Electrophysiological identification of nonrecurrent laryngeal nerves

The triad of non-recurrent laryngeal nerve; three associated predicting variants in the era of nerve monitoring: A case report

INTRODUCTION AND IMPORTANCE

The prediction and early identification of non-recurrent laryngeal nerve (RLN) may minimize risk of injury. It could be associated with other coincident variants that predict non-RLN, leading to its proper identification.

CASE PRESENTATION

A patient with multinodular goiter underwent total thyroidectomy under intraoperative neuromonitoring (IONM) guidance. Preoperative thoracic computerized tomography (CT) scan/angiography revealed aberrant right subclavian artery (ARSA). During thyroid surgery, the vagus nerve (VN) was identified in the neurovascular bundle. An anatomic variation of the VN was observed, as it was medially placed in relation to the common carotid artery (CCA). Pre-dissection electrophysiological stimulus of the VN (V1) was negative. Thus, a right non-RLN was identified with careful surgical dissection. The branching point of the non-RLN on the VN was identified, and non-RLN was fully exposed until the laryngeal entry. IONM revealed that V1 signal was negative if derived distal to the non-RLN separation, and positive if derived proximal to the non-RLN separation.

CLINICAL DISCUSSION

ARSA detected by preoperative CT scan is associated with non-RLN. The medial course of the VN in relation to the CCA was found as a coincident anatomic variant with the non-RLN. Absence of pre-dissection V1 signal by IONM was an electrophysiological variant associated with the non-RLN.

CONCLUSION

ARSA is a reliable variant for predicting the non-RLN. VN medial to the CCA and absence of electrophysiological V1 signal could precisely predict the non-RLN. Therefore, the coincidence of three anatomical and electrophysiological variants with non-RLN could lead to the prediction of non-RLN.

The Effect of Patient Positioning on Intraoperative Neuromonitoring During Thyroid and Parathyroid Surgery

BACKGROUND

Intraoperative neuromonitoring (IONM) has been accepted as a routine adjunct among surgeons who perform thyroid and parathyroid surgeries. Thyroid and parathyroid surgeries use various patient positioning strategies that have poorly understood effects on IONM. The aim of this study was to compare IONM signals between the transaxillary and transcervical approaches.

METHODS

In this retrospective cohort study, we evaluated 463 adult patients who underwent a total of 502 procedures. The procedures performed included total thyroidectomy, right or left hemithyroidectomy, and parathyroidectomy. Vagus nerve and recurrent laryngeal nerve (RLN) latency and amplitude measurements were analyzed intra-operatively. The distances between the vagus nerve and the trachea were measured via ultrasound during transaxillary procedures.

RESULTS

Compared to the transcervical approach, the right vagus nerve latency was significantly decreased in the transaxillary approach. Transaxillary surgery was not associated with increased latency or decreased amplitude on IONM. The distance between the vagus nerve and trachea was significantly decreased post-positioning during transaxillary approaches.

DISCUSSION

Despite differences in patient positioning, a transaxillary approach was not associated with increased stress on the vagus nerve or RLN, according to IONM data. The decreased right vagus nerve latency associated with a transaxillary approach highlights the importance of considering patient positioning and laterality while interpreting IONM data.

Anatomical, Functional, and Dynamic Evidences Obtained by Intraoperative Neuromonitoring Improving the Standards of Thyroidectomy

The use of intraoperative neuromonitoring (IONM) is getting more common in thyroidectomy. The data obtained by the usage of IONM regarding the laryngeal nerves’ anatomy and function have provided important contributions for improving the standards of the thyroidectomy. These evidences obtained through IONM increase the rate of detection and visual identification of recurrent laryngeal nerve (RLN) as well as the detection rate of extralaryngeal branches which are the most common anatomic variations of RLN. IONM helps early identification and preservation of the non-recurrent laryngeal nerve. Crucial knowledge has been acquired regarding the complex innervation pattern of the larynx. Extralaryngeal branches of the RLN may contribute to the motor innervation of the cricothyroid muscle (CTM). Anterior branch of the extralaryngeal branching RLN has always motor function and gives motor 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 to the anterior 1/3 of the thyroarytenoid muscle which is the main adductor of the vocal cord through the human communicating nerve. Furthermore, approximately 1/3 of the cases, EBSLN may contribute to the innervation of posterior cricoarytenoid muscle which is the main abductor of ipsilateral vocal cord. RLN and/or EBSLN together with pharyngeal plexus usually contribute to the motor innervation of cricopharyngeal muscle that is the main component of upper esophageal sphincter. Traction trauma is the most common reason of RLN injuries and constitutes of 67–93% of cases. More than 50% of EBSLN injuries are caused by nerve transection. A specific point of injury on RLN can be detected in Type 1 (segmental) injury, however, Type 2 (global) injury is the loss of signal (LOS) throughout ipsilateral vagus-RLN axis and there is no electrophysiologically detectable point of injury. Vocal cord paralysis (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 and permanent 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-RLN axis is evaluated in real time and C-IONM is superior to intermittent IONM (I-IONM). During upper pole dissection, IONM makes significant contributions to the visual and functional identification of EBSLN. Routine use of IONM may minimalize the risk of nerve injury. Reduction of amplitude more than 50% on CTM is related with poor voice outcome.

Intraoperative Neuromonitoring During Thyroid Surgery: The Effect of Surgical Positioning

Intraoperative neuromonitoring during thyroid surgery has been used to successfully prevent permanent neurological injury by early identification of anatomical variants. Proper interpretation of neuromonitoring data requires knowledge of what factors might affect the data. In this study, we examined the effect of surgical positioning on the latency and amplitude of neural recordings made from the vocalis muscle during thyroid surgery. A retrospective review was performed of 145 patients who underwent thyroid surgery. Eighty-three had open cervical procedures, and 62 had robotic-assisted transaxillary procedures. Intraoperative neuromonitoring recordings were made by stimulation of the vagus and recurrent laryngeal nerves for both groups. Ultrasound measurements were made of a subset of the transaxillary patients immediately before and after arm positioning. Groups differed only on right-sided recordings. Patients with transaxillary surgeries had significantly shorter latencies evoked from the vagus nerve. We found that vagus nerve-evoked latencies were also correlated to ultrasound measurements of the nerves. Surgical positioning during thyroid surgery is a factor that may affect intraoperative neuromonitoring data and should be taken into account by the surgeon during interpretation.

A Review of Methods for the Preservation of Laryngeal Nerves During Thyroidectomy

The recurrent laryngeal nerve (RLN) provides motor innervation to the abductor and adductor muscles of the vocal cord, whereas the external branch of the superior laryngeal nerve (EBSLN) provides motor innervation to the cricothyroid muscle, which is the tensor muscle of the vocal cord. Both the RLN and the EBSLN are anatomically close to the thyroid and are therefore at risk of injury during thyroidectomy. These 2 laryngeal nerves must be carefully preserved during surgery to ensure that the function of the vocal cord is not impaired. Currently, complete exposure of the RLN during thyroidectomy is accepted as the gold standard method for the preservation of RLN. Sufficient knowledge of surgical anatomy, clinical experience, and meticulous surgical techniques are key factors in the identification and safe dissection of the RLN. During a thyroidectomy, the RLN can be identified using four different approaches, depending on the type of thyroid growth and choice of the surgeon: There are lateral, inferior, superior, and medial approaches.

The lateral approach is the most commonly used technique in primary thyroid surgery. The RLN is usually found by dissection around the inferior thyroid artery at the level of the middle lobe of the thyroid. RLN is generally found at the site of its entry into the neck region devoid of scar formation when the inferior approach is used especially in cases with secondary surgery. The superior approach is recommended for patients with an huge goiter or large substernal goiter. In this approach, the upper pole of the thyroid is first released and then pulled forward and laterally, and the RLN is exposed on the nerve’s entry point (NEP), into the larynx, under the cricopharyngeus muscle. The medial approach is preferred for patients with substernally or retropharyngeally enlarged goiters. In this approach, the isthmus is first dissected and divided, and then the isthmus and the medial part of the lobe are dissected away from the trachea to reveal the anterolateral part of the trachea. The fibers between the lateral aspect of the second or third tracheal rings and the thyroid, and the fibers of the Berry ligament are gradually dissected cranially, to allow RLN to enter into the field of view lateral to the trachea. The preservation of the anatomical integrity of the RLN does not indicate that its functional integrity is also preserved. IONM is a tool for the functional assessment of RLN, and so this method is an addition to visually identifying RLN, which is the gold standard. IONM significantly contributes to visual identification of the RLN, determination of its anatomical variations, intraoperative recognition of RLN injury, prevention of bilateral vocal cord paralysis, and detection and preservation of electrical activity in the nerve in patients with preoperative vocal cord paralysis.

Although there is no standardized method for the preservation of the EBSLN, 3 methods have been defined during the release of the upper pole of the thyroid. These methods involve dividing the branches of the superior thyroidal artery one by one on the capsule without visually identifying the EBSLN, searching and visually identifying the EBSLN before the dissection of the upper pole vessels, or detecting the EBSLN and dissecting the upper pole under the guidance of IONM. IONM also significantly contributes to the detection and confirmation of the EBSLN and dissection and preservation of the upper pole of the thyroid gland.

Nonrecurrent Laryngeal Nerve: Precise Detection by Electrophysiological Nerve Monitoring

Complication-free thyroid surgery is mainly based on the motor integrity of the recurrent laryngeal nerve (RLN). The nonrecurrent laryngeal nerve (non-RLN) is a rare anatomical variation that may increase the risk of vocal cord palsy. Early identification and exposure of the non-RLN may minimize injury risk. This case report presents functional detection of the non-RLN by intraoperative neuromonitoring (IONM).

Total thyroidectomy was performed under the guidance of IONM on a patient with bulky multinodular goiter. The first step of IONM is pre-dissection stimulation (V1) of the right vagus nerve (VN). V1 at a standard distal point was negative as indicated by the absence of both a sound signal and wave amplitude. The right VN was then followed proximally and dissected under the guidance of IONM. This dissection established a proximal point creating a positive signal that led us to determine the separation point of the non-RLN. The right non-RLN arising from the proximal VN was identified and fully exposed until laryngeal entry. Its motor integrity was confirmed with post-dissection signals. The left RLN was identified at the usual anatomical position that was fully exposed and preserved during thyroid surgery. Total thyroidectomy was then accomplished without complication. The postoperative period was uneventful. Postoperative laryngoscopy confirmed normal vocal cord function.

The non-RLN is accurately identified by IONM during the early part of the thyroid surgery. The absence of a distal VN signal is predictive of the non-RLN. IONM-guided proximal dissection of the right VN leads to the identification of the non-RLN. The prediction of the non-RLN by the absence of a VN signal during an early stage of surgery may prevent or minimize the risk of nerve injury.