Avoid These Peripheral Nerve Monitoring Mistakes...

In peripheral and cranial nerve neuromonitoring cases , we are often asked to monitor those specific nerves and corresponding end organs. Many surgeons are using us for information in regards to EMG activity signifying?nerve root injury or irritation. Others are using us for the identification of the nerve during exposure.

While there is merit to using us for that purpose, I believe some may be doing so without considering the following:

1. Free-run EMG activity is due to during and/or after the fact trauma, which may be too late to prevent injury when using sharp surgical instruments
2. Identification of the nerve does not tell you about the ability of the nerve to conduct through the entire length in question.

Both?are?known shortcomings.

To really know the functional integrity of the cranial or peripheral nerve, you need to stimulate along the nerve proximal and distal to the operative site.

Peripheral and Cranial Nerve Neuromonitoring

Here’s an example:

You are monitoring free-run EMGs of bilateral recurrent laryngeal nerve in a thyroid cancer case and the surgeon has successfully stimulated and identified the nerve. However, the cancer is intrusive to the nerve, and identifying cancer vs nerve is difficult.

During surgery, the surgeon looks up at you panicked, and asked how things look. You tell him that EMG has been “quiet.” He stimulates the nerve and you let him know that you have a good response.

He sighs in relief and decides to continue to the other side based on the following facts:

1. He knows that he has removed enough of the tumor on the first side
2. The nerve did not show a typical response to irritation/injury
3. He thinks he has an intact nerve according to your simulation
4. He visualizes what he thinks is a?nerve

But on the other side, his luck runs out. The nerve is encompassed in the tumor, and every time he tries to peel it off, train EMG sounds off. But since the first side is intact (or so he’s assuming with the peripheral nerve stimulation forming a proper compound muscle action potential), he feels a more aggressive approach is warranted to take as much tumor as possible. During the tumor removal, train EMG runs throughout, and then he’s finally done.

You’ve told him about the EMG, but it’s his decision to continue taking out this tumor.

And then… Yes! The train EMG has disappeared. The surgeon stimulates the second nerve again and it gives a proper CMAP.

Success!

The case got a little hairy there for a second. And as you wait for the patient to wake up, you agree with the surgeon that there would be no real surprise if the patient wakes up with vocal paresis because you utilized cranial nerve neuromonitoring. That one side did show signs of irritation, so having a temporary weakness isn't out of the question.

But you both are surprised when the patient has complete vocal paralysis and the follow-up studies show that it was the first side with the complete nerve injury, not the side with the train EMG.

Oops.

And since he only stimulated distal to the lesion on the second side, he doesn’t know the extent of the integrity of that side's recurrent laryngeal nerve. Or if there will be enough regeneration of the nerve to restore motor function to the vocal muscles.

Scratching your head, you figure out that today wasn’t the shining moment in your IOM career.

Here’s what you had to offer:

1. False negative EMG on the first side, since the nerve was cut and you didn’t get any activity.
2. False positive tEMG on the first and second sides (ok, debate if you want to label this as a real false positive or not since he didn’t stimulate the whole nerve), since the surgeon only stimulated distal to the lesion to get a response.
3. True positive EMG on the second side, since the patient woke up with an injury on that side.

Even though your EMG was correct on the second side, it missed the severing of the nerve on the first side. And in this case, we aren’t sure as to what extent it helped in preventing injury. For the second side, we’ll never know if it was the initial insult, the second, or a compounding effect. I'm guessing I'm not the only one to notice nerves becoming more reactive to manipulation after the first significant train activity. EMG does a poor job at qualifying the extent of the injury.

You also realize that the disappearance of train EMG and stimulating the portion of the nerve distal to the operative site offers poor clinical data.

What’s The Peripheral and Cranial Nerve Monitoring Lesson Learned?

It’s very likely that the surgeon severed the nerve on the first side, and there was not enough activity for the EMG to pick up a response (reports of this are seen in the literature as well). So if the surgeon stimulated the proximal and distal nerve on the first side, he would have realized that the nerve was severed completely.

The surgeon might not have risked injury to the other nerve since vocal paresis is not as detrimental as vocal paralysis. He might have second-guessed his opinion to aggressively remove the tumor on the second side with a severed nerve on the other and train EMG?on the second side. But again, this is all hypothetical.

Another option is to use the probe during exposure and tumor debulking to keep a good idea of where the nerve is located and what impact the surrounding tissue might have on the nerve. Some areas might stick to the nerve, adding caution not only to the dissection of that portion of the nerve but also to potential tethering. In these surgeries, it's actually traction injuries that are more common than direct nerve injury from a surgical tool.

While I made this whole scenario up, it’s not far-fetched to find yourself in this very predicament. The best way to handle something like this is to prevent it before it happens in the first place.

1. Let the surgeon know what the best stimulating techniques are for identifying a cranial nerve.
2. Talk during the surgery so you can better interpret your free-running EMG by knowing what the surgeon is doing.
3. Inform the surgeon of the strengths and weaknesses of your modalities.
4. Be better than the automated system.

Anyone else ever have a case similar while performing cranial nerve monitoring? What was done differently, or the same?