Abstract
Purpose
The nonrecurrent laryngeal nerve (NRLN) is a rare anatomical variant but associated with high risk of nerve injury during thyroid and parathyroid operations. Therefore, intraoperative detection and verification of NRLN are necessary.
Method
A total of 390 consecutive patients who underwent thyroid and parathyroid operations (310 RLNs dissected on the right side and 293 nerves on the left side) were enrolled. Electrically evoked electromyography was recorded from the vocalis muscles via an endotracheal tube with glottis surface recording electrodes. At an early stage of operation, vagal nerve was routinely stimulated at the level of inferior thyroid pole to ensure normal path of RLN. If there is a negative response from lower position but positive response from upper vagal stimulation, it indicates the occurrence of a NRLN, and we localize its separation point and path.
Results
Four right NRLNs (1.3%) without preoperative recognition were successfully detected at an early stage of operation. Three patients were operated on for thyroid disease, one for parathyroid adenoma and all were associated with right aberrant subclavian artery. All NRLNs were localized and identified precisely with intraoperative neuromonitoring. Functional integrity of all nerves was confirmed by the intraoperative neuromonitoring and postoperative laryngeal examination.
Conclusions
Vagal stimulation at the early stage of operation is a simple, useful, and reliable procedure to detect and identify the NRLN.
1
Introduction
The nonrecurrent laryngeal nerve (NRLN) is a rare anatomical variant, with an incidence of 0.3% to 1.6% on the right side and 0.04% on the left side . The risk of nerve injury was reported with very high rate up to 75% in case the occurrence of NRLN was not recognized preoperatively .
Most NRLNs were reported to be associated with a right aberrant subclavian artery. Therefore, some methods such as the symptom of dysphagia, chest x-ray, barium swallow test, esophageal endoscopy, computed tomography, magnetic resonance imaging, angiography, and neck ultrasonography were reported to be used to predict the presence of NRLN preoperatively by discovering the aberrant subclavian artery . However, the NRLN was reported very difficult to identify preoperatively: 0% in the series of Toniato et al and Brauckhoff et al , 6% diagnosed with certainty, and 24% merely suspected in the series of Henry et al . Furthermore, NRLN can occur without subclavian artery anomaly or occur on the left side . During the operation, a large sympathetic RLN anastomotic branch can be mistaken as NRLN , and the NRLN may be associated with a second smaller right RLN in the normal RLN position that may be mistaken as the entire RLN . Therefore, intraoperative detection and verification of NRLN are necessary.
Intraoperative neuromonitoring (IONM) has been commonly applied in thyroid and parathyroid operation as a means to localized and identify RLN, to predict the outcome of vocal function and to elucidate the mechanism of RLN injury , but the usefulness of IONM for detecting and identifying NRLN was only described by Brauckhoff et al in 2002. This study aimed to verify the value of vagal neurostimulation at an early stage of operation in detecting and identifying NRLN during thyroid and parathyroid operations.
2
Materials and methods
From April 2006 to May 2010, 390 consecutive patients who underwent thyroid or parathyroid operations with the application of IONM were enrolled. There were 163 unilateral and 227 bilateral procedures. Fourteen nerves were excluded from this study due to preoperative palsy and/or cancer invasion noted intraoperatively. Thus, in all, 603 RLNs (310 right, 293 left) were at risk in this study. All patients were intubated with Nerve Integrity Monitor (NIM) Standard Reinforced EMG Endotracheal Tube (6.0 mm for women and 7.0 mm ID for men) (Medtronic Xomed, Jacksonville, FL) for general anesthesia. The middle of the exposed electrodes was placed in contact with the true vocal cords. A Pass monopolar stimulation probe (Medtronic Xomed) was used for nerve stimulation during the operation. Electromyography (EMG) activity was recorded and displayed on a NIM-response 2.0 monitor (Medtronic Xomed). At the early stage of operation, the vagus nerve was routinely tested to ensure functional monitoring system and the normal pathway of RLN after the space between the thyroid and carotid sheath was open. The vagus was typically stimulated at the level of inferior thyroid pole with current of 2 mA for direct stimulation or 3 mA for indirect stimulation (without dissection of carotid sheath). A case of negative EMG signals at lower position and positive EMG signals at upper position indicates the occurrence of a NRLN. The separation point and path of NRLN will be localized and identified precisely at upper trachea-esophageal groove ( Video ).
All patients received preoperative and postoperative laryngofibroscopic examination of cord mobility. During the operation, functional nerve integrity is documented and confirmed with the registration of EMG signals, and all exposed RLNs or NRLNs are routinely photographically documented to show visual nerve integrity. The study was approved by the institutional review board of Kaohsiung Medical University Hospital and the ClinicalTrials.gov ( http://www.clinicaltrials.gov . [identifier: NCT00629746]). Written informed consent was obtained from each patient. Patients were informed of the intent to use this monitoring system potentially to aid in the localization and identification of the RLNs and in the assessment of their function during operation. There was no financial or professional association between the authors and the commercial company whose nerve-monitoring product was used.
2
Materials and methods
From April 2006 to May 2010, 390 consecutive patients who underwent thyroid or parathyroid operations with the application of IONM were enrolled. There were 163 unilateral and 227 bilateral procedures. Fourteen nerves were excluded from this study due to preoperative palsy and/or cancer invasion noted intraoperatively. Thus, in all, 603 RLNs (310 right, 293 left) were at risk in this study. All patients were intubated with Nerve Integrity Monitor (NIM) Standard Reinforced EMG Endotracheal Tube (6.0 mm for women and 7.0 mm ID for men) (Medtronic Xomed, Jacksonville, FL) for general anesthesia. The middle of the exposed electrodes was placed in contact with the true vocal cords. A Pass monopolar stimulation probe (Medtronic Xomed) was used for nerve stimulation during the operation. Electromyography (EMG) activity was recorded and displayed on a NIM-response 2.0 monitor (Medtronic Xomed). At the early stage of operation, the vagus nerve was routinely tested to ensure functional monitoring system and the normal pathway of RLN after the space between the thyroid and carotid sheath was open. The vagus was typically stimulated at the level of inferior thyroid pole with current of 2 mA for direct stimulation or 3 mA for indirect stimulation (without dissection of carotid sheath). A case of negative EMG signals at lower position and positive EMG signals at upper position indicates the occurrence of a NRLN. The separation point and path of NRLN will be localized and identified precisely at upper trachea-esophageal groove ( Video ).
All patients received preoperative and postoperative laryngofibroscopic examination of cord mobility. During the operation, functional nerve integrity is documented and confirmed with the registration of EMG signals, and all exposed RLNs or NRLNs are routinely photographically documented to show visual nerve integrity. The study was approved by the institutional review board of Kaohsiung Medical University Hospital and the ClinicalTrials.gov ( http://www.clinicaltrials.gov . [identifier: NCT00629746]). Written informed consent was obtained from each patient. Patients were informed of the intent to use this monitoring system potentially to aid in the localization and identification of the RLNs and in the assessment of their function during operation. There was no financial or professional association between the authors and the commercial company whose nerve-monitoring product was used.
3
Results
Four right NRLNs (1.3% in the right and 0% in the left side) without preoperative recognition were detected due to negative response from the lower portion vagal stimulation but positive response from the upper portion vagal stimulation. The separation point and path of the NRLNs were localized and identified precisely at upper trachea-esophageal groove with IONM. One nerve was type I anomaly ( Fig. 1 ) in which the nerve directly arises from the vagus nerve above the laryngotracheal junction and descends into the larynx, running parallel to the trunk of superior thyroid artery. Coexisting small nerve branches in the normal RLN position was also found; they could be small collateral branches to the trachea and the esophagus and could be mistaken as the normal RLN if no IONM was applied. Another 3 were type II anatomical variant ( Figs. 2–4 ) arising from the vagus nerve below the laryngotracheal junction and running parallel to the trunk of inferior thyroid artery. Two patients were operated on for thyroid cancer, one for multiple nodular goiter and the other for right superior parathyroid adenoma ( Table 1 ). All patients were further confirmed to have right aberrant subclavian artery on computed tomography scan, so-called arteria lusoria, but none of them reported the symptoms preoperatively. Visual and functional integrity of all NRLNs was confirmed during the operation, and symmetric vocal cord movement was also confirmed after the operation.
