Visualization Versus Intraoperative Neuromonitoring of the Recurrent Laryngeal Nerves in Thyroid Surgery

Apart from hypoparathyroidism, dysfunction of the recurrent laryngeal nerve (RLN) is the most common complication following thyroid surgery. In consequence, the voice impairment leading to communication work-related problems and affecting psychological and social aspects of the individual's functioning diminishes the overall quality of life, being the common reason for medicolegal claims and litigation. The reported RLN palsy rate varies in the literature from 0% (for first-time thyroid surgery performed by an experienced endocrine surgeon) to as much as 20% (for reoperative thyroid surgery or thyroid malignancy surgery performed in low-volume centers), depending mostly on the type of thyroid disease (benign vs. malignant goiter), type (first-time vs. reoperation) and the extent of thyroid resection (subtotal vs. total thyroidectomy), surgical technique (with or without routine RLN identification) and the surgeon's experience (low-volume vs. high-volume thyroid surgery center.

In 1938, Lahey from Boston reported a significantly lower incidence of RLN injuries following thyroidectomy with dissection and visualization of the nerves as compared to operations without nerve identification. Since that time, many prospective studies have confirmed this observation, advocating routine RLN identification as the gold standard in safe thyroid surgery. But even in the most experienced hands RLN palsy occurs occasionally, with an average frequency below 1% of nerves at risk due to variability in RLNs anatomy and difficulties in nerve identification by visual or palpation control in challenging conditions (e.g. advanced thyroid malignancy or reoperative thyroid surgery). On the other hand, the use of intraoperative electrical stimulation for identifying the RLN nerve was described in 1966. However, the technique of intraoperative neuromonitoring (IONM) of RLN did not gain any widespread popularity until the late nineties of the last century, when several commercial user-friendly systems based on electromyographic signal recording became available. In these IONM systems, the RLN nerve stimulation is registered by elicited laryngeal muscles activity through the endoscopic insertion of the electrodes into the vocal cords, open insertion of the needle electrodes into the vocal muscles through the cricothyroid ligament or with the use of endotracheal tube surface electrodes. In addition to a plethora of signal acquisition techniques used in IONM, there is no consensus regarding the optimal method for nerve activity recording (continuous recording of spontaneous nerve activity versus repetitive stimulation) and no agreement as to which quantitative electromyographic parameter should be used as a predictor of postoperative vocal cord dysfunction (supramaximal versus minimal stimulation of the nerve at the end of the operation).

Some recent studies have shown that IONM can aid the RLN identification. However, the role of IONM in reducing the incidence of RLN injury rate and the value of this method in predicting postoperative RLN function remain controversial. Only a few published series represent level III of evidence and grade C of recommendation according to the evidence-based criteria (Sackett's classification, modified by Heinrich). Large, prospective, randomized trials addressing these issues and allowing for grade A recommendations are lacking due to some ethical concerns and large numbers of patients in each arm (more than 7000 patients) needed to reach the appropriate power of the study. To fulfill this gap in evidence, we designed a medium-size, single-center, prospective randomized study suitable for drawing more meaningful conclusions. Thus, the aim of this study was to compare the impact of RLN visualization versus IONM on their morbidity following thyroid surgery.

Inclusion criteria

• thyroid pathology qualified for first-time bilateral neck surgery

Exclusion criteria

• previous thyroid or parathyroid surgery,
• unilateral thyroid pathology eligible for minimally invasive approach (MIVAT),
• mediastinal goiter,
• preoperatively diagnosed RLN palsy,
• pregnancy or lactation,
• age below 18 years,
• high-risk patients ASA 4 grade (American Society of Anesthesiology),
• and inability to comply with the scheduled follow-up protocol.