Speaker: Chris Martin, Surgical Neurophysiologist (6:26) speaks at TxANA Conference Part V
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:
Using some other modalities, here is another case: spinal fusion T-10 to sacrum, the whole lumbar spine. This patient had 10 previous surgeries, so there was a lot of old hardware in there that the surgeon was going to remove and replace. Patient symptoms were back pain and leg pain with the left leg worse than the right.
Here is the baseline somatosensory evoked potentials from the ulnar nerve in the arms and from the posterior tibial nerve in the legs, left side and right side. Good subcortical responses and cortical responses all the way through. This looks okay, good baselines. The way to read this is that the blue line is the base line in all these cases, and the purple one is the one that's currently being recorded. So we're continually comparing purple to what we recorded before there was any surgical manipulation whatsoever. Again, this is an example of using the patient as their own control.
We're doing motor evoked potentials in this case, so here's the baselines for that. Again, the blue is the baseline recordings, and the purple is the real-time recording. This is from muscles in the hand, the rectus abdominis, iliopsoas, quadriceps, tibialis anterior, gastroc, something in that foot, the hallucis probably, and then they had anal sphincter. It's not so unequivocal that there's a good response here, but there is all the way up from the foot up to hand. The hand is used as a control. Obviously it's not going to be affected by something they're doing in the low lumbar sacral area, but it helps us figure out if we do see changes from that area, we can ask if it's also changing in the hand, is it generalized, or is it specific to the site of surgery?
So here is the course of surgery: the baseline pressure mean was (MAP) 69, 0.8% Sevo, 150 micrograms of propofol. The EMG became valid at 10:40 a.m., so what does that mean? That's because rock was given with induction and so the train of four didn't come all the way back to four out of four until 10:40 a.m. Some people will say, "Well I have two twitches, that's fine for you to record EMG." That's not correct. You can have even with three twitches still 75% of the neuromuscular junction blocked. It's just a variable that introduces an element of uncertainty into whether your EMG is really going to detect any kind of irritation. We look for a good four out of four, and that's what we document and annotate in our log is that the EMG's valid when there's four out of four twitches. That's what that is indicating there.
A surgeon starts taking out all the old hardware. The mean is now up to 94. At 11:52 a.m., the SSEP from the left posterior tibial nerve cortical and transcortical signals begins to decrease in amplitude, which we can see here - blue being pre-surgery manipulation and purple being a major change - it's almost gone. It's still okay on the right leg, and it's still okay on the bilateral hands.
We figured it's not a technical issue, so we're still delivering current. All of our electrodes are in place, and they haven't fallen out yet. We told the surgeon that we have this unilateral single extremity change on the left leg. He said, "Run a motor." Here are the motors. We did a series of them, a train of them. What was here is that the baseline is now gone. What was here is now gone. The foot is gone and that's a bilateral, so gastroc and foot have now been lost in the motor evoked potentials, even though the sensory change was only on the left leg. Again, we're troubleshooting this. All of this is happening at the same time. We're asking you guys if anything has changed, which it hadn't. You have a good mean that's adequate for a spinal cord profusion, propofol has been stable, there is narcotic which is great. That doesn't have any effect, Sevo's okay. We jacked up the stimulation to 800 volts to see if we could drive something through there and get that gastroc and foot back, but still nothing.
We're pretty convinced this is a real surgical event at this point, but the surgeon didn't agree. He thought it was something technical and wanted to conduct a Stagnara wake-up test, so now we've got this potential catastrophe of having to do some wake-up test where the patient wasn't prepared ahead of time that this might be the case. It's really becoming sort of an emergent situation potentially.
His assistant, though, said, "You know what? Let's explore the field a little bit before we go through this wake-up test." He explored the area a little bit and then said, "Okay, can you run some motors again?" There they are again, they're back. The foot and the gastroc, there's a little bit in there too. Now the motors have come back after this exploration, and we didn't have to do this wake-up test.
How about the sensory evoked potentials? By 1:00 p.m., the left tibial has returned to the nice, big, strong, robust responses again. That is basically back to baseline.
So what is happening here is that as he was removing the hardware, one of the facet joints actually became lodged down into the canal and was compressing the cord 30%. He said, "It wasn't easily visible," and he wouldn't have done that exploration and found that and fixed it if we hadn't had our signals changing. This is a solid catch that actually saved the patient from having some really serious post-op sequela. We convinced the surgeon that it was true too and he actually found what he had done.