Experimental study of needle recording electrodes placed on the thyroid cartilage for neuromonitoring during thyroid surgery.

backgroundNeedle electrodes placed on the thyroid cartilage (TC) are an alternative to endotracheal tube (ET) electrodes for assessing recurrent laryngeal nerve (RLN) function during thyroid surgery. Needle electrodes placed on the TC were evaluated in an experimental porcine model.

methodsContinuous intraoperative neuromonitoring was used to record the electromyogram. Each TC side was delineated into nine areas to determine the optimal placement of the electrode, and needle electrode area, depth and orientation for optimal electromyographic (EMG) amplitudes were evaluated. RLN root locations were stimulated at four locations: vagus nerve distal to the neuromonitoring electrode, and most proximal, middle and laryngeal entry points of the nerve. A nerve retraction injury model was adapted to compare RLN monitoring by TC versus ET electrodes.

resultsAn optimal site for placement of needle electrodes was identified, and electromyograms obtained from the various needle insertion depths and orientations were similar. Latencies recorded from the TC and ET electrodes were similar. The amplitude profile of TC electrodes responded earlier to RLN injury than that of ET electrodes. Amplitude and drop to loss of signal were also registered earlier.

conclusionEMG amplitudes obtained using TC electrodes were higher, and identified RLN injury earlier than ET electrodes. Surgical relevance Needle electrodes placed on the thyroid cartilage (TC) are an alternative to endotracheal tube (ET) electrodes for assessing the function of the recurrent laryngeal nerve (RLN) in thyroid surgery. This study used an experimental porcine model to evaluate the use of needle electrodes inserted in the TC, compared with ET electrodes, for producing an electromyographic (EMG) profile of the RLN. Nine areas of the TC, with various needle insertion depths and orientations, were compared. Perichondral insertion into the avascular area of the TC was found to be safe. The EMG amplitude and latency features recorded via the TC and ET electrodes were compared, using both intermittent and continuous monitoring. Changes in EMG amplitudes in response to nerve traction injury were registered earlier with TC electrodes than with ET electrodes, and the amplitudes were higher and more stable. Latencies obtained via the TC and ET electrodes were similar. These results indicate that the development of a non-invasive monitoring electrode with improved function, easy placement and low cost is possible.