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3 - Basic principles and standardization of intraoperative nerve monitoring in thyroid surgery
Mehmet Uludağ, Nurcihan Aygün, Cemal Kaya, Mert Tanal, Sibel Oba, Adnan İşgör
Voice changes after thyroid surgery are frequent and one of the most important complications. Both the recurrent laryngeal nerve (RLN) and the external branch of the superior laryngeal nerve (EBSLN) must be preserved to minimize the patients voice and respiratory problems after surgery. Intraoperative neuromonitoring (IONM) is a method based on dynamically evaluating motor function of the nerve during surgery in addition to the visual identification of the nerve. Intraoperative neuromonitoring was introduced 50 years ago in thyroid surgery and IONM via endotracheal tube with surface electrodes has become a standard applied method for reasons such as convenience, simplicity, non-invasiveness and safety, and nowadays is used in thyroid surgery. The use of IONM for RLN and EBSLN is increasing in thyroid surgery. Experience and standardization is essential for proper use of IONM for both anesthesiologist and surgeon. In this context, the learning curve for both surgeons and anesthetists is about 50-100 cases. Intraoperative neuromonitoring makes a significant contribution to the identification and functional evaluation of both RLN and EBSLN. RLN monitorisation can be performed intermittently with the monitoring probe or continuously with the aid of a probe applied to the vagus. Standardization of RLN monitoring includes the vocal cord examination via preoperative laryngoscopy (L1), getting signals from ipsilateral vagus prior to RLN dissection (V1), stimulation of RLN at the first point found in the tracheoesophageal groove (R1), stimulation of the RLN from the most proximal point it was revealed after the dissection was completed (R2), vagus stimulation after surgical site bleeding control is complete (V2), vocal cord examination via postoperative laryngoscopy (L2). V2 is the most appropriate test to predict postoperative vocal cord function. In the intermittent IONM of RLN, only the nerve stimulated by the probe and the point that the nerve is stimulated inform about the function of the distal part. Continuous IONM allows continuous follow-up of RLN function while dissecting the thyroid gland by continuous stimulation of the RLN with the probe applied to the vagus at the neck before RLN leaves the vagus. Primarily in EBSLN monitoring, the contraction of the cricothyroid muscle, which is located in the surgical field and whose motor neuron is EBSLN, is evaluated. Intraoperative neuromonitoring is a method that contributes to many aspects of thyroidectomy and increases the standards of thyroidectomy, together with significant contribution to the detection and functional evaluation of both RLN and EBSLN.
Keywords: Intraoperative neuromonitoring, recurrent laryngeal nerve, external branch of the superior laryngeal nerve
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