Thyroid surgery: RLN monitoring

The recurrent laryngeal nerve provides motor innervation to all intrinsic laryngeal muscles except the cricothyroid muscle. Intraoperative monitoring of the recurrent laryngeal nerve during thyroid surgery is possible with the use of an ETT with two electrodes embedded in its side. The recording electrodes are positioned at the level of the vocal cords. Goals of RLN monitoring include identifying the RLN, predicting postoperative RLN function, and avoiding bilateral vocal cord paralysis.

Updated definition 2020:

The recurrent laryngeal nerve (RLN) innervates all intrinsic laryngeal muscles except the cricothyroid. An incomplete injury to the RLN preferentially limits vocal cord abduction (opening) on the affected side, while a complete nerve palsy prevents both abduction and adduction (closing). This results in a fixed vocal cord in the median or paramedian position. RLN injury is a rare complication of thyroid surgery (0.4-12% of cases) and usually results from transection, traction, suture entrapment or thermal injury. In order to prevent RLN injury during thyroid surgery, intraoperative neuromonitoring has become the gold standard to supplement direct visualization of the nerve. The neuromonitoring system consists of both a stimulation probe and surface electromyography (EMG) electrodes embedded in an endotracheal tube (ETT) placed in close proximity to the vocal cords. The stimulation probe is grounded to the patient and transmits electrical current to the nerve or its innervated muscles. The current is recorded at the surface electrodes in the ETT, and the recording surface is also grounded to the patient. Both the stimulation probe and ETT electrodes are combined via an interconnected box and transmitted to a screen that records amplitude changes and makes an audible sound when the RLN is stimulated. The most commonly used neuromonitoring system for thyroid surgery in the US is NIM-Response by Medtronic.

Several anesthetic considerations play an important role in neuromonitoring for thyroid surgery. Neuromuscular blocking agents interfere with monitoring by reducing EMG amplitude, so short-acting agents like succinylcholine or smaller doses of rocuronium should be used for intubation. During laryngoscopy, the largest size NIM ETT that will pass between the patient’s vocal cords should be used. The NIM ETT should be passed under direct visualization so that the blue marked region of exposed electrodes is touching the true vocal cords. After the patient’s neck is hyperextended for surgery, the tube position should be rechecked and the system impedance should be tested to ensure adequate contact between the electrodes and the vocal cords. Sufficient reversal of neuromuscular blockade should be obtained prior to monitoring, and paralytic agents should be avoided for the duration of monitoring. Should loss of signal occur during the case, the most common causes are ETT malpositioning, displacement of the grounding electrodes, and malfunctioning of the ETT electrodes. Both the stimulation probe and ETT electrodes should be checked prior to determining if a RLN injury has occurred. Given the low incidence of RLN injury during thyroidectomy, randomized controlled trials have failed to show a difference in injury rate when using an intraoperative neuromonitoring system.