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Ann Thorac Surg 2007;83:388-392
© 2007 The Society of Thoracic Surgeons

Original Articles: General Thoracic

Recurrent Laryngeal Nerve Monitoring During Mediastinoscopy: Predictors of Injury

John R. Roberts, MD*, James Wadsworth, BS

The Surgical Clinic, The Sarah Cannon Cancer Center, Nashville, Tennessee

Accepted for publication March 31, 2006.

* Address correspondence to Dr Roberts, 2400 Patterson Street, Suite 309, Nashville, TN 37203 (Email: roberts{at}tsclinic.com).

Presented at the Fortieth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 26–28, 2004.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Discussion
 References
 
BACKGROUND: Recurrent nerve injuries occur during mediastinoscopy despite assiduous technique. We evaluated mediastinoscopy by monitoring laryngeal nerve stimulation during the surgery. These techniques utilize sensing electrodes on laryngeal masks to evaluate stimulus of the larynx, and are used to identify recurrent nerves during redo neck surgery.

METHODS: Fifteen patients were monitored during the entire mediastinoscopy. The laryngeal sensor was placed just before intubation. All patients had a suprasternal incision, digital dissection along the anterior wall of the trachea, and harvest of the nodes in the left paratracheal (4L), right paratracheal (4R), and subcarinal (7) positions. Cautery was used when needed in the subcarinal space and the right paratracheal groove.

RESULTS: Surprisingly, 14 of 15 patients demonstrated intense recurrent nerve stimulation during digital dissection along the anterior wall of the trachea. This dissection activated the right and left recurrent nerves. Though the use of cautery on the left caused significant laryngeal nerve activity, cautery in the subcarinal space and on the right caused very little activity. One patient was found to have a (transient) recurrent nerve injury after surgery. She demonstrated intense activity both during dissection along the anterior wall of the trachea, and during removal of a left paratracheal node.

CONCLUSIONS: Our data demonstrate that traction in the anterior mediastinum causes the greatest stimulation to the nerves, even greater than direct stimulation with current. Thus, these data suggest that injuries could result only from traction. Traction on both recurrent nerves can occur with dissection along the trachea. Laryngeal nerve monitoring can be used to direct biopsies in the left paratracheal groove.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Discussion
 References
 
Since its introduction by Carlens in 1959 mediastinoscopy has become and remains an important tool for the diagnosis of mediastinal lesions and staging of mediastinal malignancies. Cervical mediastinoscopy remains more accurate at staging lung cancer than the newer imaging modalities (computed tomographic scans and positron emission tomography scans) [1]. The procedure can be used to diagnose extrathoracic metastases to the mediastinum, to identify inflammatory and infectious lesions in the chest.

From the beginning, a low rate of certain complications from mediastinoscopy has been identified and includes mediastinal hemorrhage [2], incisional metastases, and recurrent nerve pareses or vocal cord palsies [3]. Though the rate of vocal cord palsies after mediastinoscopy has been generally reported at less than 1%, Widstrom [3 found that laryngoscopy before and after mediastinoscopy revealed vocal cord palsies in 6% of patients. Thoracic surgeons report that vocal cord palsies develop unexpectedly in patients in whom no dissection of the recurrent nerve, or even the left paratracheal groove, was done. We chose to study the activation of the left recurrent nerve during mediastinoscopy and report our evaluation of the intraoperative monitoring of vocal cord activity during mediastinoscopy.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Discussion
 References
 
Fifteen patients underwent recurrent laryngeal nerve monitoring using an endotracheal tube equipped with monitoring electrodes for electromyography. These tubes are used for redo neck surgery and have become the standard of care for redo neck surgery to avoid recurrent nerve injury. After induction of general anesthesia, the sensors (recording electrodes) were positioned to monitor both the right and left vocal cords. An identical reference electrode was occasionally needed and positioned in the pharyngeal folds. To document appropriate positioning of the endotracheal tube, the laryngeal nerves were stimulated transcutaneously at the neck on both sides alternatively with constant current stimuli of 0.1 ms duration. Using bipolar electrodes, 20 mA were delivered at 1.1Hz. At least three consecutive action potentials were recorded until a stable baseline was obtained.

Mediastinoscopy was performed in the usual way, with a transverse incision one fingerbreadth above the sternal notch, division of the subcutaneous tissue, and vertical division of the tissue between the strap muscles to separate the strap muscles. The trachea was then identified, and blunt dissection along the anterior wall of the trachea done to make space for the mediastinoscope. This dissection was carried down to index-finger depth.

Nodes were always harvested from at least three stations; right paratracheal, left paratracheal, and subcarinal space. The procedure was changed slightly to allow monitoring of the left recurrent nerve by stimulating the nerve with low doses of current upon first identification, after touching with the suction dissector, and finally after harvesting a left paratracheal node.

Action potentials were recorded at multiple points:

1 The neck skin incision
2 Blunt digital dissection along the anterior border of the trachea
3 Harvest of the right paratracheal nodes
4 Harvest of the subcarinal nodes
a Cautery to dry the subcarinal space

5 Harvest of the left paratracheal nodes
a Study of the left recurrent nerve
i Dissection to identify the nerve
ii Touching/stimulating the nerve with a probe and low current
iii Touching the nerve with suction—no current
iv Stimulating the nerve again
v Dissecting and harvesting left paratracheal nodes
vi Stimulating the nerve again


The study was presented to the chief of the Institutional Review Board at Vanderbilt University Hospital. Because these same techniques are commonly used to monitor recurrent nerve function in other surgeries, the study was considered observational and approved.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Discussion
 References
 
Twenty patients underwent electromyography during mediastinoscopy. Data from five patients were discarded because the left recurrent nerve could not be stimulated on the left due to bulky nodal disease in the left paratracheal groove. Demographics of the fifteen patients who are reported here are given in Table 1.


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  		Table 1. Demographics of Patients Undergoing Electromyography During Mediastinoscopy
 
Action potentials at the larynx were recorded at the points of the surgery demonstrated above. There was no significant stimulation with the skin incision or the cautery division of the subcutaneous tissue.

Surprisingly, the greatest amplitudes at the vocal cords were found with the digital dissection along the trachea, with intense stimulation of both the right and left recurrent nerves and thus vocal cords. This stimulation was so intense that some of the recordings from the initial patients had to be discarded because the scale had been set too low.

The right paratracheal nodes and subcarinal nodes were harvested next. Cautery was used as needed to dry the right paratracheal space and the subcarinal space. There was little stimulation of either vocal cord.

Extra analysis was done on the left side. The recurrent nerve was identified when possible; this report is limited to those patients in whom the nerve was identified. The nerve was then stimulated with a probe and low current, a left paratracheal node harvested, the nerve stimulated again, the nerve touched with the suction dissector, and then stimulated for a third time. After each intervention (node harvest and contact with the nerve), the response evoked in the vocal cord by the stimulating current was significantly less than the baseline stimulation (Table 2).


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  		Table 2. Response During Mediastinoscopy
 
One patient developed a transient (five months) vocal cord paresis after mediastinoscopy with monitoring of the recurrent nerve. Her surgery and evoked potentials were no different than the majority of the other patients, though the pathology was different among all the patients.


   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Discussion
 References
 
The anatomy of the recurrent nerves is relatively, though not completely, constant [4]. Most series find a small but significant incidence (1%) of direct, rather than recurrent, laryngeal nerves on the right. The nerves on the left are almost always recurrent, though there may be multiple branches of the recurrent nerves on either side. Injury of the branches of the recurrent nerve to the larynx result in speech difficulties; injury of the recurrent nerve branches to the esophagus result in swallowing difficulties [5].

Vocal cord paralysis secondary to lung cancer involvement is well known; bilateral paralysis may even result, and is more dangerous. Other tumors may more rarely cause paresis, and include glomus body tumors. Radiation for head and neck cancers can cause palsies of the hypoglossal and recurrent nerves [6].

Medical causes (Table 3) of vocal cord paralysis include histoplasmosis, Reye’s syndrome, relapsing polychondritis, vincristine, Shye-Drager syndrome, rheumatoid arthritis, silicosis, and amyloidosis. Anatomic causes of vocal cord paralysis include cervical osteophytes, thoracic aortic aneurysms, thoracic aortic rupture, sarcoidosis, benign thyroid adenomas and goiters, and tuberculosis (Table 4).


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  		Table 3. Medical Causes of True Vocal Cord Paralysis
 

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  		Table 4. Anatomic Causes of Recurrent Nerve Paralysis
 
Operative Causes of Vocal Cord Paralysis
Thyroid surgery is the most common cause of recurrent nerve injury, and gives a rate approaching 5% in larger series with close follow-up [7]. These complications are probably more common in redo neck surgery. Many different causes of operative injury to the vocal cords have been demonstrated, including intubation and placement of esophageal stethoscopes [8], pacemaker placement [9], median sternotomy, carotid surgery (2.31% to 4%), patent ductus closure (5.2%) [10], internal mammary artery harvest (<1%), reoperative aortic coarctation repair (36%), esophagectomy (5% to 22%), and anterior approach to the cervical spine (5%) (Table 5) [11].


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  		Table 5. Surgical Causes of True Vocal Cord Paralysis
 
Incidence of Vocal Cord Paralysis After Mediastinoscopy
The incidence of vocal cord paralysis probably depends upon the intensity of the search for the problem. Most series have reported a very low rate of less than 1%, but Widstrom [3] found a 6% rate of at least temporary paralysis when all patients underwent preoperative and postoperative indirect laryngoscopy.

Our proposed mechanism of traction injury to the recurrent nerve due to digital dissection is supported by the findings of stretch injury to the right recurrent nerve with the anterior cervical approach [11]. Many thoracic surgeons have described a surprising finding of recurrent nerve paralysis when little or no dissection was done in the left paratracheal groove, and limited or no biopsies were done on the left side. Together, these data suggest that indirect, or traction, tension on the nerve may be as likely or as important a mechanism of injury to the nerve as biopsy or cautery injury.

Neural Monitoring in the Chest
Endolaryngeal devices for monitoring the recurrent nerve were first described in the 1980s [12] and have become common in neck cancer surgery [13]. Laryngeal electromyography has been useful in predicting the likelihood of recovery of function in patients with paralyzed nerves [14]. Recurrent nerve and facial nerve monitoring has been shown to decrease the incidence of permanent nerve injuries in patients undergoing thyroid, parathyroid, and parotid surgery [15]. However, the technique appears to be predominantly useful to identify the exact position of the nerve, as even a loss of activity with repetitive stimulation (as we found) has not consistently demonstrated paresis of the nerve in other studies [16]. Our findings do not indicate that recurrent nerve monitoring should be used during mediastinoscopy, but suggest the following: (1) recurrent nerve injuries may result from traction on the nerve; (2) recurrent nerve monitoring may be useful during mediastinoscopy when bulky nodes in the left paratracheal groove must be harvested.

Phrenic nerve monitoring during coronary artery bypass surgery has become more common. The use of these techniques is not common in thoracic surgery, perhaps because a recurrent nerve injury is a relatively minor problem compared with lung cancer and myocardial infarction, but a big problem compared with removing what proves to be a benign neck mass. However, electromyography is becoming the standard of care for redo neck surgery, and may be useful in many thoracic procedures, especially redo procedures around the aortopulmonary window. Our data do not indicate that recurrent nerve monitoring is needed for standard cervical mediastinoscopy.

Therapy of Recurrent Nerve Paralyses
The most common methods of therapy for unilateral cord paralyses are different medialization techniques using fat, Teflon, medialization thyroplasty, or arytenoids adduction [17]. In the short term, vocal cord injection using absorbable substances is the most reasonable as many patients with symptomatic recurrent nerve palsies will recover, though they may require six months to do so. Medialization thyroplasty utilizes silicone struts to medialize the paralyzed vocal cord, and is the procedure of choice for many otolaryngologists. This procedure may be combined with an arytenoids adduction, depending upon the patient’s anatomy.


   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Discussion
References
 
DR MALCOLM M. DECAMP (Cleveland, OH): We have had a little bit of experience using this monitor in working with our ENT (ears, nose, and throat) colleagues in the management of bulky head and neck tumors and in reoperative surgery at the thoracic inlet. One of the things that I’ve learned is that, in order to use this monitoring device, you can’t use muscle relaxants. This is interesting during a long case because the anesthesiologist has to stay focused. We encounter what might be interpreted as false-positive results as the patients lighten. As patients begin to emerge from anesthesia, the cords actually start to move and you get a signal and a read-out. Could you comment on that phenomenon and how you handled it? Was that a contributing reason to your learning curve? Finally, would you recommend that we move towards more routine use of this tube or just more selective lymph node sampling? I appreciated the opportunity to read the abstract and hear the talk.

DR ROBERTS: Thank you, Mack. For the first question, that’s definitely true, there is a learning curve to it. This is a mediastinoscopy rather than a big head and neck case, so we had a shorter time period to have to deal with. But there is no question that position of the tube is not absolutely straightforward. These electrodes are wrapped around the ET (endotracheal) tube for about a 3 cm to 4 cm length. If the tube gets positioned too far in or too far out, then they don’t contact the vocal cords anymore and then they won’t pick up the muscle activity. So yes, there is no question that there is a little bit of a learning curve to it, but it is relatively short, and for a short procedure like this, you can get at them. Now I use this when I’m trying to biopsy bulky things in the left paratracheal space, and I’ve used it a time or two when I’ve had a large tumor in the left upper lobe or the left hilum and I’m worried about the nerve in the AP window. So I use it now much more than I did, although I’ve really changed my practice more by selecting which patients have to have harvest of the 4L node.

DR JOSE RIBAS M. DECAMPOS (São Paulo, Brazil): I appreciate your paper. In Sao Paulo we are using a video mediastinoscope. This can improve your image a lot, and maybe we are changing the way to perform mediastinoscopy. If you have this equipment here, would you maybe also think that you would change the way of doing the mediastinoscopy?

DR ROBERTS: I appreciate your comments. I actually had thought about that in preparing the paper, when I was writing it. I had gotten into the habit of making a lot of room in the mediastinum with my finger in order to put the rigid scope in and that conceivably that might be part of the problem, and by using a videoscope where you have more visualization as you’re going in, conceivably you might put less traction on the nerve.

DR JAMES D. LUKETICH (Pittsburgh, PA): I enjoyed the paper. During the anterior dissection of the pretracheal plane, I wouldn’t generally think of being in the area of damaging a nerve. Do you think we may be stimulating it due to the intense tracheal pressure and stimulation and bouncing up and down on the endotracheal tube? For example, If you just move the endotracheal tube back and forth within the trachea, does it give you a stimulation reading of the recurrent nerve, could this be possibly a false-positive just due to tracheal stimulation.

Secondly, have you tried using this on another case where you’re really worried about nerve injury; for example, esophagectomy or other operations. Have you any experience with it and perhaps avoiding injury in other scenarios?

DR ROBERTS: Thank you very much. Just moving the tube around won’t cause you to get firing of this, although I take your point that we move the trachea enough occasionally when you’re dissecting that it’s possible you get it. Because of that, we went back and tried to see if we could recreate it, and it really seems to be more the dissection, because once you get in, once you have space for your finger and then try to move up and down, we weren’t able to get to the same activity. The second question, we have done it in a couple of folks with esophagectomies and didn’t get enough data to be able to understand what it means.

DR ROBERT B. LEE (Jackson, MS): The other case that I think it’s real useful in, and this was brought to my attention sort of as to you, in the head and neck cases, we do a fair number of large substernal goiters and it’s very useful in mobilizing the thyroid and getting it up off the left side when it’s down on that side and obviously off to the right when it comes around the subclavian, and I have found it very useful. We have a series of over 24 where we used it. Not a published series, but we have not had a single recurrent nerve injury, and that’s where I think it has been most beneficial to me.

DR ROBERTS: Yes, and I think the literature now is such that this type of monitoring decreases the risk of injury for neck surgery and redo surgery such that it has become the standard of care for those tumors.

DR DAVID RICE (Houston, TX): I enjoyed your presentation on this novel method of monitoring of the recurrent nerve. However, one patient in 15, despite monitoring, still had a recurrent nerve injury, and I was just wondering, since suction as well as cautery seems to result in activation of signals, how do you propose to use this to prevent injury to the nerve?

DR ROBERTS: Well, at least the way I have described or ascribed this complication is due to the fact that we were doing a lot more manipulation to the left nerve than I would normally do for the purposes of studying it. I think that, again, the main point of the study is trying to avoid any activity on the left side. Exposing the nerve, putting any sort traction on it will ding it.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Discussion
 References
 

  1. Cerfolio RJ, Ojha B, Bryant AS, Bass CS, Bartalucci AA, Mountz JM. The role of FDG-PET scan in staging patients with nonsmall cell carcinoma Ann Thorac Surg 2003;76:861-866.[Abstract/Free Full Text]
  2. Carlens E, Hambraeus GM. MediastinoscopyIndications and limitations. Scand J Respir Dis 1967;48:1-10.[Medline]
  3. Widstrom A. Palsy of the recurrent nerve following mediastinoscopy Chest 1975;67:365-366.[Medline]
  4. Katz A. Extralaryngeal division of the recurrent laryngeal nerveReport on 400 patients and the 721 nerves measured. Am J Surg 1986;152:407-410.[Medline]
  5. Bhattacharyya N, Kotz T, Shapiro J. Dysphagia and aspiration with unilateral vocal cord immobility: incidence, characterization, and response to surgical treatment Ann Otol Rhinol Laryngol 2002;111:672-679.[Medline]
  6. Lin Y, Jen Y, Lin J. Radiation-related cranial nerve palsy in patients with nasopharyngeal carcinoma Cancer 2002;95:404-409.[Medline]
  7. Lo C, Kwok K, Yuen P. A prospective evaluation of recurrent laryngeal nerve paralysis during thyroidectomy Arch Surg 2000;135:204-207.[Abstract/Free Full Text]
  8. Friedman M, Toriumi D. Esophageal stethoscopeAnother possible cause of vocal cord paralysis. Arch Otolaryngol Head Neck Surg 1989;115:95-98.[Medline]
  9. Zohar Y, Buller N, Shvilly Y. Recurrent laryngeal nerve paralysis during transvenous insertion of a permanent endocardial pacemaker Ann Otol Rhinol Laryngol 1993;102:810-813.[Medline]
  10. Davis JT, Baciewicz FA, Suriyapa S, Vauthy P, Polamreddy R, Barnett B. Vocal cord paralysis in premature infants undergoing ductal closure Ann Thorac Surg 1988;46:214-215.[Abstract]
  11. Weisberg N, Spengler D, Netterville J. Stretch-induced nerve injury as a cause of paralysis secondary to the anterior cervical approach Otolaryngol Head Neck Surg 1997;116:317-326.[Medline]
  12. Hvidegaard T, Vase P, Dalsgaard SC, Jorgensen K. Endolaryngeal devices for perioperative identification and functional testing of the recurrent nerve Otolaryngol Head Neck Surg 1984;92:292-294.[Medline]
  13. Hemmerling TM, Schmidt J, Bosert C, Jacobi KE, Klein P. Intraoperative monitoring of the recurrent laryngeal nerve in 151 consecutive patients undergoing thyroid surgery Anesth Analg 2001;93:396-3993rd contents page.[Abstract/Free Full Text]
  14. Sittel C, Stennert E, Thumfart WF, Dapunt U, Eckel HE. Prognostic value of laryngeal electromyography in vocal fold paralysis Arch Otolaryngol Head Neck Surg 2001;127:155-160.[Abstract/Free Full Text]
  15. Brennan J, Moore E, Shuler K. Prospective analysis of the efficacy of continuous intraoperative nerve monitoring during thyroidectomy, parathyroidectomy, and parotidectomy Otolaryngol Head Neck Surg 2001;124:537-543.[Medline]
  16. Thomusch O, Sekulla C, Machens A, Neumann HJ, Timmermann W, Dralle H. Validity of intra-operative neuromonitoring signals in thyroid surgery Langenbecks Arch Surg 2004;389:499-503.[Medline]
  17. Chester M, Stewart M. Arytenoid adduction combined with medialization thyroplasty: an evidence-based review Otolaryngol Head Neck Surg 2003;129:305-310.[Medline]




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