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Correlating MEPs and Preop Motor Function

Posted By Lanjun Guo, MD, MSc, DABNM, FASNM, Tuesday, June 5, 2018

This blog post will summarize a paper recently published by a member of the ASNM's Board of Directors, Lanjun Guo, MD, MSc, DABNM, FASNM. Dr. Guo trained as a neurosurgeon in China and is now a prominent clinical neurophysiologist practicing in California. She is active in multiple societies, including the ASNM and ISIN. The post below was written by Dr. Guo. Thanks for reading! RV

The Correlation Between Recordable MEPs and Motor Function During Spinal Surgery for Resection of Thoracic Spinal Cord Tumor

This paper examined the association between preoperative motor function of patients’ lower extremities and intraoperative motor evoked potential (MEP) recording.

 Patients undergoing thoracic spinal cord tumor resection were studied. Patients’ motor function was checked immediately before the surgical procedure. MEP responses were recorded from the tibialis anterior and foot muscles, and the hand muscles were used as control. Electrical current with train of eight pulses, 200 to 500 volts was delivered through two corkscrews placed at C3’ and C4’ sites. Anesthesia was maintained by total intravenous anesthesia (TIVA) using a combination of propofol and remifentanil after induction with intravenous propofol, remifentanil, and rocuronium. Rocuronium was not repeated. Bispectral Index was maintained between 40 to 50.

From 178 lower limbs of the 89 patients,  myogenic MEPs (m-MEPs) could be recorded from 100% (105/105) of the patients with 5 out of 5 motor strength in lower extremity; 90% (36/40) from the patients with 4/5 motor strength; only 25 % (5/20) with 3/5; and 12.5% (1/8) with 2/5 motor strength; None (0/5) were able to be recorded if the motor strength was 1/5. Therefore, it was concluded that the ability to record m-MEPs is closely associated with the patient’s motor function. They are difficult to obtain if motor function is 3/5 motor strength in the lower extremity. They are almost impossible to record if motor function is worse than 3/5.

Excerpt: Manual Muscle Test Grading Scale

Number Clinical Exam 
 0  No muscle movement. Flaccid paralysis.
 1  Visible muscle twitch, but no movement at the joint.
 2  Able to move in horizontal plane, but not against gravity.
 3  Able to move against gravity, but not against resistance.
 4  Able to move against resistance, but less than normal.
 5  Full strength against resistance

 Generation of m-MEPs depends on the excitability of the alpha-motor neurons in the anterior horns and excitability of the neuromuscular junction. Muscle MEPs can be generated only if the resting potential of alpha-motor neurons reaches the firing threshold, and thus, transmits this activity via motor axons of the peripheral nerves and neuromuscular junctions to the muscle.

The m-MEPs are affected by anesthetic drugs. Anesthetics impair the motor cortex’s ability to generate multiple descending volleys, the I waves. They also depress the excitability of the entire spinal cord, including the alpha-motor neuron pool. Because the D wave is resistant to anesthetic depression, the anesthetic effect at the alpha-motor neurons can be overcome at low anesthetic concentrations by high-frequency multipulse stimulation through transcranial stimulation.   The multiple D waves followed by stimuli to the motor cortex summate at the anterior horn cell to generate a subsequent myogenic response. The temporal accumulation of several cortico-motoneuronal excitatory postsynaptic potentials (EPSPs) is necessary to bring motor neurons from the resting state to the firing threshold during general anesthesia.

 However, transcranial stimuli only activate a small and variable subpopulation of the lower motor neuron pool to generate MEPs. Therefore, the m-MEPs are substantially more difficult to record in patients with underlying neurological abnormalities, such as spinal cord tumor. In practice, although a patient may maintain some motor function and can move their legs, MEPs may still not be recordable from muscles of the lower extremity.  There are previous studies correlating intraoperative recordings of m-MEPs during different types of spine surgery with the preoperative motor function, although the detailed information about the relation between the recordable MEPs and the grade of motor function were not reported.

 There are a number of methodological considerations in this study. The number of lower limbs with poor grade function was relatively small, only 33 lower limbs with 3/5 grade or less compared to 145 lower limbs with 5/5 or 4/5 grade. Different stimulation methods, such as different stimulating sites on the skull, different stimulation inter-stimulus interval, and /or different stimulating pulses, were not compared. Therefore, the recordable m-MEPs rate in clinical practice may be higher if different stimulating montages were tested.

 The current study provided evidence and confirmed the clinic experience that it can be difficult to obtain m-MEPs during a surgery when the patient has motor weakness, even the patient could still move legs. It also indirectly provide the information that if MEPs lost during surgery due to surgical manipulation, the patients may still have some motor function postoperatively, but most likely that would be worse than 3/5 motor strength.

References:

Guo L, Li Y, Han R, Gelb AW. The Correlation Between Recordable MEPs and Motor Function During Spinal Surgery for Resection of Thoracic Spinal Cord Tumor. J Neurosurg Anesthesiol. 2018 Jan;30(1):39-43.

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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Major Publication Questions Utility and Value of Neuromonitoring – ASNM Responds!

Posted By Richard W. Vogel, Friday, May 25, 2018

A recent paper published by Hadley et al1 in Neurosurgery claims that IONM has very little utility and value in spine surgery. They base this claim on a rather biased review of the literature and they call it a guideline.

It would all be a hard pill to swallow, but necessary nonetheless, if their observations were objective, if their findings were valid. Unfortunately, what we have come to call The Hadley Paper is little more than a biased repudiation of IONM in spine surgery, written by 4 neurosurgeons who don’t typically use IONM and seem to have little understanding of how it works.

You should have little doubt that this paper graced the inbox, or crossed the desk, of every spine surgeon with whom you work. Make no mistake, your surgeons are talking about it in their circles. We have already observed that some surgeons have discontinued using IONM simply based on this paper alone.

If they haven’t already, your surgeons may raise it as a topic of conversation with you. Whether that happens or not, you must be prepared to proactively educate your surgeons about the flaws in this paper and why it should be summarily dismissed.

The scale of the paper is so large, the reputation of the journal so prestigious, that this paper could have a significant impact on the future of our field. Indeed, many insurance carriers will likely use this paper to deny IONM claims and this could further drive down reimbursements and leave us in ruin.

This is a serious situation, and we want you to be aware of what is going on and what the ASNM is doing to help!

First, Drs. Bryan Wilent (Chair, Research Committee) and Rich Vogel (ASNM President-Elect) started a project last year, approved by the Board, in which the ASNM would write letters to the editor of journals in response to what we call bad literature. We loosely defined bad literature as papers invalidated by serious methodological flaws and having a high enough profile to do significant harm to our profession.

A few papers have come up for discussion, but we didn’t invest the energy because they were low profile and self-published. So, we didn’t write any letters for the first year, and then we saw The Hadley Paper.

Given the obvious and significant negative implications that come from publication of The Hadley Paper, the ASNM wrote our first Letter to the Editor2, on behalf of our membership, to point out some of the most egregious flaws. Incidentally, we weren’t the only Society to write a letter, but we were the first.

If you aren’t able to access The Hadley Paper due to limited institutional permissions, then you should at least read our Letter to the Editor of Neurosurgery. We have permission to post the original on our website.

Incidentally, Hadley and Colleagues respondedto our Letter with what can only be described as an affirmation of what we knew all along: they have little understanding of IONM and how it works. They’ve actually made matters worse by responding to us, and now we’re beginning to hear from spine surgeons around the country who contact us. They say, “[Hadley et al.] don’t speak for us and don’t represent us.”. Perhaps you'll understand why upon reading their reply.

The reader may also find amusing some of their ostentatious claims. As an example, Hadley et al. asserted that one of their authors was an expert because he studied critical appraisal of the medical literature for 5 years. Just to give you some perspective on that claim, of all the authors of the Letter to the Editor that the ASNM wrote, I’m probably one of the most junior by age, and I’ve been studying critical appraisal of the medical and scientific literature for 20 years.

More vexing than amusing is the unsubstantiated claims against the ASNM, made perhaps in an attempt to belittle our society and our profession. For example, Hadley et al. said, [the ASNM] is “perhaps unfamiliar with the rigorous, and sometimes frustrating, peer review process required before endorsement by our specialty societies, which may lead to extensive revisions and in-depth questions regarding statements and approach.” I’m sure this really irritated ASNM President, Dr. Jeff Gertsch, who recently oversaw the rewrite of our Professional Practice Guidelines. Anyway, if Hadley et al. had done a basic search of the literature, or our website, they would have found quite a few such guidelines authored by the ASNM that went through the very same process. 

This brings me to the second thing the ASNM is doing to help. Several of our members are heavily involved in the North American Spine Society (NASS). We now have a Section on Intraoperative Neurophysiological Monitoring. The Section is co-founded and co-chaired by ASNM Member Dr. Adam Doan and ASNM President-Elect Dr. Rich Vogel. Other founding members include Drs. Tony Sestokas, Bob Holdefer and John Ney, among others.

At the 2018 NASS Annual Meeting in Los Angeles, we will have our first symposium on IONM in which we have an objective review of the utility and value of IONM presented by surgeons, neurophysiologists, neurologists and a health policy economist. We will also have an abstract session in which a best paper is chosen.

Other work within NASS includes authoring a coverage policy on IONM, international speaking, and developing webinars and podcasts for 2019. All of this is being done by members of the ASNM, some of whom you elected to the Board. Inter-society cooperation is certainly the way to go!

Anyway, we thank you for taking the time to read this and we strongly recommend you take the time to read our Letter to the Editor. After all, we wrote it for you.

The ASNM is doing lots of things for you, and we hope to use our blog to be better at communicating to keep you in the know. Be sure to subscribe and keep reading!

References:

  1. Hadley MN, Shank CD, Rozzelle CJ, Walters BC. Guidelines for the Use of Electrophysiological Monitoring for Surgery of the Human Spinal Column and Spinal Cord. Neurosurgery. 2017 Nov 1;81(5):713-732.
  2. Vogel R, Balzer J, Gertsch J, Holdefer RN, Lee GR, Moreira JJ, Wilent B, Shils JL. Letter: Guidelines for the Use of Electrophysiological Monitoring for Surgery of the Human Spinal Column and Spinal Cord. Neurosurgery. 2018 Jun 1;82(6):E190-E191.
  3. Hadley MN, Shank CD, Rozzelle CJ, Walters BC. In Reply: Guidelines for the Use of Electrophysiological Monitoring for Surgery of the Human Spinal Column and Spinal Cord. Neurosurgery. 2018 Jun 1;82(6):E192-E193.

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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Board Update - Committee Repopulation

Posted By Administration, Tuesday, May 15, 2018

Update from the ASNM Board of Directors:

At our last Board Meeting, it came to our attention that each of our 8 standing committees has dozens (if not hundreds) of members, many of whom don’t even know they’re on a committee. How did this happen? It turns out that the process for joining a committee was simply to click a button expressing interest during the initial ASNM membership application. Many people expressed interests in multiple committees. So, some of you may belong to multiple committees and probably don’t even know it.

In seeking to make our committees more efficient and effective as they manage their work and meet the overall mission of the Society, we realize that we need to do some repopulating. The committees and their functions will remain the same as outlined in our bylaws, but we need to start fresh with committee membership.

Here’s the process for committee repopulation and how you can get/remain involved:

  1. The glitch that allowed people to automatically join committees during initial membership application has already been fixed.
  2. We will poll each Committee Chair to determine who are the core, active members of each committee. These individuals will be contacted and given the opportunity to remain part of the committee. If the individuals are members of multiple committees, they may be asked to make a choice and focus their efforts on one committee.
  3. All “extra” committee members (i.e., those who aren’t presently participating) will be removed from all committees so we can start fresh.
  4. Each committee will meet to determine how many members they need to accomplish their work. Where there is need, openings will be made for ASNM members to fill.
  5. We will send out an email to all ASNM members asking if you are interested in committee participation. That email will give you instructions on how to join a committee if space is available.

Remember that committee work is voluntary. When considering committee participation, think about how much time and energy is required and whether or not you are able to dedicate that time and energy. It is best if you participate in only one committee so you don’t stretch yourself too thin.

Being part of a committee is definitely not just about putting something new on your resume or CV. This should be service to the profession that produces meaningful results and introduces members to networking opportunities. Everyone on a committee will be expected to participate in meetings and pull their weight to accomplish the committee’s work. Anyone who isn’t doing their part may be removed from the committee to make room for someone else.

How can you get involved? You don’t have to do anything right now. We’ll contact you in a few weeks to gauge your interest. Until then, check out the names of the different committees and the kinds of work they do. Think about where your interests lie, and what you’d like to see the ASNM accomplish. Your voice matters, and the best way to make a change is to jump in and help!

Tags:  Announcement 

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President's Message - May 2018

Posted By Joseph J. Moreira, M.D., Monday, May 14, 2018

Dear Membership,

The time has come to pass the gavel, officially, to Jeff Gertsch. I will make this brief but just wanted to thank the membership for making this past year possible. The commitment you all made to this society year after year keeps us moving forward and flourishing as a profession. We have excellent educational offerings and make progress in the field due mainly to your support and participation. Your ongoing interest and support is what drives us on the ASNM board. To that end I ask that all of you consider serving in any capacity possible. The new year comes quickly, and many hands are needed. Please consider running for office, joining a committee, speaking at a meeting, running a webinar, searching for and recruiting new members etc.

I want to thank our amazing board of directors and members of the executive committee as well as Apex Management for all their efforts and support. We accomplished a great many things this year but there is a tremendous amount of uphill work to be done. I will give all my efforts and support to the new President and board members and will do my best to keep things moving forward in our field. I ask you all to do the same.

Thank you again for this humbling experience and I am always available to help in any way. All the best to our new leadership, we are in excellent hands.

Best, Joe

Tags:  President's Message 

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ASNM Releases Position Statement on Business Practices in Neuromonitoring

Posted By Administration, Wednesday, March 14, 2018

Breaking News!

 In a newly-released Position Statement, the ASNM takes a stand on business practices related to neuromonitoring.

Read the entire position statement here.

Tags:  Announcement 

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Member Spotlight - Learn More About Joe Moreira

Posted By Richard W. Vogel, Thursday, February 1, 2018

In the Member Spotlight, we periodically introduce individual members of the ASNM to the society-at-large. We try to highlight some of their achievements and ask them interesting questions. We hope this affords members of the ASNM the opportunity to get to know each other. If you would like to recommend someone, including yourself, for the Member Spotlight, please contact Rich Vogel.

 

In this edition of Member Spotlight, we are shining the light on ASNM President Joseph Moreira, MD.

Background: Neurologist

Position: ASNM President (May 2017 - May 2018)

 

As usual, we sent some questions to Dr. Moreira so we can learn a little about our President. 

When your term as President ends, what do you hope to name as your greatest achievement?

My goal is to bring ASNM into the mainstream and work with other societies to have one voice and one consolidated set of practice guidelines. I would also like to improve communication in the field and surgeon and patient education. We should be the ones to choose what modalities are used on a given case and not have the surgeon arbitrarily select them.

 

What obstacles have you faced as President of the ASNM?

Bringing various factions and societies together on a unified front. So far so good but a lot of work left to do. I have worked on this with the current incoming president elects, Jeff Gertch and Rich Vogel and I am confident that as an executive committee going forward we will further the cause.

 

What predictions do you have about the future of the ASNM?

The ASNM’s member base will grow with all factions being well represented and involved. We will be a driving force amongst other societies and work together. We are the only pure IONM society in the US and need to lead the profession’s growth and education. If we can work with other societies and specialties, then we can flourish and grow.

 

What would you say is the best thing about attending an ASNM meeting?

The Bar! No … Apart from the educational content, the best part of the live conferences is the networking opportunity. We are all working in a vacuum most of the time, either in the OR doing our own case, or in an office. We rarely have the chance to openly discuss IONM and the industry with anyone. The ASNM meetings give us all the chance to relax, discuss, and learn new concepts. The mix of folks attending and gathering range from the new trainees to the seasoned, well experienced members at all levels

 

What is the most under-rated, or least known, benefit of being an ASNM member?

The open access to becoming involved. Anyone can join a committee, run for office, etc. This brings with it a new world of experience and accomplishment. We are a very open society to all involved in the field.

 

A little about you:

 

If I didn’t become a neurologist, I probably would have been a(n):

Anatomist, I love anatomy and its overall complexity and consistency. A fireman, besides the big trucks and awesome toys, I love to put out fires, that’s probably why I am drawn to IONM. A chef, love to cook.

 

Greatest technological advancement in neuromonitoring has been:

The development of multi-pulse train stimulation for MEPs is one of my favorites.

 

The best career advice I’ve ever received is:

Be brief, be bright, be gone!

 

A great article that everyone in the profession should read is:

Heuristic map of myotomal innervation in humans using direct intraoperative nerve root stimulation. (J Neurosurg Spine. 2011 Jul;15(1):64-70.).

I love this article because it is relevant to our everyday functioning in the OR, and expands on the range of muscles innervated by a specific root.

 

 Also:

 

Litigating Intraoperative Neuromonitoring (IOM). (University of Baltimore Law Review. 2016 45(3), #3.).

This is a great 360 degree look at IOM and litigation and how to protect all concerned.

 

One of my favorite apps is:

Open Table, the quest for great food continues! Mapster, it keeps track of all your favorite places as you travel.

 

One of my pet peeves is:

Bad Communication!

 

My favorite film(s) of all time is/are:

Godfather and Blues Brothers.

 

At the top of my travel bucket-list is:

Mars, however to be realistic Italy, Ireland, Egypt. Favorite so far is Istanbul and Buzios, Brasil.

 

My favorite hobby is:

Playing the drums and cooking.

Tags:  Dr Moreira  Member Spotlight 

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Raising Mean Arterial Pressure Alone Restores 20% of Intraoperative Neuromonitoring Losses

Posted By Richard W. Vogel, Monday, January 22, 2018

Anyone who works in neuromonitoring will tell you that the #1 reason why we lose MEPs or SSEPs during spine surgery is hypo-perfusion of the spinal cord. Indeed, the relationship between spinal cord perfusion and neuromonitoring (IONM) data is intimate: An adequately perfused spinal cord optimizes conduction-based data (i.e., MEP and SSEP), and a hypo-perfused spinal cord can cause loss of data or inability to acquire baseline data. Given that spinal cord perfusion is closely related to the mean arterial pressure (MAP), we always try to ensure that the MAP is maintained within an optimal range.

Every patient has different needs in terms of the mean arterial pressure (MAP) that will maintain adequate spinal cord perfusion, and thus function, and thus IONM signals. During spine surgery, we’re always working with anesthesia to maintain the appropriate pressure and we understand that achieving that optimal MAP can be challenging in some patients.

Sometimes the MAP slips too low, and we start to lose our signals. It’s a classic progression: MEPs deteriorate first, followed by SSEPs later. That’s because MEPs rely on conduction across the synapse between the upper and lower motor neurons. These synapses occur in the spinal cord’s ventral gray matter, they have a high metabolic demand, and they’re quite sensitive to changes in perfusion. So, the time to electrical failure is very short when blood supply is low. The SSEP pathway doesn’t synapse in the spinal cord and the dorsal column white matter tracts have a much lower metabolic demand. So, the time to electrical failure is much longer.

There’s an old saying in the world of stroke care: “Time is brain.”. The same is true in spine surgery: Time is spinal cord. When spinal cord perfusion falls below the functional threshold, the clock starts ticking as ischemia can ultimately cause an infarct. The penumbra between onset of ischemia and onset of infarct is what we call a critical window of opportunity to perform an intervention, and the first thing we usually do is request an increase in the MAP.

How frequently is this strategy effective? A recent article published in Spine1 assessed the effect of different interventions in restoring IONM signals in pediatric spine surgery. This was a multi-center prospective study of 452 patients undergoing posterior spinal deformity surgery. The results are exactly what we would expect, increasing MAP is highly effective first line of defense against spinal cord hypo-perfusion.

Of the 30 patients who had a significant IONM signal alterations in this study, 20% had return of signals due to an increase in MAP alone with no other interventions (MAP increased from x̅ = 68 to 86 mmHg). On average, signals returned to baseline after 16 min. In 60% of patients, MAP was raised from x̅ = 72 mmHg to 86 mmHg in conjunction with other interventions and signals returned to baseline after an average of 37 mins. The rest of the patients had signal changes unrelated to MAP. The authors argue that raising MAPs above 85 mmHg should be considered the first step in response to IONM signal changes, as this alone was successful in 20% of patients without sacrificing deformity correction.

A wonderful statistic not overly discussed in this paper was that there were zero bad spinal cord outcomes, meaning that performing some form of an intervention in each of the 30 alerts returned data to baseline, which was a strong predictor for success. All patients had return of signals at the conclusion of the procedure with one patient having postoperative neurological sequelae.

The rationale for having an adequate MAP extends well beyond making the neuromonitoring team happy. Increasing MAP in response to an IONM data change is universally used in spine surgery, and therefore serves as a common therapeutic intervention. However, we must never (ever) forget that an appropriate MAP during times of risk to the spinal cord is also prophylactic and makes the spinal cord more resilient to any iatrogenic or peri-surgical insult.

Interestingly, many of the alerts in this study occurred during pedicle screw placement, and not just during the correction. This suggests that MAP should be maintained at 85 mmHg during all times of risk, including screw/hook/sublaminar wire placement, osteotomies, etc. This can also include surgical exposure in certain populations like marked kyphoscoliosis patients who are at risk from positioning alone. Given that increased MAP is both therapeutic and prophylactic, maintaining MAP at 85 mmHg is likely a good idea in any cervical or thoracic spine surgery, regardless of diagnosis.

A final thought, increasing MAP doesn’t always equate to increasing spinal cord perfusion. Pure vasoconstrictors, like phenylephrine, increase MAP but their failure to increase cardiac output may do little to benefit spinal cord perfusion. In situations where hypo-perfusion is suspected, a better approach may be to consider epinephrine which increases heart rate, vascular resistance and cardiac contractility. The result of increasing the cardiac output is more blood flow to regions at risk for ischemic injury.

Adam Doan, DC, D.ABNM

Rich Vogel, PhD, D.ABNM

References:

1Yang J, Skaggs DL, Chan P, Shah SA, Vitale MG, Neiss G, Feinberg N, Andras LM. Raising Mean Arterial Pressure Alone Restores 20% of Intraoperative Neuromonitoring Losses. Spine (Phila Pa 1976). 2017 Oct 18. Find on PubMed

Tags:  In the Literature 

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Brachial Plexus Injury in Cervical Spine Surgery

Posted By Richard W. Vogel, Monday, January 15, 2018

The ASNM Monitor Blog is pleased to add a new series entitled, In the Literature. Posts published with this tag will review articles in the literature to help you follow the latest developments in the science and practice of IONM. 

 Today, we’re reviewing an article1 on the prevalence of brachial plexus injury during cervical spine surgery. The article is titled, Brachial Plexopathy After Cervical Spine Surgery. It is open access and available to download here if you wish.

 This study is a retrospective, multicenter case series of 12,903 patients who underwent cervical spine surgery at 21 different facilities. In this large sample of patients, only 1 patient experienced post-operative brachial plexopathy. So, the incidence was 0.78%.

 The results of this study suggest that brachial plexus injury is an extremely rare complication of cervical spine surgery. In their review of the literature, the authors cite a previously published paper by Hasegawa et al.2 which reported a much higher occurrence of 2.2% brachial plexopathy following c-spine surgery.

 The authors’ explanation for the disparity in the prevalence of brachial plexopathy between the two studies may be a result of several factors, such as:

  1. The retrospective nature of the study by Than et al. renders it prone to recall bias, which can decrease the prevalence rate that gets reported.
  2. The present study was composed of 12,903 patients whereas the study by Hasegawa et al. was composed of 857 patients. The larger number of patients in the present study resulted in a higher denominator when calculating the percentage of brachial plexus injuries. This may result in a more accurate estimation of the injury rate.
  3. The study by Hasegawa et al. collected data mostly in the 1980s and 1990s. This was prior to routine use of neuromonitoring in cervical spine surgery, and prior to the routine use of MEPs. The more recent study by Than et al. collected data from 2005 to 2011. Patients in this study likely received more optimal neuromonitoring and more advanced surgical techniques. This could also explain the lower prevalence of brachial plexopathy in recent years.

The one patient who presented with brachial plexopathy in the present study actually developed Parsonage-Turner Syndrome (brachial neuritis), but it didn’t appear until a few days after surgery. While there’s no mention of neuromonitoring for this patient, it is worth noting that MEPs and SSEPs can only predict neurologic function in the immediate postoperative period. Delayed deficits are rarely, if ever, detected with neuromonitoring.

Positioning issues are common in surgery. The authors of this blog post have personally detected malpositioning and compression of the extremities in hundreds of patients. Isolated changes in SSEPs or MEPs served as the warning sign. Following an intervention, such as repositioning the limb or removing compression, the data almost always returned to baseline and the patients woke without deficits. These alerts are classified most accurately as true positives. If we had not intervened, then the patient would likely have emerged from surgery with some type of deficit, such as brachial plexopathy.

 Our experience mirrors those described by Schwartz et al.3 in which the most common cause of changes in MEPs and/or SSEPs during ACDF surgery was impending injury to the brachial plexus. In fact, 65% of all data changes during ACDF were related to impending position-induced brachial plexopathy.

 How does brachial plexopathy develop in cervical spine surgery? In the present paper, the authors identify upper extremity traction as the main time of risk to the brachial plexus. This is when the surgeon is taping down the shoulders. The second greatest time of risk to the brachial plexus is extension of the neck (in anterior surgery), as this places additional traction on the brachial plexus. With this in mind, it makes the most sense to get baseline SSEPs and MEPs prior to both of these maneuvers. This would help to quickly and accurately determine the cause if there are any abnormalities in your data.

 How would this play out? First, you would establish baselines. The surgeon would then tape the shoulders in traction, and you would run another test to ensure that there are no changes from baseline. If there are changes, then you recommend that the surgeon loosen the tape on the impacted shoulder. Next, the surgeon extends the patient’s neck, and you would run another test to ensure that there are no changes from baseline. If there are changes, then you recommend that the surgeon move the patient’s neck to a more neutral position.

 Imagine if you establish baseline after these positioning maneuvers in an ACDF and you find abnormalities in the baseline data. It may be far more difficult to determine if these abnormalities in the data reflect the patient’s true baseline, or whether they are the result of one or both of the positioning maneuvers. You’d have to undo the maneuvers to help identify the root of the abnormalities. You might even have to wake the patient to perform a neuro exam. We’ve heard of this happening before. It’s quite disruptive and totally unnecessary.

 Some of these very same issues were discussed in a 2015 Editorial by Epstein and Stecker4. With respect to establishing baseline MEPs and SSEPs before positioning, they pose the question, “Why can’t we and our monitoring colleagues get this right?” They recommend establishing baselines before any maneuvers are performed that pose risk to the nervous system, regardless of whether the risk is to the spinal cord, nerve roots or peripheral nerves.

 In the context of a spinal fracture, deformity, a stenotic spinal canal, or pathology involving the spinal cord, the risk actually begins with intubation because it requires extension of the neck. Awake fibreoptic intubations are typically ordered by the surgeon in these situations so a neurological exam can be performed before unconsciousness is induced. If an awake intubation is not possible in these situations, then Epstein and Stecker recommend that MEP and SSEP baselines be established in the period of time between induction and intubation. These baselines should accurately reflect the preoperative status of the patient.

 There is at least one densely-populated geographic region of the US where almost all of the hospitals have a protocol to acquire pre-intubation baselines on all patients undergoing cervical spine surgery, regardless of diagnosis or severity of risk. This technique requires that almost all neuromonitoring electrodes be placed in pre-op holding (usually after a little Versed). All of the needles get placed in holding except in the head and feet. When the patient comes in the OR, all the electrodes get plugged in. Anesthesia induces unconsciousness with a Propofol injection and manual respiration begins through a mask as the monitorist quickly places electrodes in the patient’s head and feet. Then, the monitorist quickly tests MEPs and SSEPs to establish baseline. After that, anesthesia proceeds with intubation. The teams who do this regularly have excellent preparation and communication. The whole process takes less than a minute. This pre-intubation baseline protocol is extremely rare outside of this one particular geographic region, and probably not necessary in most patients.

 Following intubation, the next stage of risk to the nervous system is positioning of the patient. If it’s an anterior approach, then the risky maneuvers include shoulder traction and neck extension. If it’s a posterior approach, then the risky maneuver is prone positioning of the patient. We are of the opinion that MEP and SSEP baselines should always be established before performing these maneuvers. A separate paper just published in 2017 reported neuromonitoring data changes immediately following neck extension in 2.4% of patients undergoing ACDF surgery5. In most of the affected patients, signals returned to baseline immediately following repositioning of the neck. In a small portion of affected patients, signals were never recovered and they emerged from surgery with new neurologic deficits.

 Unfortunately, some surgeons don’t want to get baselines before positioning the patient, particularly in ACDF surgery, because they think it will slow them down. This is a flawed perspective, though, because it should take less than a minute to acquire baselines in a typical ACDF surgery. So, there is no good reason to not establish neuromonitoring baselines prior to positioning for cervical spine surgery. With this in mind, we believe it is best practice for the neuromonitoring team to recommend pre-positioning baselines to the surgeon before cervical spine surgery, regardless of diagnosis. If the surgeon declines, it is best practice to document this recommendation along with the fact that the surgeon declined.

 Rich Vogel, PhD, DABNM

 Adam Doan, DC, DABNM

 References:

  1. Than KD, Mummaneni PV, Smith ZA, Hsu WK, Arnold PM, Fehlings MG, Mroz TE, Riew KD. Brachial Plexopathy After Cervical Spine Surgery. Global Spine J. 2017 Apr;7(1 Suppl):17S-20S.
  2.  Hasegawa K, Homma T, Chiba Y. Upper extremity palsy following cervical decompression surgery results from a transient spinal cord lesion. Spine (Phila Pa 1976). 2007 Mar 15;32(6):E197-202.
  3. Schwartz DM, Sestokas AK, Hilibrand, AS, Vaccaro AR, Bose B, Li M, Albert TJ. Neurophysiological identification of position-induced neurologic injury during anterior cervical spine surgery. J Clin Monit Comput 2006; 20: 437–444
  4. Epstein NE, Stecker MM. Intraoperative neuro-monitoring corner editorial: The need for preoperative SSEP and MEP baselines in spinal surgery: Why can't we and our monitoring colleagues get this right? Surg Neurol Int. 2014 Dec 30;5(Suppl 15):S548-51.
  5. Appel A, Korn A, Biron T, Goldstein K, Rand N, Millgram M, Floman Y, Ashkenazi E. Efficacy of head repositioning in restoration of electrophysiological signals during cervical spine procedures. J Clin Neurophysiol 2017; 34:174-178.

Note: Opinions expressed in this blog belong to the authors of the post and do not represent the opinions of the ASNM.

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Understanding Anodal and Cathodal Stimulation

Posted By Richard W. Vogel, Friday, December 1, 2017

Whether you practice neurophysiology in surgery, in the lab, or in the clinic, you probably use electrical stimulation to activate the nervous system on a daily basis. As you probably know, cathodal stimulation works best in some applications, while anodal stimulation works best in other applications.

Armed with this knowledge, you know precisely where to place electrodes on the body, and where to plug those electrodes in - black in cathode (-) and red in anode (+). But, what's the difference? What exactly is anodal or cathodal stimulation, and why does one work better than the other in some applications?

Today I hope to answer some of those questions for you because I believe that understanding stimulus polarity is important, and it will make you a better neurophysiologist.

Before we talk about how stimulators works, it is important to have a basic understanding of how a battery works.

How a Battery Works

The correct term for what we frequently refer to as a "battery", is a "cell", but I'm going to use the word battery to keep it simple. So, a battery is a charge-separating device.  It stores electric energy by separating cations and anions into two separate compartments, or terminals (Figure 1).

  • Cations are positively-charged ions (+).
  • Anions are negatively-charged ions (-).

If you refer to the illustration in Figure 1, you will see that one terminal of the battery contains an excess of cations (+), and this is the positive terminal (+). Because it contains cations (+), the positive (+) terminal of the battery is called the cathode (+). The other terminal of the battery contains an excess of anions (-), and this is the negative terminal (-). Because it contains anions (-), the negative (-) terminal of the battery is called the anode (-).

When the battery is connected to a load, in this case a lightbulb, the device is powered by the flow of current. Conventional Current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal. This was the convention chosen during the discovery of electricity, but they were wrong! Rather, Electrical Current is what actually happens, as electrons (-) flow out of the negative terminal (anode), through the circuit and into the positive terminal (cathode). 

The take-home message is that, in a battery, current flows from anode to cathode. To learn more about batteries, go here.

How an Electrical Stimulator Works

In an electrical stimulator, the flow of anions (-) and cations (+) is controlled by the mechanics of the circuitry within the stimulator.  The stimulator is unique in that the cathode is the negative pole (-) because it discharges anions (-), and the anode is the positive pole (+) because it discharges cations (+). At the end of the day, that's the fundamental difference between a battery and a stimulator.

Depending on how we configure the polarity, the stimulator will discharge either cations or anions into the body part being stimulated.

In cathodal stimulation, anions (-) are discharged into the body as current flows from the cathode (-), through the tissue, and back to the anode (+).

In anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (-).

Now, let's imagine that we place an electrical stimulator on the surface of the skin with a nerve bundle running underneath (Figure 2). Within the nerve bundle is a single nerve fibre (axon) upon which we will focus.

At rest, the inside of a cell is more negative than the outside of a cell. This occurs because there is a slightly greater number of negative charges than positive charges inside of the cell (intracellular space), and a slightly greater number of positive charges than negative charge outside of the cell (extracellular space). Because of the electrical difference, the cell is said to be polarized - just like a magnet, one side is more positive and the other side is more negative. If the electrical gradient were suddenly reversed, the cell would be depolarized, and we might see an action potential.

Cathodal Stimulation of Peripheral Nerves

When we use the term cathodal stimulation, what we mean is that negatively-charged anions (-) flow from the cathode, into the tissue, and back to the anode (Figure 3). As the electrical current flows from cathode to anode, negative charges (anions) tend to accumulate on the outer surface of the nerve membrane as they will be repelled by the negatively-charged cathode. This makes the outside of the membrane more negative. Consequently, the inside of the membrane becomes more positive due to accumulation of positive ions on the inside. This will result in depolarization, which, if sufficient in magnitude, will result in an action potential (nerve impulse or muscle activation).

Figure 3 illustrates activation of the axon under the cathode. As a result of stimulation, an action potential is sent in both directions along the length of the nerve, starting at the cathode. Something interesting happens underneath the anode, though! All of the negative charge from the extracellular space is attracted to the anode, leaving the outside of the cell excessively electrically positive relative to the inside of the cell. The cell is thus hyperpolarized under the anode, meaning that it is very, very difficult to activate.

If you apply the information above to the median nerve SSEP (Figure 4), then you can see why the anode is always distal, and the cathode is always proximal.

What happens when you accidentally reverse your stimulating electrodes when performing an SSEP test? The difficulty that you may experience in attempting to acquire an SSEP is explained by the phenomenon of anodal blocking (Figure 3). Thus, when bipolar electrodes have tips in the same orientation as a fiber, a fiber will be depolarized under the cathode, and hyperpolarized under the anode. If the hyperpolarization is large enough, an action potential initiated under the cathode may not be able to propagate through the region of hyperpolarization. If this is the case, the action potential will propagate in only one direction. While we often talk about the phenomenon of anodal blocking, you won't see this in the clinical scenario if you use appropriate stimulation parameters. For intraoperative monitoring of SSEPs, you should be using supramaximal stimulation. The high intensity stimulus will overcome any issues that may be experience as a result of anodal blocking. 

Anodal Stimulation of Peripheral Nerves

When we use the term anodal stimulation, what we mean is that cations (+) flow from the anode, into the tissue, and back to the cathode (Figure 5). When applied to the surface of a nerve, anodal current will increase the concentration of cations (+) in the extracellular space under the anode. This will result in hyperpolarizationwhich, as I just mentioned, puts the cell in a heightened state of rest. So, what we see in Figure 5 is that the nerve axon becomes deactivated (hyperpolarized) under the anode.

The Importance of Cell Orientation

In all of the examples described thus far, the orientation of the cell under the stimulator has been horizontal with respect to the orientation of the anode and cathode (Figures 2-5). This is usually the case when stimulating nerves in the arms and legs.

What happens when the orientation of the cell is vertical with respect to the orientation of the anode and cathode? The answer is that things usually work exactly opposite to what we just discussed regarding horizontally-oriented cells.

This becomes particularly important in the brain where pyramidal cells of the cerebral cortex are vertically-oriented with respect to the surface where we stimulate.

Anodal Stimulation of Cerebral Cortex

Electrical stimulation of cerebral cortex is used for lots of reasons, but today I'm going to focus on motor evoked potentials (MEPs). If you use electricity (as opposed to a magnet) to evoke MEPs in your clinical practice, hopefully you know the following principle:

Whether you are stimulating the scalp over motor cortex, or directly stimulating the cortical surface, MEPs are always easiest to elicit and characterize when you use anodal, monopolar, pulse-train stimulation. Things change a little with subcortical stimulation, but that's a topic for a different day.

Starting with Fritsch and Hitzig (1870), many researchers have shown that monopolar stimulation of the motor cortex is more effective with an anode, as opposed to a cathode. Also, monopolar anodal stimulation seems to activate pyramidal cells directly.

One proposed mechanism is that anodal current enters (and hyperpolarizes) dendrites at the surface of the brain, then leaves and depolarizes the axon or cell body. One way to think about this illustrated in Figure 7.

Anodal stimulation is just the injection of positively-charged ions under the electrode. Because opposites attract, negatively charged ions migrate to the the very surface of cortex under the anode. You can think of this a current sink and the consequence is hyperpolarization of the apical dendrites of the pyramidal cell. In order to compensate for this current sink, a current source is generated distally such that positively-charged ions congregate around the other end of the pyramidal cell. This results in depolarization (activation) of the cell body, the axon hillock and the initial segment of the axon, which forms the corticospinal tract.

Of course, it isn't that simple! Computational simulations paint a more complex picture. As Figure 8 illustrates, the neural response to stimulation is likely a complex pattern of depolarization and hyperpolarization throughout the neural geometry of the cell, which is dependent upon stimulation parameters and the neural positions relative to the electrode. Clearly, when the long axis of the cell is oriented vertically relative to the orientation of an anodal stimulation electrode, the computation simulation supports hyperpolarization of the apical dendrites and depolarization around the axon hillock.

It all comes down to the orientation of the cell!

Think about this... when you place your monopolar stimulating electrode over the motor cortex and deliver anodal stimulation, your lowest threshold CMAPs are from the vertically-oriented cells just below your electrode. If you do transcranial MEPs, your electrode is probably C3 or C4, right? And, the electrode are just over the hand representation of the motor homunculus. You really have to increase the intensity to get MEPs from the legs, correct? This is because those "leg" cells are deep in the interhemispheric fissure and the cells are oriented horizontal to your anodal stimulating electrode. BUT, if you switch your polarity and deliver cathodal stimulation from the same electrode, MEPs from the legs are sometimes easier to elicit and hands become more challenging. This phenomenon works best when you are stimulation around threshold intensity. You can use this to troubleshoot your MEPs. If you begin stimulating and you get MEPs from the legs/feet at lower intensity than the arms/hands, then your polarity is probably reversed.

References

  • Fritsch GT, Hitzig E. 1870. Über die elektrische Erregbarkeit des Grosshirns. Arch Anat Physiol Med Wiss 300–32. Translation in Von Bonin G. 1960. Some papers on the cerebral cortex. Springfield (IL): Charles C Thomas.
  • Merrill DR, Bikson M, Jefferys JGR. Electrical stimulation of excitable tissue: Design of efficacious and safe protocols. J Neurosci Methods. 2005 Feb 15; 141(2):171-198.
  • Nair DR, Burgess R, McIntyre CC, Lüders H. Chronic subdural electrodes in the management of epilepsy. Clin Neurophysiol. 2008 Jan;119(1):11-28. Epub 2007 Nov 26. Review.
  • Ranck JB Jr. Which elements are excited in electrical stimulation of mammalian central nervous system: a review. Brain Res. 1975 Nov 21;98(3):417-40. Review.
  • Stephani C, Luders HO. Electrical Stimulation of Invasive Electrodes in Extratemporal Lobe Epilepsy. In: Koubeissi MZ, Maciunas RJ, eds. Extratemporal Lobe Epilepsy Surgery. Montrouge, France: John Libby Eurotext; 2011. 261-313. Print.

Note: This article was originally published by Richard Vogel in 2015 and is being republished here for the benefit of ASNM members. The contents of this post are the work of the author and do not necessarily represent the views of the ASNM.

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President's Message - December 2017

Posted By Joseph J. Moreira, M.D., Thursday, November 30, 2017

Greetings,

I hope this message finds you all well and that you had a wonderful Thanksgiving.  It is now a family tradition that when things are either not going well, or we are complaining about life in general, that we force ourselves to think of 5 positive things. Yes, my partner is a psychotherapist! I am proud to update you on a number of items that I hope will brighten your day a bit.

The first order of business is to congratulate everyone on a well-run and successful election. I am very grateful to all the candidates that had the courage and spent the time to throw their hats in the ring and run for an office. It is a wonderful process to undertake as it gets you thinking about our field and what needs to be done to keep it moving in the right direction. It also gives each candidate a bit of exposure and helps them to network some and be recognized for their accomplishments. I urge all our members to consider being a part of this process and get involved in an election, a committee, a meeting, or whatever you have time for.

Our new President-Elect is Rich Vogel. He will become president after the Annual Meeting in 2019.

We also have 4 new Directors who will take office in 2019: 

I am excited about our new board members and am looking forward to working with all of them. This society is in excellent hands and we owe the entire group that participated in this election a thank you for getting involved.

 

The second item I am grateful for is the recent board approval of our new Practice Guidelines for the Supervising Professional: Intraoperative Neurophysiologic Monitoring. This was over a year and a half in the making with the great efforts of an ad hoc committee comprised of original authors, board members, executive Committee members and general members. Great pains were undertaken to assure that the new document was properly updated. We strove to have the document reflect current practices while maintaining the highest quality of patient care. We will now submit the revision to other societies for their review and accept comments from them if any. The document will then be published after a final run through and consideration of any comments generated during the review period.

 

Item number three. We had one of our most successful Fall meetings ever in Baltimore. The main thrust was IONM and Medical-legal issues. Our Mock trial was a tremendous success and is the start of an all new type of creative format for our future meetings playing off a theme and using a more practical method of delivering the message.

 

That brings me to number four. Our Annual meeting will be held February 23-25 at the Swan and Dolphin in Disney World, Orlando, Florida. This will be held during President’s week off so please consider bringing the family. The meeting will be a new format of 2 tracks on day 1. Each session will have its own lectures, panel discussions and E poster presentations. The topics are cutting edge and all promise to be excellent. Please check out the agenda on our website.

 

Finally, number five. In keeping our promise to collaborate with other societies, May 1-6, 2018 we will be contributing to the 31st International Congress of Clinical Neurophysiology (ICCN) in Washington, DC. This is a great opportunity to hear from international neurophysiology experts and attend workshops and lectures. Note that this meeting is taking place during our usual Annual Meeting and that our Winter Meeting in Orlando is our Annual Meeting.

 

I look forward to seeing you all at the meetings and wish everyone a Happy, Joyous and Healthy Holiday Season.

 

Best Wishes, Joe

 

Joseph J. Moreira, MD

President, American Association of Neurophysiologic Monitoring

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