jasiukaitis P, Berven SH, Burch S, Feiner J The reliability of motor evoked potentials to predict dorsiflexion injuries during lumbosacral deformity surgery: the importance of multiple myotomal monitoring”, The Spine Journal, 19: 377-385, 2019
  • Tamkus A, Rice KS, Hoffman G, “Transcranial motor evoked potential alarm criteria to predict foot drop injury during lumbosacral surgery”, Spine, 15;43(4):E227-E233, 2018

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Tags:  In the Literature 

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