Vagal versus recurrent laryngeal nerve monitoring in thyroid surgery

Abstract

Recurrent laryngeal nerve (RLN) palsy is considered a serious complication of thyroid surgery. Permanent lesions are still occurring in about 1% of patients, despite the standardized surgical approach to the nerve, and the availability of RLN monitoring [1]. Intraoperative RLN monitoring is based on the visual or acoustic registration of evoked electromyography of the laryngeal muscles. Primarily, it proves conductivity of the stimulated nerve segment towards the muscle. However, a recent meta-analysis did not demonstrate a statistically signiWcant diVerence in the rate of transient or persistent vocal cord palsy after using intraoperative neuromonitoring versus RLN identiWcation alone during thyroidectomy [2], conWrming the anecdotal views of many experienced thyroid surgeons on this issue. In addition, following an iatrogenic lesion of the RLN in a porcine model, the distal nerve segment showed unchanged amplitude of the electrophysiological response for an observational period of more than 1 h [1], thus revealing a potential pitfall for the neuromuscular monitoring of the RLN in the human surgical setting; the false assumption of an anatomically intact nerve even after transection. Newly developed vagal stimulation probes permit continuous intraoperative neuromonitoring of the RLN during thyroid surgery. We had recently performed a hemithyroidectomy for a pedunculated right thyroid lobe lesion, which was preoperatively classiWed as THY 2 with groups of follicular thyrocytes on U/S-guided FNA. During the operation a monitoring probe was attached onto the right vagal nerve (Fig. 1). In detail, after identiWcation of the right carotid sheath medial to the right sternomastoid muscle, the omohyoid was divided, and the vagus nerve accessed, behind and lateral to the internal jugular vein. The monitoring system was the NIM-Response 3.0 (Medtronic Inc.), which uses the automatic periodic stimulation (APSTM) electrode. The obtained waveform amplitude (the amount of current going through the nerve) was 111 V, and was above the standard amplitude setting of 103 V, according to the manufacturer’s instructions. The system is designed in such a manner that a decrease of 50% in nerve conduction, which is shown as an amplitude drop below the level of 50 V, sets oV an alarm (Fig. 2). In the presented case, the alarm was indeed set oV intraoperatively before the visual identiWcation of the right RLN (Fig. 2), indicating related strain, however, the standard RLN probing (also incorporated to the system) after the