In the Literature: Solutions to the technical challenges embedded in the current methods for intraoperative peripheral nerve action potential recordings
The Big Question/Background:
This paper begins by giving an overview of the challenges of recording intraoperative nerve action potentials (NAPs). The objectives of the authors, in a nutshell, was to test the current method for stimulating and recording NAPs and see if a more effective way to obtain reliable responses could be found. In their own words, “The authors’ goal was to improve intraoperative NAP recording techniques by revisiting the methods in an experimental setting” (p. 1).
Animal testing on non-human primates was used initially to attempt to remove the typically confounding stimulus artifact caused by recording potentials so close to the stimulation source. They used the standard method of lifting the nerve from the surrounding tissue but faced the same stimulus artifact problems that negatively affects NAP recording. They then tried a novel technique where they used a saline-soaked gauze under or around the portion of the nerve between the stimulation and recording sites. The end result was that the stimulus artifact was removed and the NAPs were recorded at significantly larger amplitudes at lower stimulus thresholds and with very little stimulus induced interference.
The authors hypothesized that the saline gauze created a salt bridge between the outside of the nerve and the surrounding tissue thus preventing the stimulus current from looping back on, around, or through the nerve and confounding the equipment’s average/amplifier (termed “the loop effect” (p.6)). Next the authors, based off the information they had obtained through the gauze salt bridging, successfully recorded NAPs with the same conclusions by simply using insulated stimulation and recording electrodes and not lifting (“nonlifting technique” p. 6) the nerve from the surrounding tissue. The authors suggest that by isolating both the stimulation and recording mediums in the electrodes the current loop that the gauze prevented was prevented in the same fashion.
Finally they verified their results through a “stimulus polarity switch test and by the intensity-response function test” (p. 3). This is done by reversing polarity of the stimulation delivered to the nerve. Only the deflection of the stimulus artifact should change direction thus verifying your NAP. However, they also noted that when polarity was switched the stimulus threshold needed to generate the same NAP approximately doubled when stimulating anodally versus cathodally. Something to be aware of if the reader plans to attempt this method.
Finally they tested the “nonlifting technique” in the OR setting on patients and similar results occurred and the results were again verified with the intensity-response deflection test (p. 8).
Briefly explained, and again in the authors’ own words, “We identified exaggerated stimulus artifacts being a major problem and found bridge grounding to be a simple and effective solution. Ultimately, we brought our new methodology forward into clinical practice, where clinical rather than research equipment was used. The outcome was the same, validating the principal concept shared by recordings in these different settings” (p. 9).
The authors were able to consistently record action potentials in both the experimental and clinical settings by removing “the loop effect” with either “bridge grounding” with a saline-soaked gauze or by insulated stimulating and recording electrodes and not lifting nerve from the surrounding tissue (the grounding source) thus allowing that tissue to shunt the stimulus before looping back through the nerve.
The authors acknowledge the biggest limitation of this study was that, in the clinical setting, this technique (or more specifically the “bridge-grounding” version of this technique) was only tested on four patients intraoperatively. They encourage the IONM community to verify this technique through “systematic and quantitative evaluations of these methods, additional investigations in healthy and, more importantly, chronically injured nerves” (p. 9).
This method minimizes the major confounding factor in recording NAPs and could improve the confidence of technologists, neurophysiologists, and surgeons in the testing being done and the results displayed. If further testing found it to consistently work intraoperatively this research could have a major impact on the reliability and use of NAP recording.
Wu G, Belzberg A, Nance J, Gutierrez-Hernandez S, Ritzl EK, Ringkamp M. Solutions to the technical challenges embedded in the current methods for intraoperative peripheral nerve action potential recordings. J Neurosurg. 2019 Aug 16:1-10.
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