BAER (Brainstem Auditory Evoked Potentials) During Microvascular Decompression Surgery
You might remember when I was complaining about using ABR in the operating room and how to adjust the click polarity to help obtain a more reliable BAER. But my first gripe, having a big honking tumor pressing up against the nerve, isn’t a problem in MVD surgery. The other problems revolving around the waves being small and susceptible to noise from a hostile electrical environment (the operating room) and larger physiologic responses (EEG).
But there is a new paper out by Joo et. al. that might help address this problem.
The Search To Make BAER Better
Because of the poor signal-to-noise ratio of BAER, we have been forced to collect thousands of sweeps in order to get a reliable response. During that collection period, even small disturbances could severely distort the responses collected up to that point. So not only is there a long collection period, clearing out traces and starting over isn’t something uncommon.
The Rep Rate
So faster collection rates would be more favorable. Joo looked at the collection of BAER using rep rates of 10 to 100 Hz, going up by 10 Hz. They looked for the sweet spot where there was still a reliable wave V collected as fast as possible. They found that between 40-50 Hz had reliable waves without surgical artifact. They then came up with 43.9 Hz as their “go to” rate to better avoid overlap with common noise frequencies.
They then looked at the necessary number of sweeps in order to record a reliable response. They found that 400 traces was as reliable as 1000 traces. Some quick math (400/43.9) shows that they were able to collect a reliable BAER in 9.11 seconds, or about 6 every minute.
What’s The Up Side?
Reducing time to collect is always a good thing. I can think of 4 ways this is very helpful:
- You’re able to identify problems faster
- You’re able to verify that it isn’t technical faster
- You’re better able to collect in between use of the bovie
- You’re less likely to have to clear out a trace and start all over
What’s The Catch?
But there has to be something more to the story, right? I mean who hasn’t already considered just turning up the rep rate for faster collection?
The last part of their study was to compare amplitude change in wave V > 50% vs. changes in interpeak latency of > 1.0 ms. Their outcomes showed that it was only the change in wave V that was useful, although a shift and latency may also be seen.
That changes things a little bit. If only amplitude change of wave V is significant, then we might elect to give up good looking waves I and III, as well as appropriate interpeak latencies in order to have a reliable wave V at a collection speed that allows for faster observation of changes. Most people are hesitant to go above a rep rate of 20 Hz because of it can distort waveform, but staying below 50 Hz is not a new suggestion either.
There are some points that need to be made about this paper:
- This technique was used on patients without hearing problems. I would not carry this over to acoustic neuroma cases, where you might already battling with poor waveforms. Increasing the rep rate and lowering the sweeps this much might make any data collection impossible.
- This protocol is still new. It looks like obtaining their objective (a reliable wave V) is doable, but I’d like to see more cases with changes in order to call this a superior technique.
- I would like to see a larger sample of patients with changes before abandoning looking at interpeak latency as something that should be reported to the surgeon. I’ve always felt that amplitude changes are a better marker than latency changes, but I have seen a latency shift only with almost no amplitude change have post op deficits. This was with SSEPs, but enough reason for me to continue to use latency changes as something that needs to be looked at.
What do you think about this technique? Anyone using it themselves?
Joe Hartman DC, DACNB, DABNM
Vice President Of Clinical Services - Sentient Medical
Joe came into the neuromonitoring field as a board-certified chiropractic neurologist with an interest in neurodiagnostics. After earning a diplomate from the American Board of Neurophysiologic Monitoring, he started IntraoperativeNeuromonitoring.com as a resource for other surgical neurophysiologist looking to learn and discuss all things neuromonitoring. Once he started work at Sentient Medical in 2016, the website was relaunched and expanded to include webinars, polls, practice test and a forum. He spends more time tinkering on this website in his spare time than he cares to admit.
Want new articles before they get published?
Subscribe to our Awesome Newsletter.
Here are some related guides and posts that you might enjoy next.
Double-Train MEP On A Comeback Kick Using transcranial electric motor evoked potentials in the operating room has become routine practice for spinal cord monitoring. Recent improvements in the ability to record tcMEP have resulted in increased use during other...read more
How Resourceful Of A Neuromonitoring Tech Are You? First off, let me start this topic off by saying that I'm not a big fan of the term neuromonitoring tech (I prefer surgical neurophysiologist or SNP). But I really want to address those in the field that might embrace...read more
Loss of Cortical SSEP Due To Loss of Receptor Activation Earlier in this 30 Days of Neuromonitoring series, I wrote about cortical and subcortical reorganization after a loss of afferent feedback from carpal tunnel syndrome and how that affects intraoperative...read more
The CNIM vs The Intraoperative Neuromonitoring Degree (Joe's notes: This is a GUEST POST by Josh Mergos, who is the director of the Intraoperative Neuromonitoring Program at the University of Michigan - School of Kinesiology. We met for the first time during a small...read more
Optimizing Sub-cortical SSEP There is 1 electrode that I see get misused in somatosensory evoked potentials more so than any other electrode in any modality. This is the electrode placed over the cervical spine (or sometimes around the ear or mastoid) and generally...read more
The Trapezius Muscle In Intraoperative Neuromonitoring If we follow the same medical textbooks we used in school to pass a test, then we know for sure that the motor portion of the trapezius is innervated by the spinal accessory nerve (CN 11) and the sensory portion...read more