Preoperative ultrasound mapping of the vagus nerve in thyroid surgery

Backgrounds: intraoperative neuromonitoring (IONM) in thyroid surgery requires electric stimulation of the vagus nerve to verify correct electrode placement. Classically the nerve is found deep to or in-between the common carotid artery and internal jugular vein, but previous studies have shown that the nerve can sometimes be found superficial to the vessels. Our aim was to determine the incidence of a superficial vagus nerve using ultrasound (US) and study possible clinical factors associated with an anteriorly-located vagus nerve. Methods: Retrospective study of patients undergoing thyroid surgery (lobectomy or total thyroidectomy) with intermittent IONM. Substernal goiters, locally invasive tumors or bully lymph nodes were excluded. The vagus nerve was identified at the level of the mid-thyroid lobe on each side on preoperative US performed by two specialized radiologists, and its location according to 6 possible positions in relationship to the common carotid artery was recorded. The anatomic variability of the vagus nerve was analyzed in relationship to patient demographics and thyroid pathology. Results: Five-hundred twenty-seven patients were included. The right vagus nerve (n=522) was in-between, superficial or deep to the vessels in 92.3%, 6.1% and 1.5% and of cases, respectively, and the left vagus (n=517) in 80.2%, 18.6% and 1.2% of cases, respectively, with a statistically significant difference between right and left vagus nerves (P<0.001). The type of pathology, size of the dominant nodule or the volume of the thyroid lobe were not correlated to finding a superficial vagus nerve. Conclusions: The vagus nerve was identified in all cases on US and found to be anterior to common carotid artery at the level of the thyroid lobe in 18.6% of cases on the left and 6.1% of cases on the right. Identifying this anatomic variant preoperatively may facilitate IONM and avoid inadvertent trauma to the vagus nerve during thyroid surgery.