Opportunities and challenges of intermittent and continuous intraoperative neural monitoring in thyroid surgery

Abstract

The number of thyroid operations and there radically continues to rise in the western hemisphere, bringing prevention of recurrent laryngeal nerve (RLN) palsy to the fore. Overall, the incidence of RLN palsy is fairly low but continues to prompt litigation for malpractice. In an effort to diminish transient, and more importantly permanent, RLN palsy rates, intraoperative neuromonitoring (IONM) has been advocated as a risk minimization tool. Recent meta-analyses of studies, many of which were limited by poor study design and the sole use of intermittent nerve stimulation, were unable to demonstrate superiority of IONM over mere anatomic RLN dissection. This is where continuous IONM (CIONM) comes into play: this technology enables the surgeon to (I) identify impending nerve injury as it unfolds; (II) release distressed nerves by reversing causative surgical maneuvers; and (III) verify functional nerve recovery after intraoperative loss of the electromyographic signal. Despite this superiority, CIONM is not devoid of methodological limitations, which need to be accounted for. This review summarizes the current key achievements of IONM; outlines opportunities for improvement regarding clinical implementation; and suggests areas of future research in this rapidly evolving field.

Keywords: Intraoperative neuromonitoring (IONM); continuous intraoperative neuromonitoring (CIONM); nerve monitoring; thyroidectomy; vagus nerve stimulation (VN stimulation).

Figure 1

Hierarchical model of RLN monitoring. The model encompasses three interdependent steps of RLN evaluation: preoperative, intraoperative, and postoperative RLN assessment. At the top of the hierarchical pyramid come voice, breathing, and swallowing. CIONM, continuous intraoperative neural monitoring; HR-QL, health-related quality of life; IONM, intermittent intraoperative neural monitoring; LOS, loss of signal; postop postoperative; preop preoperative; RLN, recurrent laryngeal nerve; VBS, voice, breathing, swallowing; VF, vocal fold.

Figure 2

Examples of special electromyographic tracings under continuous neural monitoring. (A) ‘Normal’ EMG tracing in the presence of pre-existent vocal fold palsy; (B) artificial decrease in amplitude <50% of baseline (but >100 µV) with concomitant increase in latency >110% of baseline caused by tube rotation, mimicking a “combined event”; normal vocal fold function; (C) global LOS type 2 with amplitude ≤100 µV after ‘combined event’ (decrease in amplitude <50% of baseline (but >100 µV) and increase in latency >110% of baseline), caused by traction on the thyroid lobe; normal vocal fold function (false-positive result); (D) segmental LOS type 1 with plunge in amplitude ≤100 µV without concomitant increase in latency) on bipolar coagulation near the recurrent laryngeal nerve; transient vocal fold palsy; (E) nascent segmental LOS type 1 with a 50% decrease in amplitude (arrow) without concomitant increase in latency, produced by bipolar coagulation near the recurrent laryngeal nerve; ceased after halting the surgical maneuver; normal vocal fold function; (F) ‘normal’ EMG tracing with dislocation (arrow) and repositioning of the vagus electrode: isolated increases in latency >110% of baseline with normal amplitude; transient vocal fold palsy (false-negative result).