Intraoperative Neurophysiological Monitoring for the External Branch of the Superior Laryngeal Nerve: Case Illustrations

The superior laryngeal nerve is a branch of the vagus nerve (CN X) and divides into the internal and external branches of the superior laryngeal nerve. The external branch of the superior laryngeal nerve (EBSLN) supplies motor enervation to the cricothyroid muscle. During thyroid surgery the EBSLN can be monitored by placing paired subdermal needle electrodes into the cricothyroid muscle. An endotracheal tube (ETT) incorporating electrodes which lie at the level of the vocal cords after intubation can also be utilized for monitoring of the EBSLN. Utilizing recording intraoperative monitoring equipment, free running EMG activity from the cricothyroid muscle and the ETT can be monitored for any indirect activation of the EBSLN. Direct stimulation of the EBSLN can also be performed with the use of a Prass monopolar cathode stimulator with a reference anode electrode placed in the trapezius muscle. Stimulation intensities of 0.5-4 milliamps (mA) are used. All waveforms and data were obtained utilizing Cadwell Cascade® intraoperative monitoring recording equipment.