The latency periods progressively shorten from the proximal vagus nerve to the midcervical vagus nerve and then to the recurrent laryngeal nerve (RLN) (horizontal calibration, 2 milliseconds/division; vertical calibration, 100 µV/division).
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Suh I, Yingling C, Randolph GW, Duh Q. A Novel Method of Neuromonitoring in Thyroidectomy and Parathyroidectomy Using Transcutaneous Intraoperative Vagal Stimulation. JAMA Surg. 2016;151(3):290–292. doi:10.1001/jamasurg.2015.3249
Intraoperative neuromonitoring (IONM) of recurrent laryngeal nerve (RLN) function is commonly performed for patients undergoing thyroidectomy or parathyroidectomy.1 Although its routine use remains controversial, IONM has demonstrated utility in selected situations, such as cases of reoperation.2,3 Intraoperative neuromonitoring using specific stimulation of the more proximal vagus nerve is thought to provide more complete anatomic analysis of RLN integrity than that of isolated distal segments of the RLN alone.4 However, current vagal IONM methods typically entail placing an electrode around the vagus for continuous electrical stimulation. This requires additional dissection of, and fixation to, the nerve, which, in theory, adds time and increases risk.5 We hypothesize that a novel method of transcutaneous intraoperative vagal stimulation (TIVS) in the upper neck is less invasive and is feasible in thyroidectomy and parathyroidectomy.
This study was approved by the institutional review board of the University of California, San Francisco. Four patients (2 undergoing thyroidectomy and 2 undergoing parathyroidectomy) with intermittent TIVS provided written informed consent and were examined. The patients were intubated with an electromyography (EMG) endotracheal tube to monitor vocal cord function. A pair of tiny subdermal needle electrodes were placed at the right and left upper neck overlying the mastoid insertion point of the sternocleidomastoid muscle, at the approximate level of the jugular foramen and proximal vagus nerve. These transcutaneous electrodes were easily placed using standard anatomic landmarks for head/neck neuromonitoring procedures, and their small caliber and superficial location ensured safe placement. Each transcutaneous site was stimulated independently with pulses of 0.2-millisecond duration at a sufficient intensity (approximately 25 mA) to produce a consistent EMG response from the vocal cord–recording electrode, signifying integrity of the circuit from proximal vagus nerve to RLN. Spontaneous EMG waveforms and responses to both TIVS and direct surgical probe stimulation (1-2 mA) of the vagus nerve and the RLN were recorded at intermittent time points throughout the procedure. The main measures for confirmation of a meaningful TIVS signal were (1) the presence of amplitude and waveform patterns characteristic of vagal stimulation and (2) an appropriately longer latency (ie, time of action potential migration along the nerve) from stimulation at the proximal vagus nerve to the detection of the EMG response at the vocal cord.
Transcutaneous intraoperative vagal stimulation produced consistent vagus waveforms (Figure), with minimal background twitching from contraction of muscles around the stimulating electrodes. Vocal cord responses from left-sided TIVS had longer latency periods (mean latency period, 12.9 milliseconds) than those from right-sided stimulation (mean latency period, 8.7 milliseconds), consistent with the longer anatomic path of the left RLN; this provided consistently correct lateralization information. On each side, latency was longest with TIVS—owing to its proximal point of stimulation and longer path of nerve conduction to the vocal cord—and decreased sequentially at more distal sites along the surgically dissected vagus nerve and RLN (Figure). In addition, the waveform characteristics from both vagal and RLN stimulations were consistent with normative data reported in the literature (Table).6 These findings, in aggregate, are consistent with successful lateralized vagal stimulation with TIVS. In one patient, stretching of the RLN during critical dissection at the ligament of Berry produced increased spontaneous EMG activity and a 30% decrease in stimulated waveform amplitude, which returned to normal after removal of the specimen. There were no complications associated with this IONM method. The results of all the patients’ postoperative voice assessments and laryngeal examinations were normal.
To our knowledge, this is the first report to describe the successful application of TIVS at the upper neck for vagal IONM in thyroidectomy and parathyroidectomy. This method allows minimally invasive monitoring of vagal-RLN function without the need to deliberately dissect and directly stimulate the vagus nerve. Further studies are needed to confirm the utility of TIVS as an accurate, less-invasive, and cost-effective method of IONM of patients undergoing thyroidectomy or parathyroidectomy.
Corresponding Author: Insoo Suh, MD, Endocrine Surgery Section, Department of Surgery, University of California, San Francisco, Medical Center at Mount Zion, 1600 Divisadero St, Room A-725, San Francisco, CA 94115 (email@example.com).
Published Online: November 4, 2015. doi:10.1001/jamasurg.2015.3249.
Author Contributions: Drs Suh and Duh had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: All authors.
Acquisition, analysis, or interpretation of data: All authors.
Drafting of the manuscript: All authors.
Critical revision of the manuscript for important intellectual content: All authors.
Statistical analysis: Suh.
Administrative, technical, or material support: Suh, Randolph.
Study supervision: Suh, Randolph, Duh.
Conflict of Interest Disclosures: None reported.