Recurrent Laryngeal Nerve Stimulation Research Articles

The role of nerve monitoring to predict postoperative recurrent laryngeal nerve function in thyroid and parathyroid surgery

To determine the role and efficacy of intraoperative recurrent laryngeal nerve (RLN) stimulation in the prediction of early and permanent postoperative nerve function in thyroid and parathyroid surgery. A retrospective review of thyroid and parathyroid surgeries was performed with calculation of sensitivity and specificity of the response of intraoperative stimulation for different pathological groups. Normal electromyography (EMG) response with 0.5 mAmp stimulation was considered a positive stimulation response with postoperative function determined by laryngoscopy. No EMG response at >1-2 mAmps was considered a negative response. The rates of early and permanent paralysis, as well as sensitivity, specificity, and positive and negative predictive values for postoperative nerve function were calculated for separate pathological groups. The number of nerves at risk analyzed was 909. The overall early and permanent paralysis rates were 3.1% and 1.2%, respectively, with the highest rate being for Grave's disease cases. The overall sensitivity was 98.4%. The specificity was lower at 62.5% but acceptable in thyroid carcinoma and Grave's disease patients. The majority of nerves with a positive stimulation result and postoperative paralysis on laryngoscopy recovered function in 3 to 12 weeks, showing positive stimulation to be a good predictor of eventual recovery. Stimulation of the RLN during thyroid and parathyroid surgery is a useful tool in predicting postoperative RLN function. The sensitivity of stimulation is high, showing positive stimulation to be an excellent predictor of normal nerve function. Negative stimulation is more predictive of paralysis in cases of thyroid carcinoma and Grave's disease. 2b.

Value of recurrent laryngeal nerve monitoring in the operative strategy during total thyroidectomy and parathyroidectomy

ObjectiveThe purpose of this study was to assess the role of recurrent laryngeal nerve (RLN) monitoring in the operative strategy during total thyroidectomy and parathyroidectomy. Due to the risk of serious respiratory complications of bilateral recurrent nerve paralysis, two-stage surgery may be considered in the case on negative stimulation of the first side. Patients and methodsThis prospective study was conducted in 100 consecutive patients between May 2007 and March 2011. Translaryngeal monitoring was performed. When stimulation of the RLN on the first side dissected was negative, dissection of the other side was deferred to avoid the risk of bilateral RLN paralysis. ResultsThe main surgical indications were thyroid carcinoma (34%), Graves’ disease (27%), multinodular goitre (27%) and parathyroid hyperplasia (9%) with seven cases of redo surgery. Four RLN identified on the first side gave a negative response to stimulation and surgery to the other side was therefore deferred. Transient unilateral RLN paralysis was observed in these four patients and two cases of RLN paralysis were observed among patients with positive RLN stimulation. Among the 96 contralateral RLNs tested, two were not visualized (one case of transient RLN paralysis, one case of permanent RLN paralysis), two gave a negative response to stimulation (two cases of permanent RLN paralysis) and 92 gave a positive response to stimulation (nine cases of transient RLN paralysis, including two cases associated with transient paralysis of the first side, and one case of permanent RLN paralysis). The incidence of RLN paralysis by nerve was 9.6% for transient RLN paralysis and 2% for permanent (unilateral) RLN paralysis. ConclusionWhen bilateral RLN dissection is planned, RLN monitoring is particularly useful to limit the risk of bilateral RLN paralysis. Two-stage thyroidectomy, following functional recovery of the damaged RLN, can therefore be proposed. The risk of bilateral RLN paralysis was avoided in four patients, while transient bilateral RLN paralysis was observed in two patients despite positive stimulation.

Recurrent Laryngeal Nerve Monitoring during Thyroid Surgery

Injury to the recurrent laryngeal nerve (RLN) remains a significant cause of morbidity during thyroid surgery. Intraoperative nerve monitoring (IONM) is being applied in many centers to facilitate nerve identification. The aim of this study was to elucidate normative human vagal and recurrent laryngeal nerve electromyograhic (EMG) parameters during standard IONM application. A prospective IONM study conducted over an 8-month period. Internal review board (IRB) approval was obtained. Department of Otolaryngology, Division of Thyroid and Parathyroid Surgery, Massachusetts Eye and Ear Infirmary, Boston. All patients who were scheduled for routine thyroid, parathyroid, or neck exploration surgery were invited to participate. All patients had a preoperative and postoperative laryngeal examination to assess vocal cord function. Any patient with an abnormal preoperative laryngeal examination was excluded. Fifty-eight patients participated in this study. The right and left RLN latencies were similar. The left vagus latency was greater than the right vagus but was not significant. The RLN latency was significantly less than the vagus nerve. The right vagus nerve amplitude was significantly greater than the left. There was no difference between male and female amplitudes for either the RLN or vagus nerve. This study highlights the electrophysiological/EMG differences and similarities between the RLN and vagus nerve. Normative amplitude measurements for bilateral RLN and vagus nerve stimulation are presented. There are limited data available in the literature on normal RLN and vagal EMG signals generated during thyroid surgery.

Robotic Transaxillary Thyroid Lobectomy of a Follicular Neoplasm

Minimally invasive thyroid surgery using various techniques is well described. The purpose of this video is to show a robotic-assisted transaxillary right thyroid lobectomy for a follicular neoplasm with intraoperative nerve monitoring and stimulation of recurrent laryngeal nerve. Herein, we show our experience with the technique and its safety and feasibility. We performed a right thyroid lobectomy on a 33-year-old patient using the da Vinci-Si-HD Surgical System. The operation was done via a single axillary incision, 5cm in length. The flap creation time was approximately 26min. The robot docking time was 6min. The recurrent laryngeal nerve was identified and nerve stimulation was used to stimulate with 0.5milliamps. The operative console time was 21min. Total operative time was 69min. The procedure was successfully completed. Blood loss was minimal. Postoperative laryngoscopy showed intact and mobile bilateral vocal cords. There were no complications. Patient was discharged 4h after surgery. Robotic transaxillary endoscopic gasless thyroid surgery with monitoring and stimulation of the RLN is feasible and safe. This technique eliminates a visible neck scar and affords excellent high definition optics of the cervical anatomy. This new technique can be accomplished on an outpatient basis.

Neuronal Mechanisms Underlying the Laryngeal Adductor Reflex

Electromyographic studies of the laryngeal adductor reflex, glottal closure occurring in response to laryngeal stimulation, have demonstrated an early ipsilateral response (R1) and a late bilateral response (R2). To better define the physiologic properties of these responses, we recorded responses from expiratory laryngeal motoneurons (ELMs) in rats during stimulation of the superior laryngeal nerve (SLN). Single unit extracellular recordings were obtained from 5 ELMs, identified by their antidromic responses to recurrent laryngeal nerve stimulation and postinspiratory firing pattern, in 4 Sprague-Dawley rats. Unilateral stimulation of the SLN (at 20 Hz) stopped both phrenic nerve inspiratory activity and ELM postinspiratory activity. However, the ELMs displayed robust tonic firing, consisting of non-respiratory burst activity and single action potentials. The single action potentials were identified as short-latency ones (5 to 10 ms) activated by ipsilateral SLN stimulation, with an occurrence rate of 90%, and long-latency ones (20 to 50 ms) activated by bilateral SLN stimulation, with occurrence rates of 47% on the ipsilateral side and 58% on the contralateral side. The R1 response appears to be the result of the short-latency action potentials, orthodromically activated by ipsilateral stimulation of the SLN. The R2 response is likely to be a result of the long-latency action potentials that can be recorded from ELMs on both sides.

Feasibility on the Use of Intraoperative Vagal Nerve Stimulation in Gasless, Transaxillary Endoscopic, and Robotic-Assisted Thyroidectomy

Intraoperative nerve stimulation (IONS) could potentially reduce the incidence of recurrent laryngeal nerve (RLN) injury in thyroidectomy. The current study aimed at demonstrating the technical feasibility of using IONS in gasless, transaxillary endoscopic thyroidectomy (GTET) and robotic-assisted thyroidectomy (RAT) with conventional nerve stimulator probe and comparing the overall accuracy between two different nerve stimulation techniques, namely the direct RLN stimulation and the indirect stimulation via the vagus nerve (VN group), in predicting postoperative RLN function. From 2009 to 2010, 60 (17.1%) patients underwent endoscopic thyroidectomy using IONS. Thirty-three (55.0%) patients had direct intraoperative RLN stimulation (RLN group), whereas 27 (45.0%) patients had stimulation to the VN (VN group). Total number of nerves at risk was 76. The results of IONS were confirmed by the postoperative vocal cord movement on laryngoscopy. Patient demographics, surgical indications, resection type, size of dominant nodule, excised gland weight, and final pathology were similar between the two groups. The number of vocal cord palsies in the RLN and VN groups was 3 (7.3%) and 2 (5.7%), respectively. Compared with the VN group, the RLN group had a significantly lower percentage of true negatives (78.0% vs. 94.3%, P=.045) and higher percentage of false positives (14.6% vs. 0.0%, P=.018). Overall accuracy was higher in the VN group. The current study demonstrated the technical feasibility of using conventional open-nerve stimulator probe in both GTET and RAT. Indirect stimulation via the VN produced more reliable and accurate IONS test results than direct RLN stimulation.

Functional role of human laryngeal nerve connections

Current knowledge of the functional role of human laryngeal nerves is based on traditional laryngeal neuroanatomic descriptions or contradictory electromyographic studies. The aim of this study was to clarify the functional role of neural connections between laryngeal nerves by correlating the different electromyographic patterns observed after laryngeal stimulation and the existence of different neural connections. Descriptive. Electromyographic and morphologic study in 13 patients during total laryngectomy procedure. Seven patients showed an additional evoked response from the cricothyroid muscle after recurrent laryngeal nerve stimulation. External laryngeal nerve stimulation resulted in additional responses from the posterior cricoarytenoid muscle in three cases and from the arytenoid muscle in one. The presence of a neural connection was confirmed in all patients who showed an unexpected electromyographic response. The different connections between laryngeal nerves are at least partially of motor nature and play a role in the mobility of vocal folds.

Signalstabilität als Grundvoraussetzung für kontinuierliches intraoperatives Neuromonitoring

Postoperative recurrent laryngeal nerve (RLN) palsy is a well-known and dreaded complication of thyroid surgery. Continuous intraoperative neuromonitoring (CIONM) has been developed in order to provide an effective real-time surveillance of the RLN to its full extent and to detect subtle changes in nerve conductivity. A key requirement for a reliable interpretation of CIONM is signal stability. In a prospective randomized controlled pilot study 24 patients corresponding to 30 nerves at risk (NaR) were included to compare a newly developed, flexible, saxophone-shaped backstrap electrode for vagal stimulation (16 NaR) to a commercially available CE-marked cylindrical and rigid electrode (14 NaR). Electrode applicability, safety and signal stability were analyzed by assessment of electrode implantation times, stimulation currents, EMG amplitudes and electrode displacement rates. Implantation and extraction was significantly faster and easier with the saxophone-shaped backstrap electrode. Accidental electrode displacement occurred up to eight times per operation when applying the cylindrical electrode, while a total of two displacements resulted using the backstrap electrode in this study. Stimulation currents necessary for supramaximal RLN stimulation were significantly lower using the newly developed electrode. At the same time, significantly greater stable EMG amplitudes resulted using the new saxophone-shaped electrode. No RLN palsy occurred during the study. According to the data, only the saxophone-shaped backstrap electrode provided the signal stability required for CIONM. The closed electrode geometry with isolated contacts for nerve stimulation and defined current entry provide the prerequisites required for reliable continuous intraoperative neuromonitoring.

Errata

Equine Veterinary JournalVolume 43, Issue 1 p. 122-122 Free Access Errata This article corrects the following: Considerations for pacing of the cricoarytenoid dorsalis muscle by neuroprosthesis in horses N. G. DUCHARME, J. CHEETHAM, I. SANDERS, J. W. HERMANSON, R. P. HACKETT, L. V. SODERHOLM, L. M. MITCHELL, Volume 42Issue 6Equine Veterinary Journal pages: 534-540 First Published online: June 7, 2010 First published: 10 December 2010 https://doi.org/10.1111/j.2042-3306.2010.00345_2.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat In the article by Ducharme et al. (2010)‘Considerations for pacing of the cricoarytenoid dorsalis muscle by neuroprosthesis in horses’ published in the September issue of EVJ, there are the following 3 errors: Under Table 1 should read: *This horse did not have transection of adductor branch of recurrent laryngeal nerve. The legend for Fig 3b should read: b) minimal electrical stimulation (24 Hz, 0.427 ms and 3.5 mA) of the recurrent laryngeal nerve leading to maximal abduction angle of the left arytenoid cartilage after ipsilateral transection of adductor branch in a 2-year-old Thoroughbred colt. In the Results section, Stimulation-abduction trials, lines 8–11 should read: Conversely, stimulation of the entire recurrent laryngeal nerve after transection of the adductor branch resulted in clinically appropriate arytenoid cartilage abduction without evidence of vocal cord adduction (Fig 3b). We apologise for these errors. References Ducharme, N.G., Cheetham, J., Sanders, I., Hermanson, J.W., Hackett, R.P., Soderholm, L.V. and Mitchell, L.M. (2010) Considerations for pacing of the cricoarytenoid dorsalis muscle by neuroprosthesis in horses. Equine vet. J. 42, 534- 540. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Volume43, Issue1January 2011Pages 122-122 ReferencesRelatedInformation

Intraoperative Neuromonitoring for Early Localization and Identification of Recurrent Laryngeal Nerve During Thyroid Surgery

Early and definite identification of the recurrent laryngeal nerve (RLN) is an important step to avoid inadvertent nerve injury during complicated thyroid operations. This study aimed to determine the feasibility of routine use of intraoperative neuromonitoring (IONM) to localize and identify the RLN at an early stage of thyroid surgery. This prospective study enrolled 220 consecutive patients (333 RLNs at risk) who underwent thyroid operations with application of IONM. The RLN was localized and identified routinely with a nerve stimulator after opening the space between the thyroid and carotid sheath. The success rates of early RLN localization and identification were evaluated. The current for localization and the amplitude of evoked laryngeal electromyographic signals were also recorded and analyzed. All RLNs, including 87 (26%) nerves that were regarded as difficult to identify, were successfully localized and identified. The stimulation level for RLN localization was 2mA in 315 nerves (95%) and 3mA in the other 18 nerves (5%). The signal obtained from RLN localization (amplitude = 932 ±436μV) showed a clear and reliable laryngeal electromyographic response that was similar to that from direct vagus (amplitude=811±389μV) or RLN stimulation (amplitude=1132±472μV). The palsy rate was 0.6% and no permanent palsy occurred. RLN injury is rare if the nerve is definitely identified early in the thyroid operation. The conclusion of this study is that IONM is a reliable tool for early RLN localization and identification, even in complicated thyroid operations.

Investigation of optimal intensity and safety of electrical nerve stimulation during intraoperative neuromonitoring of the recurrent laryngeal nerve: A prospective porcine model

Intraoperative neuromonitoring (IONM) of the recurrent laryngeal nerve (RLN) has recently been more frequently applied in thyroid surgery. However, concerns have been raised regarding the safety and optimal intensity of electrical nerve stimulation. Eight piglets were enrolled, and electrically evoked electromyography (EMG) was recorded from the vocalis muscles via endotracheal surface electrodes. The baseline EMG was measured and continuous pulsatile stimulations were performed on the vagus nerve and RLN for 10 minutes. Changes of EMG waveform and cardiopulmonary status were analyzed. A dose-response curve existed with increasing EMG amplitude as stimulating current was increased, with maximum amplitude elicited on vagal and RLN stimulation at <1 mA. No obvious EMG changes and untoward cardiopulmonary effects were observed after the stimulation. Electrical stimulation is safe during IONM in this porcine model. Minimal current that required generating the maximal evoked EMG, approximately 1 mA in this study, can be selected to minimize the risk of nerve damage and cardiopulmonary effects.

Spontaneous and Evoked Laryngeal Electromyography of the Thyroarytenoid Muscles: A Canine Model for Intraoperative Recurrent Laryngeal Nerve Monitoring

We sought to determine the feasibility of performing spontaneous and evoked intraoperative laryngeal electromyography (L-EMG) using nerve monitoring equipment and to compare recording electrode configurations and methods of recurrent laryngeal nerve (RLN) stimulation in dogs. Four beagles underwent crush injury of the left RLN, and 2 beagles underwent left RLN transection. Serial spontaneous and evoked L-EMG was recorded with the NIM-Response nerve monitoring system under sedation. Transesophageal, percutaneous, and direct open RLN stimulation was performed. Recordings of spontaneous and evoked responses were made with endotracheal tube surface electrodes and bipolar vocal fold needle electrodes. The L-EMG procedures were repeated every 1 to 2 weeks after injury, and intersubject and intertrial differences were evaluated. Low-amplitude motor unit action potentials, polyphasic potentials, fasciculations, and fibrillations were detected in injured animals with bipolar needle recording electrodes with this system of spontaneous L-EMG. The surface recording electrodes did not detect pathologic waveforms. Percutaneous needle stimulation of the RLN is possible at currents slightly higher than those used for direct stimulation. Consistent, discrete, transesophageal stimulation of the RLN could not be reliably performed. Recording evoked responses with needle electrodes generated sharper waveforms, facilitating calculation of latency and wave duration. Evoked L-EMG utilizing surface recording electrodes limited the intertrial and intersubject variability of evoked amplitude. Typical patterns of nerve injury can be detected with this system of intraoperative L-EMG in a canine model. Quantitative measures of amplitude, latency, and wave duration in healthy and injured canine RLNs may be determined with this system.

Serial Electromyography of the Thyroarytenoid Muscles Using the NIM-Response System in a Canine Model of Vocal Fold Paralysis

We sought to determine whether serial intraoperative laryngeal electromyography (L-EMG) or evoked L-EMG predicts vocal fold (VF) recovery following iatrogenic injury. Six beagles were sedated, and bipolar needle electrodes were inserted into each thyroarytenoid (TA) muscle. Endotracheal tube surface electrodes were also placed. As the sedation lightened, L-EMG activity was recorded from all electrodes with an intraoperative nerve monitoring system. The neck was opened, and direct recurrent laryngeal nerve (RLN) stimulation was performed. Subsequently, 4 animals underwent crush injury of the left RLN,and 2 animals underwent nerve transection. The L-EMG procedures were repeated every 1 to 2 weeks until left VF motion was observed in the dogs that suffered RLN crush injury. At each time point, the neck was opened and both RLNs were stimulated. Fibrillation potentials were detected in all animals after RLN injury. A change to electrical silence was seen in the animals in the crush injury group that were evaluated during the week preceding VF recovery. Fibrillation potentials and VF immobility persisted in the transection group throughout the complete time course of these experiments. The first appearance of an evoked response coincided with or occurred after the return of left VF motion in the crush injury group. The threshold, latency, and amplitude differed from those of the controls and approached normal values over time. No response was detected in the transected nerves. The disappearance of fibrillations on intraoperative L-EMG was noted in the animals tested the week before the return of VF motion, and the return of motor unit action potentials was seen along with return of VF function. Evoked L-EMG was not helpful in predicting the return of VF mobility, but it may help quantify degrees of RLN injury and predict the speed of recovery.

Control of vocal fold cover stiffness by laryngeal muscles: A preliminary study

To perform preliminary measurements of the shear modulus of the vocal fold cover layer during intrinsic laryngeal muscle contraction. Shear modulus was measured in an in vivo canine larynx and an ex vivo human larynx. Shear stress was applied to the transverse axis of the vocal fold using a modified linear skin rheometer (LSR) via an attached suction probe. The probe displacement in response to the applied force was measured at various levels of laryngeal muscle contraction. The force-displacement data were used to derive the shear modulus using a simple shear model. In the ex vivo human larynx, lateral cricoarytenoid (LCA) muscle and cricothyroid (CT) muscle activity was simulated with arytenoid adduction and cricothyroid approximation sutures, respectively. In the in vivo canine, adductor muscle and CT muscle contraction was induced with graded stimulation of the recurrent laryngeal nerve (RLN) and the superior laryngeal nerves (SLN), respectively. : Baseline shear modulus was between 1,076 and 1,307 Pascals. In the ex vivo human larynx, the shear modulus increased gradually to a maximum of 1.6 times baseline value with graded arytenoid adduction and 3.7 times baseline value with cricothyroid approximation. In the in vivo larynx, the shear modulus increased to a maximum of 1.6 times baseline value with RLN stimulation and 2.5 times baseline value with SLN stimulation. Findings are in agreement with the cover-body model in that cricothyroid muscle activity generates a greater change in cover stiffness than laryngeal adductors. The role of the individual laryngeal adductors (thyroarytenoid [TA] vs. LCA) in control of vocal fold cover stiffness remains to be further elucidated.

Assessment of Recurrent Laryngeal Nerve During Thyroid Surgery With Laryngeal Mask Airway

To study the feasibility of using laryngeal mask anesthesia (LMA) with bronchoscopic evaluation of recurrent laryngeal nerve (RLN) integrity when stimulated. Single-institution prospective case series. A single, mid-Atlantic region academic medical center. Twenty-seven adult volunteers. Laryngeal mask anesthesia for thyroid surgery, monitored by flexible laryngoscopy and nerve integrity testing. Success rates for LMA use in thyroid surgery, bronchoscopic visualization of laryngeal glottis, and documentation of RLN integrity following surgery. We report our experience on 27 consecutive cases in which LMA with RLN stimulation was used for thyroid surgery. Twenty-five of 27 patients underwent successful LMA and visual documentation of RLN integrity by bronchoscopic inspection of nerve stimulation. Direct visualization of vocal cords using a fiberoptic bronchoscope via an LMA provides a safe and feasible method of laryngeal assessment following thyroid dissection. Continuous real-time video monitoring may be the next step in development of this technique as a patient safety measure for thyroid and parathyroid surgery.

Artificial Voice Modulation in Dogs by Recurrent Laryngeal Nerve Stimulation: Electrophysiological Confirmation of Anatomic Data

We hypothesized that voice may be artificially manipulated to ameliorate dystonias considered to be a failure in dynamic integration between competing neuromuscular systems. Orderly intrinsic laryngeal muscle recruitment by anodal block via the recurrent laryngeal and vagus nerves has allowed us to define specific values based on differential excitabilities, but has precluded voice fluency because of focused breaks during stimulation and the need to treat several neural conduits. Such problems may be obviated by a circuit capable of stimulating some axons while simultaneously blocking others in the recurrent laryngeal nerve, which carries innervation to all intrinsic laryngeal muscles, including the arguably intrinsic cricothyroideus. In 5 dogs, both recurrent laryngeal nerves received 40-Hz quasi-trapezoidal pulses (0 to 2000 microA, 0 to 2000 micros, 0 to 500 micros decay) via tripolar electrodes. Electromyograms were matched with audio intensities and fundamental frequencies recorded under a constant flow of humidified air. Data were digitized and evaluated for potential correlations. Orderly recruitment of the thyroarytenoideus, posterior cricoarytenoideus, and cricothyroideus was correlated with stimulating intensities (p < .001), and posterior cricoarytenoideus opposition to the thyroarytenoideus and cricothyroideus was instrumental in manipulating audio intensities and fundamental frequencies. Manipulation of canine voice parameters appears feasible via the sole recurrent laryngeal nerve within appropriate stimulation envelopes, and offers promise in human laryngeal dystonias.

Sensitivity and Specificity of Intraoperative Recurrent Laryngeal Nerve Stimulation Test for Predicting Vocal Cord Palsy after Thyroid Surgery

Recurrent laryngeal nerve (RLN) palsy after thyroidectomy, although infrequently encountered, can decrease quality of life. In addition to the hoarseness that occurs with unilateral RLN palsy, bilateral RLN palsy leads to dyspnea and often to life-threatening glottal obstruction. Therefore, intraoperative awareness of the nerve's status is of great importance. This study examined the sensitivity and specificity of a palpation technique to detect contraction of the posterior cricoarytenoid muscle (PCA) through the posterior hypopharyngeal wall while the RLN was being stimulated with a disposable nerve stimulator during thyroid surgery (the laryngeal palpation test) to predict postoperative RLN deficits. A total of 2197 RLNs in 1376 patients were identified to be at risk of injury during thyroidectomy performed between July 2003 and August 2004. Postoperative RLN integrity was assessed using direct laryngoscopy or laryngofiberoscopy to visualize vocal fold mobility. Altogether, 76 RLNs failed to elicit a PCA contraction in response to nerve stimulation, and 80 cases of temporary vocal cord palsy and 21 cases of permanent vocal cord palsy were recognized on postoperative evaluation. For postoperative vocal cord palsy, the sensitivity and specificity of the laryngeal palpation test were 69.3% and 99.7%, respectively, with a positive predictive value of 92.1% and negative predictive value of 98.5%. For permanent vocal cord palsy, the sensitivity and specificity were 85.7% and 97.3%, respectively, with a positive predictive value of 23.7% and negative predictive value of 99.8%. The laryngeal palpation test is not a particularly useful method for predicting the level of RLN function after thyroidectomy. All patients must be examined postoperatively by direct laryngoscopy or laryngofiberoscopy to check vocal cord mobility. Even if there is no contraction of the PCA and we detect vocal cord palsy immediately after surgery, vocal cord palsy often recovers within 1 year when visual preservation of RLN is successful.

Evoked Electromyographic Technique for Quantitative Assessment of the Innervation Status of Laryngeal Muscles

The purpose of this study was to develop a minimally invasive, noninjurious evoked electromyographic technique that could accurately quantitate the level of innervation of laryngeal muscles with recurrent laryngeal nerve stimulation. A four-phase study was conducted in 24 canines, including 1) identification of the best stimulation-recording configuration, 2) statistical analysis of sensitivity and accuracy, 3) evaluation of safety, and 4) identification of the laryngeal muscle(s) that contribute to the evoked response. The results demonstrated that an entirely noninvasive technique is not feasible. The stimulating cathode must be invasive to ensure discrete activation of the recurrent laryngeal nerve, whereas both recording electrodes should remain on the surface with one overlying the thyroid ala. This configuration proved to be highly accurate, with an error rate of only 6% to 7%, and with sensitivity sufficient to detect a signal in a nerve with fewer than 1% of the axons intact. There was no evidence of nerve injury in any animal over the course of 350 stimulus needle penetrations. By use of neuromuscular blockade to identify those muscles generating the surface response, the thyroarytenoid muscle was found to be the primary contributor, whereas the posterior cricoarytenoid muscle was uninvolved. This evoked electromyographic technique could provide quantitative information regarding the extent of muscle innervation during denervation and regeneration in case of laryngeal paralysis.

Nerve Monitoring and Stimulation During Endoscopic Neck Surgery in the Pig

To determine the feasibility of recurrent laryngeal nerve monitoring and stimulation during endoscopic neck surgery in an animal model. Prospective, nonrandomized experimental investigation in a porcine model. Bilateral recurrent laryngeal nerve monitoring and stimulation was accomplished during endoscopic neck surgery in five domestic pigs. Each pig was intubated with an electromyography endotracheal tube. Recurrent laryngeal nerve function was monitored throughout the endoscopic neck surgery with a nerve integrity monitor system. An endoscopic surgical pocket was created in the neck using blunt dissection followed by low-pressure carbon dioxide insufflation. Under direct endoscopic visualization, the trachea, thyroid gland, and associated vasculature were identified. The recurrent laryngeal nerve was identified on each side of the animal and was successfully stimulated with a monopolar stimulator probe. Ten of ten recurrent laryngeal nerves were successfully monitored and stimulated. No significant complications were encountered during the procedures. Recurrent laryngeal nerve monitoring and stimulation may be successfully accomplished during endoscopic neck surgery.

Intraoperative Monitoring of the Recurrent Laryngeal Nerve Using Acoustic, Free-Run, and Evoked Electromyography

The purpose of this study was to explore advantages and disadvantages of electrodes used for monitoring of the recurrent laryngeal nerve (RLN) and to determine the postoperative outcome in 135 patients. A pilot study examined 11 patients to compare the clinical performance of two commercially available recording electrodes and three stimulation electrodes for RLN neuromonitoring. After determining the most reliable and consistent stimulating/recording electrode combination, 124 patients were then monitored. In a total of 135 patients there was no permanent iatrogenic nerve damage to the RLN, although 2 patients developed transient vocal hoarseness that resolved shortly postoperatively. An initial "searching" current for evoked electromyography (EMG) was delivered at 0.7 mA and then decreased to a value of 0.35 mA, which reliably elicited RLN stimulation with a minimal incidence of false-positive results. The combination of auditory feedback from the mechanically elicited EMG and a control channel involving EMG monitoring of an additional peripheral muscle improved the detection of artifact and improved the clinical efficacy of intraoperative electromyography. A video camera showed the surgical manipulation on the EMG screen, allowing the neurophysiologist to correlate dissection with RLN status and location. Auditory responses from the nerve helped to determine the type of distress the RLN was encountering.