Anatomical, Functional, and Dynamic Evidences Obtained byIntraoperative Neuromonitoring Improving the Standards ofThyroidectomy

Abstract

The use of intraoperative neuromonitoring (IONM) is getting more common in thyroidectomy. The data obtained by the usage ofIONM regarding the laryngeal nerves’ anatomy and function have provided important contributions for improving the standardsof the thyroidectomy. These evidences obtained through IONM increase the rate of detection and visual identification of recurrentlaryngeal nerve (RLN) as well as the detection rate of extralaryngeal branches which are the most common anatomic variations ofRLN. IONM helps early identification and preservation of the non-recurrent laryngeal nerve. Crucial knowledge has been acquiredregarding the complex innervation pattern of the larynx. Extralaryngeal branches of the RLN may contribute to the motor innerva tion of the cricothyroid muscle (CTM). Anterior branch of the extralaryngeal branching RLN has always motor function and givesmotor branches both to the abductor and adductor muscles. In addition, up to 18% of posterior branches may have adductor and/ or abductor motor fibers. In 70–80% of cases, external branch of superior laryngeal nerve (EBSLN) provides motor innervation tothe anterior 1/3 of the thyroarytenoid muscle which is the main adductor of the vocal cord through the human communicatingnerve. Furthermore, approximately 1/3 of the cases, EBSLN may contribute to the innervation of posterior cricoarytenoid musclewhich is the main abductor of ipsilateral vocal cord. RLN and/or EBSLN together with pharyngeal plexus usually contribute to themotor innervation of cricopharyngeal muscle that is the main component of upper esophageal sphincter. Traction trauma is themost common reason of RLN injuries and constitutes of 67–93% of cases. More than 50% of EBSLN injuries are caused by nervetransection. A specific point of injury on RLN can be detected in Type 1 (segmental) injury, however, Type 2 (global) injury is the lossof signal (LOS) throughout ipsilateral vagus-RLN axis and there is no electrophysiologically detectable point of injury. Vocal cordparalysis (VCP) develops in 70–80% of cases when LOS persists or incomplete recovery of signal occurs after waiting for 20 min.In case of complete recovery of signal, VCP is not expected. VCP is temporary in patients with incomplete recovery of signal andpermanent VCP is not anticipated. Visual changes may be seen in only 15% of RLN injuries, on the other hand, IONM detects 100%of RLN injuries. IONM can prevent bilateral VCP. Continuous IONM (C-IONM) is a method in which functional integrity of vagus-RLNaxis is evaluated in real time and C-IONM is superior to intermittent IONM (I-IONM). During upper pole dissection, IONM makessignificant contributions to the visual and functional identification of EBSLN. Routine use of IONM may minimalize the risk of nerveinjury. Reduction of amplitude more than 50% on CTM is related with poor voice outcome.