Well, once again, I'd like to thank the organizers for inviting me to talk about somatosensory evoked potentials and motor evoked potentials and brain mapping procedures. Here are my conflict of interests, none of which are going to be related to what I'll be talking about today. So, when we talk about interoperative neuromonitoring, there are three classes and modalities that we can look at. There are electrical stimulation evoked potentials that include the sensory and motor, of which we're going to be talking about today. There are task-related evoked potentials, which are down there. They're more the complex processing that goes on. And then finally, there are the free-running biologic signal recordings, our single and multi-unit recordings, electroencephalography, both surface and intracranial, and electromyography. There are, though, some issues that do complicate interoperative monitoring in the operating room, and they include environmental issues, external noise brought in by other pieces of equipment. One of the pieces of equipment that causes some noise is the microscopes as it goes up and is brought into the area, and it interferes with some of our recording potentials a little bit. There are patient-related issues, such as disease-related, such as muscle atrophy. When we're trying to do motor evoked potentials, there are two problems with muscle atrophy. The first is we may not be able to get our electrodes into the muscles, so our potentials may not be as large as they normally should be. And the second one is usually muscle atrophy is related with some damage to the motor unit. So for transferring signals that we actually are stimulating part of the cortical spinal tract, they may not be getting to the muscles due to the damaged motor unit. There are technical issues in the IOM equipment, which we need to make sure that our IOM equipment is actually functioning. And then there are anesthetic issues. We need to know what anesthesia to use specifically for motor evoked potentials. The inhalational gases tend to be problematic in being able to get our motor evoked potentials, so we like to use a pure total intravenous protocol if we actually can for those. What I'm going to be talking about today are somatosensory evoked potentials and motor evoked potentials monitoring. And really what I'm going to be discussing is the anesthetized patient, doing mapping in the anesthetized patient with these techniques. So the benefits of IOM during these procedures is that it's the most accurate localization technique at the time it's used, and that's in the operating room. So it's giving you that millimeter accuracy in the operating room when you're doing that testing. There are other ones. Extradorally, if you're doing motor cortical stimulators, for instance, you have no visual access to the central sulcus, so it's a way of mapping those areas of the central sulcus. And then intradorally, if there's distorted anatomy due to the tumor, you may not have good anatomic landmarks, and this is a nice way to give you those pictures. So we'll start with the phase reversal potential. And what I'll start with down here, I want you to focus on these images down here that we can look at. And you'll see that we get what we call a P20 and an N22 on contacts 0 and 1, and we get an N20 and a P22 on contacts 1 and 2. And if you take a look at those two wave shapes, they look like they're almost mirror images of each other. And that's the phase reversal that we talk about, finding those mirror images. One of them sits on the pre-central gyrus, and one of them sits on the post-central gyrus lining up. Now, they're not exact mirror images, and that has to do with the way the dipoles are actually formed on the surface of the brain, and so the electricity has a direction to it, and that's because the direction may be a little bit off. The peak positivity anterior to the central sulcus occurs at a slightly longer latency than the peak negativity, but it's really that type of phase reversal that's really important for what we're looking at. So technically, the optimal phase reversals with the perpendicular placement of the electrode, the less the relative central sulcus electrode angle, the less sharp the somatosensory evoked potential phase reversal localization potential will be, to getting a complete loss of phase reversal if you're lying parallel, although that can give you some information also as to where you are in space. We stimulate the median and ulnar nerve at the wrist. Some people have stimulated the common peroneal or posterior tibial nerves at the knee or ankle, although the results there are not as clear-cut as stimulating at the median or ulnar nerve at the wrist. We only need to average about 10 to 20 trials when doing this, so unlike the standard SSCPs that are done during spinal surgery, where it can take two minutes to get a response at four hertz, we're getting our results in about five seconds from doing this, because we don't have the skull attenuating our signal. How do we go about doing this structure over here? Here's our central sulcus located, and here's what we do. We see these images on our screen, the physiology. We can see that on these first two channels, 0, 1, and 1, and 2, we have an upward-going wave and then a downward-going wave, and then on this one we have a downward-going wave and then an upward-going wave, and so our phase reversal is somewhere in between here, usually at contact 2, so we can place the electrode. We figure the electrode's sitting around there in space, and we continue to do this. We can see over here that we have an upward-going wave and a downward-going wave on 0, 1, but on 1, and 2, and 2, and 3, we have the downward wave and the upward wave, showing that our phase reversal is probably sitting on contact 1. We can continue to do this. Now, there may be some complex wave shapes that we're looking at. We may get multi-phasic waves that are occurring there, but it's really that first peak and that last peak that we're looking at when we're doing this phase reversal potential that we're trying to see. We can line it up over here, and over here we get the same one. We can see that, depending upon, you can get a very nice picture of where the central sulcus is. Now, what does the phase reversal potential do? It helps us find the motor cortex to get to the next stage of what we're doing, which is the motor mapping, and it limits the time that we actually have to stimulate the brain, because if we know where the motor strip is first, we can stimulate the brain less when we're just trying to map out the specific regions of the motor strip. Now, what do we do if we get all the waves facing in the same direction? This is what I said before. If we're parallel to the sulci, all the waves will face in the same direction. What we end up doing in that case, we just ask the surgeon to turn the electrode 90 degrees, and we start to get our phase reversal potentials back over here, and then we can continue to map as we mapped before to do the continual phase reversal mapping. Really, about four or five of these phase reversals are needed to get a pretty good idea of where the central sulcus is in space. The next step that we do is we do the motor mapping procedures. For motor mapping, there are some considerations that we have to think about, especially because we're delivering an electrical stimulus to the brain. We like to know about the histories of seizures the patients may have had, because that will affect the initial stimulation level. We'll probably go at a little lower level at first and start to come up, but it also, more importantly, affects the speed of motor mapping. A lot of times, motor mapping is done by just we turn the stimulator on, and the surgeon places the probe on the brain and then moves that probe around. If there is a history of seizures, we're usually going to recommend that we give a few seconds, maybe five seconds between stimulation points to give the brain time to recover. We always use EEG monitoring during this, looking for after discharges or looking for seizure activity in the brain. You need to use EEG during motor mapping procedures. Anesthetics, as I say, do interfere with the recording, some more than others. As I said, it's the inhalational agents or the muscle relaxants that really make motor mapping difficult. We really like to use the intravenous with no muscle relaxant on board. We also do keep ice-cold saline available, because if we do get a seizure, you can actually put the ice-cold saline right onto the brain surface, and it will inhibit that seizure. The nice thing about the ice-cold saline compared to some of the other techniques that may be used to stop a seizure is in about 10 minutes, you can go back to motor mapping, because the brain has recovered by that time. There are two types of stimulation. When we do motor mapping on the surface of the head, we use a cathodal stimulation. When we do motor mapping inside, when we're looking to stimulate the cortical spinal tract directly, we do anodal stimulation, and I'll show you why we do that in a little bit. I like to use this high-frequency technique, which is 250 to 500 hertz short bursts of 5 to 9 pulses. There is the low-frequency technique also, but there are a few reasons why I like to use it. A, it has a lower seizure threshold. There's been some literature that's talked about the low frequency having about a 25% chance of seizures. This is under 5%. The other one is it doesn't obscure what I'm looking at. You'll see the example of a stimulus in the eye or in the eye muscles, and we're not getting that obscured by stimulation artifact, because it's a short pulse. Our average responses when we're extradorally are between 8 milliamps to 25 milliamps, although I usually see them between 8 and 10 milliamps, but I have seen them as high as 25. Intradorally, we're about 1 to 5 milliamps when we're on the region of interest. When it comes to using monopolar versus bipolar stimulation, there's this paper from Phillips and Porter in 1962 that I like to reference when talking about it. I like to use monopolar, as I said, anodic stimulation for the surface of the head, or monopolar cathodic for when I'm inside looking at the cortical spinal tract. That has to do with the threshold of activation. We can see that the anodic stimulator down here, its threshold of activation is close to 1 milliamp on average when you're sitting directly over a region of interest, whereas when you use bipolar, depending upon where you are, if you're on the side lobe, so one of your bipolar tips is at the center of interest, it's about a half a milliamp higher, whereas if your bipolar is in the center, it can be a whole milliamp higher. Once again, since we are trying to minimize seizures, anything that will reduce the amount of energy that we need to deliver the brain, I think, is better for looking at that. Here's an example of what we do in the operating room. We can see over here that we have all of our muscles. We monitor down from the face all the way down to the foot when we're looking at this over here. On this first response, we can see that we get activity over here in the face muscle. You can see that I have this artifact of the pulses showing up over there. That's the train of five stimuli that I use, whereas if I were to use a continual stimulus using the low frequency, that artifact would continue along the trace, and I'd have to try to pick up my muscle response inside that artifact. You can see it's much easier to see that muscle response there. It tells us we're probably stimulating somewhere in this area. In reality, the surgeon knows where the stimulus is going on the brain, and I as a physiologist know what the response is, so there has to be a lot of communication going back and forth. This isn't like the standard IOM procedures where we're monitoring and looking for after the problem has occurred. We're actually highly interactive here. Actually, one of the neurosurgeons that I know, Mark Sindel, coined the term interventional neurophysiology for these because we're helping to form the surgical plan as we go along. Over here, we get a response in the bicep and tricep muscle, and they're focal responses. I'm not getting muscle responses in anywhere else, so I'm probably stimulating up in this area over here. Over here, I get a focal response in the hand region, so I'm stimulating probably down around that area. Now up here, I get the whole arm to activate, so I'm probably stimulating with a little bit more current. Anywhere inside this purple circle is where I can get that response. You can see I lose focality when I'm doing that in that particular way. Over here, I'm getting everything from face all the way down to hand, so I'm stimulating anywhere in this blue region. You can see that you lose focality, and even I can stimulate on part of the sensory strip, and still I can get motor responses if I stimulate too high. It really is important to look for your thresholds to be able to figure out where you are, and knowing that your thresholds when you're on the motor strip should be between 1 and 5 milliamps. If you have to go stimulate much higher than that, you may not be stimulating on the motor strip. We can talk about, as I said, there's a cortical versus subcortical motor mapping. Cortical is anodic stimulation, because what we're stimulating, we're stimulating along the axis of the Betz cell, on layer 5 of the cortex. When we stimulate along the axis, anodic stimulation tends to give a better response than cathodic stimulation. It would take eight times more cathodic stimulation to get that type of response. We can use either a strip or a probe, and we're going to talk about a paper that talks about a strip or a probe, because the next step from mapping is to monitor. Where mapping requires a surgical intervention of putting the probe on, we can monitor these procedures while the surgeon is going on by laying something over it. Subcortically, it's cathodal stimulation, and we do use a monopolar probe, and that's because we're placed perpendicular to the axon, and that's where cathodal does better. As I say for this short, I use three to nine pulses, an interstimulus interval of two to four milliseconds, which is about two to 400 hertz, and a pulse width of 500 microseconds, preferably on those, and anywhere from one to 15 milliamps is our response that we're looking at. Now, as I said before, the next level after mapping of a procedure is looking at the monitoring of the technique, and monitoring really is the continual interrogation and recording of the system. So it takes place during the surgical resection. It does not require any specific intervention from the surgeon once the strip is placed. So while the surgery is occurring, we can be continually monitoring the motor system while the surgery is going on without having to require any input from the surgeon, and so if something occurs, we can jump in and say that there's a problem. Now, depending upon the stimulation amplitude, the cortex may be missed. If you stimulate too high, if you're stimulating at a large level, you're going to jump below and eventually stimulate parts of the cortical spinal tract directly. Mapping is an intermittent interrogation of the system. It only takes place at fixed surgical points. It requires surgical intervention, and it's really used to localize motor cortex and subcortical motor tracts. So here's a paper from 2013 that really looked at the criteria for mapping and monitoring in the brain, and they wanted to correlate monitoring and mapping techniques with new postoperative motor deficits. They used a mapping technique, which was subcortical monopolar stimulation, and they used a monitoring technique, which was direct cortical stimulation using motor-evoked potential responses. They did 100 patients with intrinsic brain tumors, metastatic lesions, and cavernous malformations. Now, originally on DTI fiber tracking, they picked a point that was 10 millimeters from the cortical spinal tract, and that was going to be their initial safety point of what they were going to look at. And then stimulation testing was going to go and do a little bit. They included only patients where the strip electrode could be placed on the presensual gyrus. So the criteria for alarm, because that's important when you're doing these procedures, how do you know when you're getting too close to an eloquent area? So for a motor-evoked potential, that's when you did the strip directly on the cortex, any sudden increase of 4 milliamps to obtain a response. So what they would do is they would test and get a threshold for a motor response, and then say that was 4 milliamps. If during the surgery that response jumped to 8 milliamps or higher, that would be an alarm criteria, and when that alarm criteria was met, the surgical response was suspension of tissue excision, removal of retractor if it was used, increasing cerebral perfusion, and normalizing anesthesia if it had changed. Now for the subcortical mapping motor threshold, they would start at 1 milliamp and slowly increase the stimulus up to 22 milliamps. If they didn't get anything, they were safe. They were far enough away. In general, it's believed that when you stimulate the subcortical looking at cortical spinal tract, it's about 1 milliamp per 1 millimeter. That's the general distance to stimulation level ratio that you're looking at. At 10 milliamps, the testing was started to be performed every 2 millimeters, so they wouldn't perform it every 2 millimeters before that. Surgery was stopped when motor's threshold reached 3 milliamps or below. If alterations in MEP were noted, surgery was stopped immediately, and that'll be important when we look at some of the results of this study. Gross total resection was in 71% of patients, subtotal resection in the remaining 25%. In 68% of those subtotal resection patients, surgery was stopped based on the physiology. Either the criteria of 3 milliamps on the mapping criteria met or an MEP alarm had occurred. In the rest of the surgery, the surgery was stopped based on neuronavigation as preoperatively planned, and that's possibly because there are other areas of brain, such as the thalamus or the basal ganglia, that we can't monitor with the MEP motor mapping technique. We can take a look at some of the results. We can see if we were within 1 to 3 milliamps of stimulus intensity. Post-op day 1, a decent amount of patients had injury, but only about 10 patients had some type of injury, or 10% of patients had a permanent deficit, the closer you were. When you got to 4 to 5 milliamps, none of the patients had a permanent deficit. At 6 to 10 milliamps, there was a low percentage of patients, about 5% of patients or 4% of patients had a motor deficit over there. One of the things that they discussed in the paper was these patients had occurred, the alarm had occurred from the motor evoked potential mapping, and so they stimulated at 6 to 10 milliamps, and they started to do their resection, and then all of a sudden the MEP criteria was met of 4 milliamps or a loss, but they never went back and re-stimulated to see how close to the cortical spinal tract they were. So the thought was that they had brought up in the paper was that these probably fit down in these groups over here, they just don't know exactly what those thresholds were. And then when you get much further out, there are no permanent deficits after surgery. When you look at the results for the motor evoked potentials, it's even greater to see. If there were normal or no changes in the motor evoked potentials, no patients at discharge had motor deficit. If there was a reversible or unspecific change after one of the interventions had occurred, then no patient had a permanent motor deficit. It was only during irreversible or signal loss situations that there were permanent motor deficits. And actually with the signal loss, you can see that 3 out of the 4 patients had a permanent motor deficit when the MEP was gone, and that's because the cortex was probably damaged or the cortical spinal tract fibers were damaged there. So in conclusion, the SSCP phase reversal helps in reducing the amount of time needed for motor strip mapping and thus reducing the chances of seizures due to excess stimulation. Using MEP monitoring and mapping can be helpful in reducing postoperative neurologic deficits, and all of these techniques are not independent. They should be used together and interpreted similarly. So thank you. applause

somatosensory evoked potentials

motor evoked potentials

brain mapping procedures

interoperative neuromonitoring

precise localization

motor mapping