Speaker: Chris Martin, Surgical Neurophysiologist (10:03) speaks at TxANA Conference Part II
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:
This slide, it’s the circle of communication. It’s actually been gratifying – as I’ve met some of you at our booth – to hear how important communication is for you, between neurophysiologists, and you, and also the surgeon as the third element of that. So really, all three teams in the operating room with, obviously, the patient being the center of our focus for having this circle of communication.
We’ll see this slide a couple of times in the talk. Things like, “Does the surgeon want pre-incision baselines or pre-positioning baselines” – that sort of thing – “What actually is at risk? What structures are at risk from that operation? And can we put a signal through them to monitor them? If so, what modalities does that mean we’re going to be using given those modalities, and what effect are the drugs that you are going to be giving going to have on those modalities?” It really is a circular communication that needs to be all done ahead of time before we’re flailing around trying to figure it out at the last minute.
A really good example of this complete circle of communication is a place that I’ve worked at where in certain tight cervical cases the surgeon wants post-induction but pre-intubation motor baselines. That takes a real world class level of cooperation between all three of these people and these teams. In fact, what they’ll do is they’ll induce mask and ventilate, put some bite blocks in really quickly. The neurophysiologist will have his motor stimulators ready to go, fire off a quick set, take the bite blocks out, do their intubation, and then they’ll reset up and get some post-intubation baselines. This is even in the context of using a glide scope or fiber optic where there’s not that much extension, but still that’s what the surgeon wanted. So, something like that even is possible if this circle is just really well carried through.
How do you know if your case is going to have neuromonitoring in it? Maybe on the posting sheet or the schedule you may see any number of these acronyms, IONM, neuromonitoring, just intraoperative monitoring. It may just say evoked potentials, which doesn’t necessarily mean that’s what the modality is that’s going to be used, but that’s just sort of the way they think about it, spinal cord monitoring, any number of these. It may say the company’s name; it may just say with Neuro Alert or something like that. You want to try to take a look for these sorts of things before your case starts, so you have an awareness that you’re maybe going to have some neuromonitoring going on, or maybe the first time you find out about it is when one of us rolls in with a cart full of cables and wires and stuff like that. In order for that circle of communication to work you need to know ahead of time whether you’re going to have neuromonitoring for your case.
If you are going to have it what does that mean? Our society, the ASNM, conveniently enough has published a definition of what is neuromonitoring. I want to just highlight some of it: “Any measure employed to assess the functional integrity of the central or peripheral nervous system” is one part of this definition. Central and peripheral nervous system is going to lead us into therefore I just want to do a quick review of anatomy since it mentions that in the definition. I have that broken down into the brain, the sensory pathways, motor pathways, and a little bit on the auditory pathways. These are the common neural structures that we can monitor if they’re going to be at risk, either directly or indirectly.
So, anatomically in the cortex, what are we looking at? It’s basically populations of all the cells in the cortex are contributing some sort of charge input. As I mentioned earlier, a negative deflection of overall charge will cause an upwards deflection and overall positive will be downwards. Pyramidal cells tend to be the largest cells, and they tend to fire synchronously so most of the input that our EEG is recording is going to be generated from the excitatory and inhibitory post-synaptic potentials of groups, populations of these pyramidal cells and the various layers of the cortex. That’s where the electrical charge is. It’s just a passive brain beat. The activity the brain’s passively making all the time. We’re just sitting, recording electrodes over it, and listening to it. It’s nothing that we’re provoking or stimulating or anything like that. This is picking up on Berger, but we have multiple channels covering different areas of the cortex. It’s a little more comprehensive than his earlier stuff, but that’s what an output of all those EPSPs and IPSPs would look like over time.
For the sensory anatomy, the arrow is moving up so let’s start from the bottom. Some sort of sensation stimulation at the nerve endings will cause an ascending volley to go up, and this is not pain. This is not C fibers or temperature, this is more proprioception, two point discrimination, light touch, stereognosis. Those sorts of sensory inputs are what we’re monitoring with our SSEPs. We could sit under the table and just tap their wrist for the entire case and that would provoke an evoked response, but that’s not really feasible to do so we use electrical current instead and deliver a pulse at a peripheral nerve. So, then that goes in through a plexus. If it’s from a lower extremity nerve, the lumbosacral plexus or the brachial plexus, in through the nerve roots and dorsal root gang, where it goes into the fasciculi, the gracilis and cuneatus, so legs kind of midline here in the dorsal column and arm area, more out lateral. Up the dorsal column and to the medulla where it’s nuclei then decussate over, these tracks up through here into the internal capsule are called the medial lemniscus. It radiates out to different areas of primary sensory cortex. That’s the basic pathway from site of stimulation into the cord and up to the brain.
The decussation is important from a technical point of view, and we’ll look at this a little bit later on, but if we wanted to record an evoked potential off of the left brain we would stimulate the right wrist. On the primary sensory cortex, once it gets up to the level of the brain then the cortex is laid out somatotopically where there’s proportional representation of different parts of the body along that post-central gyrus, the green one here. Yellow is rolandic fissure there. Here’s the homunculus, our friend homunculus, with the proportional representation of very much in the hands, a lot in the face, tongue, and lips area. These are the sensory parts of the body that are most represented on the sensory strip, and surprisingly not so much down in there.
From the motor point of view it’s kind of the opposite direction. It’s starting at primary motor strip here, precentral gyrus, heading down through radiations again through the internal capsule, decussating over to the other side again, and going down through the corticospinal tracts now in the spinal cord, so different area in the cord than the fasciculi coming up from the sensory.
The corticospinal tract into the nuclei out through the alpha motor neuron and finally down to the neuromuscular junction where you could record something off of the muscle distally to that. The corticospinal tracts are here in green. Dorsal column is here, gracilis and cuneatis. You can see that they’re more lateral and somewhat anterior as well. It’s a completely different set of pathways in the spinal cord even though it’s a pretty tight space. They’re definitely discreet areas.
I want to take a second and talk about the vascular supply of the spinal cord too. Here’s the dorsal column. It gets two posterior spinal arteries for profusion whereas the front, the ventral column, only gets one, the anterior spinal artery or artery of Adamkiewicz lower down. There definitely is a possibility for ischemia to one area of the cord versus the other. It’s not that you’re going to infarct your entire cord necessarily. If you nick the ASA you could really only knockout some of the corticospinal stuff and the dorsal column could be intact from the two redundant or a posterior spinal arteries.
Quickly on the auditory path anatomy, starting at the cochlea. The signal, it goes into the distal eighth nerve into the cochlear and nucleus through the superior olive up to the inferior colliculus and then the medial geniculate and then ultimately out to the auditory cortex in the temporal lobe. Each one of these anatomic locations generates its own unique response to any kind of auditory stimulation. The way we would do this in the operating room was to present a clicking sound and then we would record the response that each of these generators makes. This is the general anatomy for how we would do the brain stem evoked potentials.
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