Post-auricular Muscle Artifact In BAER

This is a question that I got about post-auricular muscle artifacts on intraoperative brainstem auditory responses. This is one of those responses that I don’t have any of my own images because I’ve never seen it live. This is because I have never placed my A1 or A2 recording electrodes behind the ear, which is the cause of the artifact overlapping the middle latency responses.

I doubt many people have been fooled by this in the OR, so it isn’t something that gets a lot of attention when training someone on brainstem auditory evoked potentials. But it does end up making its rounds on questions on the CNIM and DABNM examinations.

But before I go into a more detailed explanation, let me help direct others who might have similar questions.

The Neuromonitoring Forum

One area of the website that I would encourage readers of this blog to spend some time and effort on is the neuromonitoring forum. I think it opens up a lot more possibilities than someone just asking me for a one-on-one response to a question. I answered the question (see below), but maybe there is another angle to cover that I didn’t consider. That’s where the neuromonitoring forum comes in. The way I see it, the forum can serve 3 major purposes.

• Provide answers to those who have questions. Throughout my entire career, I’ve always made sure to work for a company that had people on staff who are leaders in the field. The easiest way to fast track your own learning is by gaining knowledge from others. I understand that not everyone has been as fortunate. For those without that perk, the forum is the place to be. Ask questions and follow up.
• Give a platform for leaders to emerge. It takes some guts to stick your neck out there just to help others. But trust me, people are looking. This is a great opportunity to help yourself by helping others first.
• Join the club. The IONM community is pretty small. Less than 3700 CNIM and 200 DABNM as I write this. You can become acquaintances and friends pretty fast with a good percentage of the field. This is an excellent place to network.

I get questions all the time from people I’ve met along the way. Phone calls, texts, emails, live chats, etc. I’ve tried my best to give a complete and correct answer every time. But it is a lot of effort, so I’d like to leverage the content a little bit for all to see. If you do ask me a question, chances are I will ask you to put it on the forum, or it will land here as a blog post.

That way you have a better chance of getting a better answer. Plus, someone else is or will be wondering the same thing.

Here’s a question that I had sitting in my archives:

The Question

Feel free to share this with others in our group.

I was reading about ABR and came across the topic: Post-auricular Myogenic response and Middle latency potential.

The latency that we would expect to see them: Postauricular – 13/20 msec Middle auditory – 12/50 msec

Here are my questions: 1. What is the very large amplitude potential following the ABR waves I-V at approximately 15ms? Post-auricular myogenic artifact or Middle latency potential?

Usually, the post-auricular myogenic artifact or post-auricular muscle activity may appear toward the end of the ABR waveform if an analysis time of 15msec or longer is used.

2. When should I expect to find that response? Between 13-20msec range?

3. Does it appear on both sides or just on the ipsilateral side?

4. Are both of these responses from the upper brainstem?

5. How can I differentiate between a middle auditory response and a post-auricular myogenic artifact?

My response:

In the OR, we only monitor short latency ABR responses, the wave I, III, and V that you are used to seeing. The reason that you’re not used to even seeing these on the screen in the case you’ve been on is that we use a pretty short time base. I start with a 15 ms window and place my markers. It can also be useful to change the time base to 10ms total to better see subtle shifts in latency. You should start with 15 ms at least just to verify that you are marking the correct waves.

These middle latency potentials are upper brainstem/reticular/thalamic potentials that are unreliable in the OR but are used by an audiologist in clinical settings.

Post-auricular Muscle Artifact

The post-auricular muscle (or PAM) can cause a post-auricular myogenic response (also called post-auricular muscle artifact, or PAMR), which is an artifact picked up by the recording electrodes (in additions to these middle latency cortical potentials) from muscle activity that distorts the amplitudes and latencies of these middle latency potentials, as well as the short-latency response we are analyzing.

They are seen when you place your recording electrode behind the ear or near the neck, which is why we put our recording electrodes pre-auricular for all BAER cases. The 2 places that I’ve had success is just anterior to the tragus and just below the tragus near the ear lobe insertion. I typically start anterior to the tragus, mostly because this seems to be a more common electrode placement. If I am not satisfied with the wave I collection from the Ai-Cz montage, I may test out the second position. I do this because wave I of the brainstem auditory evoked potentials are near-field responses. This will sometimes reproduce a better wave I response without any post-auricular muscle artifact.

The PAM artifact is known for its large amplitudes and to be intermittent (since it depends on the muscle contraction to cause the artifact, which could be ipsilateral, contralateral, or bilateral).

On the CNIM or DABNM written exam, if there was a picture of ABRs with huge amplitudes (you can tell by looking at the display gain used on the picture, or the size of the waves I-V, which are very small (0.2-0.5uV), I would be thinking PAM even before I looked at the answers to choose from. The latencies you stated in the question are good ranges to know for the artifact and the middle latency responses.

To make sure that you understand the information and can put it to practical use, please answer the question below.

Guess which one of the 2 traces has the muscle artifact…

?

?PRO TIP – Beyond what’s listed above, I don’t think I would spend too much time on middle or late-latency ABRs. Your time studying for the CNIM or DABNM test would be better suited elsewhere.

OK… one last question:

In that same image (the trace without the PAM), what can you comment on about the absolute latencies of Wave II, III, and V compared to what you might see in an acoustic neuroma case in the OR, and what might you attribute this to? Please leave your responses in the comments section.