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Chris Martin Speaks at TxANA Part III: EEGs and SSEPs

Posted: Dec 29, 2016 11:50:09 AM

Speaker: Chris Martin, Surgical Neurophysiologist (11:03) speaks at TxANA Conference Part III

In August, 2016, Neuro Alert exhibited at the TxANA Annual Convention & Trade Show. Here, Chris Martin, one of Neuro Alert's Senior Surgical Neurophysiologists, was asked to present "Introduction to Intraoperative Neurophysiology" and "Intraoperative Monitoring Applications and Considerations." In this six-part presentation, Martin walks us through the origins of Intraoperative Monitoring to present-day applications.

Transcripts to follow:

Chris Martin, CNIM, Surgical Neurophysiologist:

Going back to that definition from the ASNM. We've gone over the central and peripheral nervous system part of the anatomy, but let's go on to talk about signals monitored continuously. Detection of adverse changes enables corrective action and so on, but monitored continuously.; So what does that mean? Now we're getting into more of the physiology and the modalities that are involved in actually utilizing that anatomy to record signals continuously during an operation.

There are a whole bunch of different modalities that are possible for us to run, and they can be broken down into two different categories. The spontaneous ones, which like EEG are just happening, the body is generating these signals spontaneously. We don't provoke or do anything to elicit them. Electrocorticography, which is an EEG recorded directly from the surface of the brain, and EMG, any kind of muscle activity can be spontaneous. EMG can also be stimulated, and this is the other category. These are responses that would only occur subsequent to some sort of external stimulation, causing them to occur, and that's EMG in the sense of a triggered EMG instead of a spontaneous one. Somatosensory evoked potentials, motors, brain stem evoked potentials and some of these other one which are less common. We'll just look particularly today at EEG, EMGs, Sensory evoked potentials and the transcranial motors.

Let's go through some of those modalities then. We've seen this electroencephalography again. What's the definition for that? As we said, it's signals originating from synaptic inaction potentials, which can be observed using electrodes placed in or on the scalp. Again, EEG is recording of that signal from the scalp. Corticography is recording directly from the surface of the brain with a strip that's placed by the surgeon. So using the filters that we have available, we'll see different patterns in the EEG. What you can see here is that there are a mix of different frequencies. It's hard to appreciate, but there are one second delineations, so that this entire window is ten seconds.

You have something like this, which is kind of a slower response, and then there's some overlying quick activity to it. In fact EEG is analyzed based on its frequencies and it's broken down into four different frequencies, delta being anything that's four Hertz or slower, theta being four to seven Hertz, alpha being eight to fourteen, around there and beta being faster than fourteen.

What we're doing to analyze with EEG is what are the constituent frequencies? Are they symmetric? This is left hemisphere and right hemisphere. Is each side of the brain generating about the same amount of frequencies? Are they continuous? This is a good continuous EEG. Drugs that you give can cause discontinuity and reverse suppression. These are some of the things that we're going to be looking at when we're doing electroencephalography.

This is just showing why we put the electrodes where we put them. There is a system of standardized measurements on the brain cell that the same patient can be recorded in a standardized way by different people using anatomic landmarks. It's called the ten-twenty system. Using these measurements, you can create a similar placement even if different people are doing the EEG on a patient.

This guy here in the operating room, he's skipping every other one so it's kind of a double distanced array, but this is what it's based on, so this is what you'll see at the head, when EEG is used as a modality.

Sensory evoked potentials - again the ascending dorsal column sensory pathway. What's a definition for that? A recording of volume conducted events, usually from the skin's surface of the ascending pathway of the nervous system. Dorsal column and medial lemniscal after it decussates, activated usually by electrical stimulation at the wrist and ankle you have good access to very superficial nerves at the wrist and ankle peripherally, so that's a good site to stimulate. It allows you to get a good strong beginning to the signal so that you can record it.

Each part along the pathway you record a deflection which represents a discrete anatomical site for that response. Here's an example. This is a median nerve that's been stimulated and its different deflections are broken down into three separate channels. The lowest one being a peripheral response, brachial plexus. Then a sub-cortical response, cervical medullary junction which is recorded from one of various non-cephalic electrodes, and finally a cortical response, sort of thalamic cortical radiations and primary sensory cortex is generating this bump here.

You can see that the stimulation time is at time zero. It has taken twenty milliseconds for that stimulation to get through the brachial plexus into the cervical medullary junction and up to the brain. It is therefore labeled, and twenty response because it's a negative of going deflection at twenty milliseconds and that is based on normative values for massive populations, that is the nomenclature N20.

That doesn't mean that everyone's N20 is at twenty milliseconds. But that's just what that response is called. Patients serve as their own control. As I was saying, you can have an abnormal patient that could have a neuropathy or any kind of other problem that's causing their N20, it's hard to appreciate here, but maybe their N20 is occurring at twenty-two or twenty-four milliseconds, so in absolute terms, it's abnormal by ten or twenty percent, but we don't really care about that, compared to what normative values are in a general population. We just want to know, does it stay the same throughout that case, so they are their own control. We'll take that abnormal reading into account and just say does it deviate any further from there. That's important to differentiate.

Recording sites. Again for the cortical recordings it's going to be at various areas of the scalp and this is going to be based again on the homunculus that we saw. If we're stimulating at the wrist, the wrist representation is maximal is in this area here so this is where we're going to sit an electrode so that we have it as close to that anatomic generator as we can to give us the best chance of recording that response. It's a very tiny electrical response. You see that here, and again it's based on some standardized ways of measuring using anatomic landmarks, you end up with SSEP electrodes here, here and here. If you were to overlay them with the primary sensory strip underneath, you can see that they're essentially sitting right over hand on both sides and then in the midline here.

This one is good for recording the stimulation from the ankle. That's the generator and the sensory strip is deep in the convexity so you can't really sink a depth electrode down there to get closer to that generator, but you can sit one at the vertex and catch it as it's coming up. Those are referenced to a non-cephalic lead that doesn't really have any contribution from the sensory strip into it.

That subcortical response that we saw can be really generated from any one of these areas. That's going to depend on what kind of surgery it is. If you have a posterior cervical case going on, probably any of these cervical levels are not going to be okay to use. They're going to be in the surgical field, so you're doing to have to move that electrode either to the mastoid, the chin, front of the neck, somewhere like that. Any of these spots you might see an electrode placed there for the subcortical response.

Here is the sensory evoked potentials. It's a little hard to appreciate, but he's cleaning up using some kind of filtration to clean up the signals a little bit but what I want you to look at is when he starts a new set of SSEPs, which he's going to do just now, what you've got, if you can see that, was huge deflections that were essentially saturating the entire screen and didn't look like any kind of responses at all. Now after a minute or so, thirty seconds, now you've started to see these SSEP responses resolve out.

The reason that happens is because as I said these are very tiny on the order of micro volt recordings, so any electrical activity that's present is going to dwarf those tiny little signals, so even the EEG that the brain is making, the electrical activity from the heart, certainly environmental electrical noise, are orders of magnitude larger than the tiny evoked potentials we're trying to record. The good news about that is that all that extraneous outside electrical activity tends to be random in nature. So with one recording, it's generally positive, the next recording it's generally negative. Since it's random, it's going to tend to cancel itself out over time. The SSEPs that we're recording are time-locked and occur at the same time after stimulation every time.

If you mathematically average several of these or a hundred of these together, the random outside noise is going to cancel itself out and the mathematically time-locked noise is going to build up out of that background. It does take a little bit of time sometimes to resolve out these evoked potentials, and if the surgeon says, "Hey. How are the SSEPs?" And you've just started a test and you've got all this noise in there, you can't give them an immediate feedback. We work really hard to keep our signal to noise ratio as good as possible to really make it easier to resolve these out as fast as possible.

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