I came across some papers on motor evoked potentials (MEPs) that got me thinking a little bit.

It got me thinking about small influences in the operating room that may, or may not, affect our tracings during our cases. Specifically, MEPs.

Here’re some snippets from the 2 articles:

Kamibayashi K (2009)

Specifically, to investigate the effect of load-related afferent inputs on the corticospinal excitability during passive stepping, motor evoked potentials (MEPs) in response to the stimulation were compared between two passive stepping conditions: 40% body weight unloading on a treadmill (ground stepping) and 100% body weight unloading in the air (air stepping). In the rectus femoris, biceps femoris and tibialis anterior (TA) muscles, electromyographic activity was not observed throughout the step cycle in either stepping condition. However, the TMS-evoked MEPs of the TA muscle at the early- and late-swing phases as well as at the early-stance phase during ground stepping were significantly larger than those observed during air stepping.

and Kitamura T (2012)

During passive stepping, the MEP amplitudes in FCR muscle were significantly increased in six adjacent stimulus sites of the hot spot

For chiropractors, PT’s, OT’s, etc that do physical rehab, these findings should come at no shock. Afferent bombardments are used all the time to improve neurological function. We utilize the pain gate theory by using TENS units to stimulate below pain thresholds (larger diameter afferent fibers) to inhibit the slower pain fibers (smaller diameter afferents) at the spinal and cortical levels. Chiropractors will adjust the spine to change the gain of muscle spindle fibers and restore segmental motion, thereby creating a heightened level of afferent feedback.

This kind of stuff is really old hat.

MEPs in the OR

But it got me thinking about those very fickle motor evoked potentials that we perform on an anesthetized patient in the operating room. What kinds of things would influence MEPs in that environment? Well here’s some of the much-studied causes:

In addition, here’s a short (incomplete) list of commonly accepted factors:

  1. Anesthesia
  2. Blood pressure
  3. Recording and stimulating electrode placement
  4. Case duration/Fade
  5. Blood loss
  6. Patient temperature

But according to Kamibayashi K (2009) and Kitamura T (2012), there can be an increase in the amplitude of muscles with afferent stimulation. And this can happen to the muscle being loaded, like the tibialis anterior in the weighted portion of normal gait, as well as a distant muscle, like the arm flexors during weighted gait.

And I’m aware of studies like Journee (2007) Léonard G (2012), and Lapole T (2012) where they’ve experimented with front loading MEPs with afferent stimulation and applying a double stimulation to utilize temporal facilitation.

But what I’m wondering about is the intermittent afferent stimuli affecting MEPs that is out of our control…

  • Do MEPs improve when the blood pressure cuff turns on? Is it significant?
  • What about the lower extremities with cases with intermittent sequential pneumatic compression? Do those make a difference?
  • Is there any ramped up feedback when the patient is forced into dorsiflexion at the ankle on certain tables? Does it last consistently?
  • How about if the patient is getting low levels of anesthesia and begins reacting to pain, as seen on EMGs. I wouldn’t want to shock them then, but I wonder what their MEPs look like then?

Like I said, this was just something I was wondering about. I’m not sure if my thought process was of any benefit, but maybe those were some papers you were unaware of.

What about you, ever wonder about MEPs?

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Joe Hartman DC, DACNB, DABNM

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