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Recurrent laryngeal nerve location in thyroidectomy and parathyroidectomy: Use of an indwelling laryngeal surface electrode with evoked electromyography
RECURRENT LARYNGEAL NERVE LOCATION IN THYROIDECTOMY AND PARATHYROIDECTOMY: USE OF AN INDWELLING LARYNGEAL SURFACE ELECTRODE WITH EVOKED ELECTROMYOGRAPHY J. LEE REA, MD, ANJUM KHAN, MD
Injury to vital neural structures remains a source of major morbidity in head and neck surgery. Electromyographic (EMG) monitoring has been successfully used during otologic and parotid surgery. However, its use in thyroid and parathyroid surgery has been less than routine even though preservation of recurrent laryngeal nerve function is essential to voice and airway. We describe our technique with the use of a postcricoid surface electrode for evoked EMG monitoring in thyroid and parathyroid surgeries. Technique of insertion of the electrode is described. Operative thyroidectomy is described for either close-capsular dissection or initial nerve location. Equipment required and monitoring technique is described for both methods and outcomes are briefly discussed. These results support evoked EMG by postcricoid surface electrode as a useful practical tool in identification of recurrent nerve in thyroid and parathyroid surgery.
Surgical nerve location has evolved through advances in anatomic understanding and improvements in devices available to assist the surgeon. Initially there was only anatomic nerve location, then nerve stimulation with an electronic nerve stimulator with visual or tactile observation of the resulting movement, and more recently, stimulation of the nerve and detection of the evoked electromyographic (EMG) potential from the target muscle. This pattern of improved technique has occurred with surgery of the facial nerve culminating in the use of EMG in neurotologic and skull base procedures. In this report we describe the experience of two surgeons with an indwelling laryngeal electrode (Fig 1) to allow evoked electromyography for operative identification of the recurrent laryngeal nerve.
MONITORING OF THE LARYNX WITH NEEDLE ELECTRODES Many articles have been written proposing methods of recurrent laryngeal nerve location. The technology developed for facial nerve location has been applied to the problem of laryngeal nerve location using needle electrodes. Needle electrodes may be used for diagnostic EMG of the larynx but are not suited for monitoring in this structure. Parnes 1 notes that the laryngeal muscles examined are small, the technique of (needle) elecFrom the Department of Otolaryngology and Head and Neck Surgery, St. Mary's Medical Center, Jefferson City, MO. Address reprint requests to J. Lee Rea, MD, 1021 Northeast Dr, Jefferson City, MO 65109. Copyright © 1994 by W.B. Saunders Company 1043-1810/94/0502-0006505.00/0
trode insertion is blind," and that failure of EMG may "reflect sampling error." Lipton et al 2 also demonstrate this problem. Payne et al 3 note that the exact location of laryngeal EMG needle electrodes cannot be known if they are placed after induction of general anesthesia. Mu and Yang 4 describe the high degree of skill required and also safety concerns in laryngeal needle electromyography. Rea et al 57 and Davis et al s have extensive experience in laryngeal needle electromyography for monitoring purposes and report concerns for possible electrode fragmentation, electrode displacement with possible vascular injury or laceration from sharp needles, and possible inadvertent deflation of the endotracheal tube balloon. The possibility of hook-wire electrode 2 shearing off from insertion and creation of a foreign body in the vocal cord has not been reported to our knowledge, but this complication would seem to be inevitable with routine monitoring by this technique. Vocal cord hematoma occurs frequently with needle electrode monitoring 3 and causes postoperative anxiety for possible recurrent laryngeal nerve injury. All needle electrodes without barbs or retention devices risk possible displacement or laryngeal injury by any movement of the patient during surgery. Because of these problems and risks, a concerted effort to develop a noninvasive, easily initiated method of performing recurrent laryngeal-nerve-evoked monitoring has been undertaken with results that are presented in here.
MONITORING THE LARYNX WITH SURFACE ELECTRODES The ideal laryngeal electrode would be totally noninvasive, would not interfere with intubation or operative manipulations, would provide a stable platform for signal
OPERATIVE TECHNIQUES IN OTOLARYNGOLOGY--HEAD AND NECK SURGERY, VOL 5, NO 2 (JUN), 1994: PP 91-96
91
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FIGURE 2. Evoked CAP as recorded from LSE. Courtesy of George Washington University Hospital Neurophysiology Laboratory.
FIGURE 1. Laryngeal surface electrode as provided by RLN Systems, Inc, Jefferson City, MO. collection, and would be secure from dislocation. Kimura 9 states: "Surface electrodes, in general, are better suited than needle electrodes for recording evokedmuscle action potentials. The response recorded by surface electrodes is a compound action potential with contributions from all the muscle fibers innervated by the nerve . . . . This is in contrast to recording from a needle electrode, which registers only a small portion of the muscle action potential." The typical compound motor action potential (CAP) (Fig 2) that occurs with evoked EMG is better examined with surface electrodes because the signal is several orders of magnitude larger. Correspondingly, less sensitive monitoring devices are required. The CAP is so large and so characteristic in its latency from the stimulating event that equipment can be designed for electronic identification of a stimulation event. A monitoring assistant can be employed if equipment with higher capability is to be used. A strip electrode carried either side of the endotracheal tube was fabricated in 1977 and found on testing to be "noisy" because of excessive movement of the tube with 92
respiration and the minimal surface contact. An endotracheal tube electrode contacts the vocalis muscle only at one point made by the cross of the electrode and vocalis tendon and acts more like a needle electrode in its electrical characteristics. This configuration may actually be compared more with an antenna than a surface electrode. Other endolaryngeal strategies examined included devices dipped to the posterior lamina of the cricoid, double balloon devices to straddle and fix the cord, or other even more fanciful arrangements. We read with great interest an article by Woodson 10 describing an extralaryngeal surface electrode to be carried on a nasoesophageal tube for noninvasive monitoring of the posterior cricoarytenoideus muscle (PCA) EMG. This was done on the awake patient by positioning the electrode in the postcricoid space, Payne et al i 1 also described a postcricoid laryngeal surface electrode (LSE) used perorally for awake, diagnostic laryngeal EMG. The postcricoid hypopharynx provides the ideal locus for surface electrode laryngeal EMG: a broad potential space to abut the largest muscles of the larynx. After multiple modifications and clinical trials, a LSE has been developed that meets the criterion for an ideal monitoring device. 12 The retrolaryngeal, retrocricoid location defines the construction and use of this electrode. This human adult larynx anatomy has proven to have a rather narrow range of measurements in cadaver studies. 13 The cricoid cartilage, for example, has been found to measure approximately 2.5 cm in breadth posteriorly in females and about 3.0 cm in males. Likewise, the postcricoid hypopharynx, in the anatomically normal adult, has predictable a n a t o m y [ 4"1s The vertical ridges of the lateral edge of the thyroid cartriage and inferior cornu laterally, the cricopharyngeus sphincter and introitus to the esophagus inferiorly, and the spine supporting the hypopharynx posteriorly form a space that stabilizes the electrode in every dimension except superiorly (Fig 3). The weight of the larynx resting REA AND KHAN
Endotracbeal " tube Epiglottis bone
Inf. pharyngeal -'_ constrictor m. Thyroid cartilage
electrode
Cricopharyngeus m.
Esophagus
Recurrent laryngeal nerves on the electrode in the recumbent position and superior fixation of the device at the time of surgery provides ideal, movement-free contact for evoked electromyography.
SIGNAL-TO-NOISE RATIO In evoked electromyography, the "signal" is a compound motor action potential, a massed discharge of the individual myocytes slaved in time to the stimulation event by a latency determined by the length of nerve to the target muscle. "Noise" is all other electrical events not related to signal. The dissipation of electrical signals in a volume conductor is by the inverse square law in distance from the source. A laryngeal electrode is anatomically very close to the stimulation site in the thyroidectomy wound and the stimulation event is a large electrical
RLN LOCATION BY EMG: A CLINICAL TECHNIQUE
FIGURE 3. Position of the laryngeal surface electrode for monitoring.
event. Contrary to the typical case in facial nerve location, in recurrent laryngeal nerve location simple inverse square dissipation is insufficient to separate the stimulation artifact from the signal. 16 Timing relationships can be used to electronically obliterate the stimulation artifact. Clinical trials of the electrode with careful monitoring have shown consistent reliability of this relationship allowing the separation of the stimulation artifact from wanted signal by a time "window." Also, differential electrode systems, such as the LSE, maximize the signal geographically between the two positive electrodes and minimize other signals originating outside fl~e electrode axis. Additional noise is caused by movement of the electrode/leadwire system, other electrical equipment in the operating theater, and floating static voltages. Efforts to reduce noise include: (1) physical stabilization of the electrode against movement; (2) increasing inversesquare dissipation; (3) window timing; (4) electronic noise
93
control measures (filtering, differential amplification); and (5) waveform display with analysis of signal by technician or surgeon.
CLINICAL USE OF THE POSTCRICOID SURFACE EMG OF THE LARYNX
the palate and a single 4 x 4 sponge is placed behind the handle and tongue as the laryngoscope is withdrawn so as not to displace the electrode. The lead wires are separately taped to the upper lip and then inserted into the EMG cable assembly. Typically the end of the tubed handled is just opposite the central incisor teeth after final positioning.
Insertion of the Electrode
Monitoring
In surgery of the thyroid and parathyroid the following steps are used in the clinical application of LSE to operative recurrent laryngeal nerve location. The LSE consists of a paddle-like surface with two surface electrodes on the concave side and one on the back, or convex side. There is a flexible tube handle with three wires that terminate in EMG connectors. The handle is held in the right hand with the convex surface up. The patient is completely positioned for surgery with all shoulder rolls and neck hyperextension before LSE insertion. A specially modified, notched Mackintosh blade laryngoscope is used to secure the endotracheal tube and is inserted so as to allow the tongue, endotracheal tube, and larynx to be lifted away from the postcricoid hypopharynx revealing the postcricoid space (Fig 4). The posterior cricoid and back of the arytenoid cartilages must be visually identified. The larynx being held up by the left hand and a modified Mackintosh laryP, goscope, the LSE is inserted into the pharynx, generally positioned parallel to the posterior wall of the pharynx, and gently slid inferiorly until it is visually sundowned posterior to the arytenoid cartilages (Fig 5). The handle is then buckled at 90° at the level of
The LSE has been tested for compatibility with several commercially available EMG devices. The LSE was unsuccessfully used with the Neurosign 100 (Smith and Nephew, Richards Inc, Memphis, TN) due to stimulation artifact, and u n s u c c e s s f u l l y u s e d w i t h the NIM-2 (Xomed-Treace, Jacksonville, FL) due to too high a gain. Monitoring has been successful with the Nicolet Viking 2E (Nicolet Biomedical Instruments, Madison, WI) with use of a monitoring assistant in 31 cases with 39 nerves at risk, and with the Neurovision SE (Fig 6) (RLN Systems), a dedicated nerve locating device, with 30 nerves at risk.
Method of Surgery There are two generally accepted techniques for thyroidectomy summarized here: close-capsular dissection and initial nerve location. Close-capsular dissection (Rea) consists of a direct dissection of the thyroid gland, with ligation of vessels near their entry into the gland. In this method, the recurrent nerve may be identified if exposed in the w o u n d or if readily accessible, but is typically spared by avoidance. The dissection usually proceeds from the inferior aspect
Laryngeal Eleclrode FIGURE 4. Postcricoid electrode in place, viewed through a suspension laryngoscope under general anesthesia.
94
REA AND KHAN
Tongue
Epiglottis / Thyroid cartilage Cricoid cartilage
Hard & palate
Electrode Vocal cord
FIGURE 6. Neurovision SE nerve locator as provided by RLN Systems, Inc. of the gland superiorly until the suspensory ligament of the thyroid gland to the trachea is isolated. This ligament is dissected with use of a "stimulating hemostat" (RLN Systems) as a dissector (Fig 7), the nerve warning tone being activated usually w h e n the ligament is first bluntly elevated. The Neurovision SE unit provides a simple warning tone to the surgeon when the nerve is approached. Severing of the ligament usually exposes the nerve immediately below the level of the ligament. Verification of nerve location and function can be done by resorting to further nerve stimulation. Occasionally the nerve is never seen. In cases with 30 nerves at risk done by Rea since May 1991 with the Neurovision SE nerve locator and laryngeal surface electrode, there were 26 positive identifications of the nerve. Four nerves were not visually identified but three of them were stimulated deep in the wound. One nerve was neither identified nor stimulated. These
RLN LOCATION BY EMG: A CLINICAL TECHNIQUE
FIGURE 5. Placement of the LSE using a modified Mackintosh laryngoscope.
cases represented the typical problems encountered in a private otolaryngology practice consisting of cold nodules, goiters, and a rare thyroid cancer. There were no postoperative nerve dysfunctions. A second accepted technique (Khan) of recurrent laryngeal nerve sparing in thyroidectomy, as described by Lor6,17 uses initial identification and anatomic exposure of the main trunk of the recurrent laryngeal nerve as it emerges from the superior thoracic outlet. Following the identification, the recurrent nerve is kept in constant view as dissection proceeds to remove the ipsilateral lobe. However, because of enlarged substernal goiters or massive extra-thyroidal tumors, it may not be possible to identify the nerve in the tracheoesophageal groove. In such circumstances, usually division of the isthmus and superior pole will allow enough rotation to be able to identify the nerve in that area. Preferentially, smaller blood vessels are ligated nearer to the gland rather than to the main trunk of the inferior thyroid artery. The final dissection proceeds to include exposure of the suspensory ligament, which is subsequently transected while keeping the recurrent nerve in full view. A simple nerve stimulation probe is used intermittently to verify the anatomic identification of the nerve on a Nicolet EMG with a monitoring assistant in attendance. The usefulness of nerve stimulation with a probe in the face of an already identified and exposed nerve is recognized in situations such as: (1) displaced recurrent nerve; (2) recurrent nerve surrounded by mass or tumor; (3) tethered nerve by adherent tissues as seen in thyroiditis; (4) nerve s u r r o u n d e d by scar tissue; and (5) extra-laryngeal branched recurrent nerve (seen in 39% of cases in a study by Nemiroff and Katz17). Following extubation, laryngeal nerve function is routinely assessed in the operating room by the operating surgeon with the use of a flexible scope.
95
ryngeal n e r v e paralysis; one r e c o v e r e d at 3 m o n t h s , the other has still not r e c o v e r e d at 8 m o n t h s despite the verification of n e r v e continuity at the time of s u r g e r y a n d the cause of paralysis unclear.
SUMMARY A l a r y n g e a l s u r f a c e e l e c t r o d e c o n f i g u r e d to p e r f o r m e v o k e d m o n i t o r i n g f r o m the postcricoid h y p o p h a r y n x has b e e n d o c u m e n t e d . T h e m e t h o d of m o n i t o r i n g has b e e n described a n d case results o v e r several y e a r s h a s b e e n p r e s e n t e d . This technique m a k e s the identification of the recurrent laryngeal n e r v e b y e v o k e d E M G m o n i toring a useful clinical tool.
REFERENCES
FIGURE 7. Dissecting stimulating hemostat as provided by RLN Systems, Inc. In 31 cases b y K h a n d o n e since S e p t e m b e r 1992, intermittent stimulating w a s u s e d w i t h the Nicolet Viking 2E monitor. T h e s e cases w e r e f r o m an academic, referralb a s e d practice a n d consisted m a i n l y of v e r y large or complicated m a s s e s . T h e r e w e r e 39 n e r v e s at risk. T h r e e recurrent laryngeal n e r v e s did not stimulate at surgery. O n e w a s a n e r v e that w a s w i d e n e d b y c o m p r e s s i o n b y a v e r y large p a r a t h y r o i d a d e n o m a . The other t w o w e r e in the s a m e p a t i e n t a n d m a y h a v e r e p r e s e n t e d an unrecognized electrode d i s p l a c e m e n t . Earlier configurations of the LSE w e r e smaller a n d occasionally w o u l d displace. There w e r e t w o patients w i t h p o s t o p e r a t i v e recurrent la-
96
1. Parnes SM: Laryngeal Electromyography, in ENTechnology, Boston, MA, Little, Brown, 1988 2. Lipton RJ, McCaffrey TV, Litchy WJ: Interoperative electrophysiologic monitoring of laryngeal muscle during thyroid surgery. Laryngoscope 98:1292-1296, 1988 3. Payne J, Higenbottam T, Guinde G: Respiratory activity of the vocal cords in normal subjects and patients with airflow obstruction; an electromyographic study. Clin Sci 61:163-167, 1981 4. Mu L, Yang S: A new method of needle-electrode placement in the posterior eircoarytenoid muscle for electromyography. Laryngoscope 100:1127-1131, 1990 5. Rea JL, Davis WE, Templer JW: United States Patent No. 4,155,353: Electrode and Method for Laryngeal Electromyography. May 22, 1979 6. Rea JL, Davis WE, Templer JW: Recurrent nerve locating system. Ann Otol Rhinol Laryngol 88:92-94, 1979 7. Rea JL, Davis WE, Templer JW: Design and testing of a new electrode for laryngeal electromyography. Arch Otolaryngol 104:685686, 1978 8. Davis WE, Rea JL, Templer JW: Recurrent laryngeal nerve location using a microlaryngeal electrode. Otolaryngol Head Neck Surg 87: 330-333, 1979 9. Kimura J: Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. Philadelphia, PA, Davis, 1983, p 86 10. Woodson G, Gayle E: Respiratory activity of the cricothyroid muscle in conscious humans. Laryngoscope 100:49-53, 1990 11. Payne JK, Higenbottam T, Guindi GM: Surface electrode for laryngeal electromyography. J Neurol Neurosurg Psych 43:853-860, 1980 (letter) 12. Rea JL: Postcricoid surface laryngeal electrode. Ear Nose Throat J 71:267-269, 1992 13. Ajmani ML: A metrical study of the laryngeal skeleton in adult Nigerians. J Anat 171:187-191, 1990 14. Woodburne RT: Essentials of human anatomy. Toronto, Ont, Oxford, 1969, p 164 16. Tucker GF: Human Larynx Coronal Section Atlas. Washington, DC, Armed Forced Institute of Pathology, 1971 16. Licht S: Electrodiagnosis and Electromyography. Baltimore, MD, Wavefly, 1971, p 384 17. Lor6JM Jr: An Atlas of Head and Neck Surgery (ed 3). Philadelphia, PA, Saunders, 1988, p 726-736
REA AND KHAN
Injury to vital neural structures remains a source of major morbidity in head and neck surgery. Electromyographic (EMG) monitoring has been successfully used during otologic and parotid surgery. However, its use in thyroid and parathyroid surgery has been less than routine even though preservation of recurrent laryngeal nerve function is essential to voice and airway. We describe our technique with the use of a postcricoid surface electrode for evoked EMG monitoring in thyroid and parathyroid surgeries. Technique of insertion of the electrode is described. Operative thyroidectomy is described for either close-capsular dissection or initial nerve location. Equipment required and monitoring technique is described for both methods and outcomes are briefly discussed. These results support evoked EMG by postcricoid surface electrode as a useful practical tool in identification of recurrent nerve in thyroid and parathyroid surgery.
Surgical nerve location has evolved through advances in anatomic understanding and improvements in devices available to assist the surgeon. Initially there was only anatomic nerve location, then nerve stimulation with an electronic nerve stimulator with visual or tactile observation of the resulting movement, and more recently, stimulation of the nerve and detection of the evoked electromyographic (EMG) potential from the target muscle. This pattern of improved technique has occurred with surgery of the facial nerve culminating in the use of EMG in neurotologic and skull base procedures. In this report we describe the experience of two surgeons with an indwelling laryngeal electrode (Fig 1) to allow evoked electromyography for operative identification of the recurrent laryngeal nerve.
MONITORING OF THE LARYNX WITH NEEDLE ELECTRODES Many articles have been written proposing methods of recurrent laryngeal nerve location. The technology developed for facial nerve location has been applied to the problem of laryngeal nerve location using needle electrodes. Needle electrodes may be used for diagnostic EMG of the larynx but are not suited for monitoring in this structure. Parnes 1 notes that the laryngeal muscles examined are small, the technique of (needle) elecFrom the Department of Otolaryngology and Head and Neck Surgery, St. Mary's Medical Center, Jefferson City, MO. Address reprint requests to J. Lee Rea, MD, 1021 Northeast Dr, Jefferson City, MO 65109. Copyright © 1994 by W.B. Saunders Company 1043-1810/94/0502-0006505.00/0
trode insertion is blind," and that failure of EMG may "reflect sampling error." Lipton et al 2 also demonstrate this problem. Payne et al 3 note that the exact location of laryngeal EMG needle electrodes cannot be known if they are placed after induction of general anesthesia. Mu and Yang 4 describe the high degree of skill required and also safety concerns in laryngeal needle electromyography. Rea et al 57 and Davis et al s have extensive experience in laryngeal needle electromyography for monitoring purposes and report concerns for possible electrode fragmentation, electrode displacement with possible vascular injury or laceration from sharp needles, and possible inadvertent deflation of the endotracheal tube balloon. The possibility of hook-wire electrode 2 shearing off from insertion and creation of a foreign body in the vocal cord has not been reported to our knowledge, but this complication would seem to be inevitable with routine monitoring by this technique. Vocal cord hematoma occurs frequently with needle electrode monitoring 3 and causes postoperative anxiety for possible recurrent laryngeal nerve injury. All needle electrodes without barbs or retention devices risk possible displacement or laryngeal injury by any movement of the patient during surgery. Because of these problems and risks, a concerted effort to develop a noninvasive, easily initiated method of performing recurrent laryngeal-nerve-evoked monitoring has been undertaken with results that are presented in here.
MONITORING THE LARYNX WITH SURFACE ELECTRODES The ideal laryngeal electrode would be totally noninvasive, would not interfere with intubation or operative manipulations, would provide a stable platform for signal
OPERATIVE TECHNIQUES IN OTOLARYNGOLOGY--HEAD AND NECK SURGERY, VOL 5, NO 2 (JUN), 1994: PP 91-96
91
200 U~!
i
J
I' r, *
t
,
Rm,o
1
!
I i} U
/ 1
,
,
'
i'
I
2 ms
!, /'
,
200 uV Rm£ 2
i
Tri9 1 . 5 ms
FIGURE 2. Evoked CAP as recorded from LSE. Courtesy of George Washington University Hospital Neurophysiology Laboratory.
FIGURE 1. Laryngeal surface electrode as provided by RLN Systems, Inc, Jefferson City, MO. collection, and would be secure from dislocation. Kimura 9 states: "Surface electrodes, in general, are better suited than needle electrodes for recording evokedmuscle action potentials. The response recorded by surface electrodes is a compound action potential with contributions from all the muscle fibers innervated by the nerve . . . . This is in contrast to recording from a needle electrode, which registers only a small portion of the muscle action potential." The typical compound motor action potential (CAP) (Fig 2) that occurs with evoked EMG is better examined with surface electrodes because the signal is several orders of magnitude larger. Correspondingly, less sensitive monitoring devices are required. The CAP is so large and so characteristic in its latency from the stimulating event that equipment can be designed for electronic identification of a stimulation event. A monitoring assistant can be employed if equipment with higher capability is to be used. A strip electrode carried either side of the endotracheal tube was fabricated in 1977 and found on testing to be "noisy" because of excessive movement of the tube with 92
respiration and the minimal surface contact. An endotracheal tube electrode contacts the vocalis muscle only at one point made by the cross of the electrode and vocalis tendon and acts more like a needle electrode in its electrical characteristics. This configuration may actually be compared more with an antenna than a surface electrode. Other endolaryngeal strategies examined included devices dipped to the posterior lamina of the cricoid, double balloon devices to straddle and fix the cord, or other even more fanciful arrangements. We read with great interest an article by Woodson 10 describing an extralaryngeal surface electrode to be carried on a nasoesophageal tube for noninvasive monitoring of the posterior cricoarytenoideus muscle (PCA) EMG. This was done on the awake patient by positioning the electrode in the postcricoid space, Payne et al i 1 also described a postcricoid laryngeal surface electrode (LSE) used perorally for awake, diagnostic laryngeal EMG. The postcricoid hypopharynx provides the ideal locus for surface electrode laryngeal EMG: a broad potential space to abut the largest muscles of the larynx. After multiple modifications and clinical trials, a LSE has been developed that meets the criterion for an ideal monitoring device. 12 The retrolaryngeal, retrocricoid location defines the construction and use of this electrode. This human adult larynx anatomy has proven to have a rather narrow range of measurements in cadaver studies. 13 The cricoid cartilage, for example, has been found to measure approximately 2.5 cm in breadth posteriorly in females and about 3.0 cm in males. Likewise, the postcricoid hypopharynx, in the anatomically normal adult, has predictable a n a t o m y [ 4"1s The vertical ridges of the lateral edge of the thyroid cartriage and inferior cornu laterally, the cricopharyngeus sphincter and introitus to the esophagus inferiorly, and the spine supporting the hypopharynx posteriorly form a space that stabilizes the electrode in every dimension except superiorly (Fig 3). The weight of the larynx resting REA AND KHAN
Endotracbeal " tube Epiglottis bone
Inf. pharyngeal -'_ constrictor m. Thyroid cartilage
electrode
Cricopharyngeus m.
Esophagus
Recurrent laryngeal nerves on the electrode in the recumbent position and superior fixation of the device at the time of surgery provides ideal, movement-free contact for evoked electromyography.
SIGNAL-TO-NOISE RATIO In evoked electromyography, the "signal" is a compound motor action potential, a massed discharge of the individual myocytes slaved in time to the stimulation event by a latency determined by the length of nerve to the target muscle. "Noise" is all other electrical events not related to signal. The dissipation of electrical signals in a volume conductor is by the inverse square law in distance from the source. A laryngeal electrode is anatomically very close to the stimulation site in the thyroidectomy wound and the stimulation event is a large electrical
RLN LOCATION BY EMG: A CLINICAL TECHNIQUE
FIGURE 3. Position of the laryngeal surface electrode for monitoring.
event. Contrary to the typical case in facial nerve location, in recurrent laryngeal nerve location simple inverse square dissipation is insufficient to separate the stimulation artifact from the signal. 16 Timing relationships can be used to electronically obliterate the stimulation artifact. Clinical trials of the electrode with careful monitoring have shown consistent reliability of this relationship allowing the separation of the stimulation artifact from wanted signal by a time "window." Also, differential electrode systems, such as the LSE, maximize the signal geographically between the two positive electrodes and minimize other signals originating outside fl~e electrode axis. Additional noise is caused by movement of the electrode/leadwire system, other electrical equipment in the operating theater, and floating static voltages. Efforts to reduce noise include: (1) physical stabilization of the electrode against movement; (2) increasing inversesquare dissipation; (3) window timing; (4) electronic noise
93
control measures (filtering, differential amplification); and (5) waveform display with analysis of signal by technician or surgeon.
CLINICAL USE OF THE POSTCRICOID SURFACE EMG OF THE LARYNX
the palate and a single 4 x 4 sponge is placed behind the handle and tongue as the laryngoscope is withdrawn so as not to displace the electrode. The lead wires are separately taped to the upper lip and then inserted into the EMG cable assembly. Typically the end of the tubed handled is just opposite the central incisor teeth after final positioning.
Insertion of the Electrode
Monitoring
In surgery of the thyroid and parathyroid the following steps are used in the clinical application of LSE to operative recurrent laryngeal nerve location. The LSE consists of a paddle-like surface with two surface electrodes on the concave side and one on the back, or convex side. There is a flexible tube handle with three wires that terminate in EMG connectors. The handle is held in the right hand with the convex surface up. The patient is completely positioned for surgery with all shoulder rolls and neck hyperextension before LSE insertion. A specially modified, notched Mackintosh blade laryngoscope is used to secure the endotracheal tube and is inserted so as to allow the tongue, endotracheal tube, and larynx to be lifted away from the postcricoid hypopharynx revealing the postcricoid space (Fig 4). The posterior cricoid and back of the arytenoid cartilages must be visually identified. The larynx being held up by the left hand and a modified Mackintosh laryP, goscope, the LSE is inserted into the pharynx, generally positioned parallel to the posterior wall of the pharynx, and gently slid inferiorly until it is visually sundowned posterior to the arytenoid cartilages (Fig 5). The handle is then buckled at 90° at the level of
The LSE has been tested for compatibility with several commercially available EMG devices. The LSE was unsuccessfully used with the Neurosign 100 (Smith and Nephew, Richards Inc, Memphis, TN) due to stimulation artifact, and u n s u c c e s s f u l l y u s e d w i t h the NIM-2 (Xomed-Treace, Jacksonville, FL) due to too high a gain. Monitoring has been successful with the Nicolet Viking 2E (Nicolet Biomedical Instruments, Madison, WI) with use of a monitoring assistant in 31 cases with 39 nerves at risk, and with the Neurovision SE (Fig 6) (RLN Systems), a dedicated nerve locating device, with 30 nerves at risk.
Method of Surgery There are two generally accepted techniques for thyroidectomy summarized here: close-capsular dissection and initial nerve location. Close-capsular dissection (Rea) consists of a direct dissection of the thyroid gland, with ligation of vessels near their entry into the gland. In this method, the recurrent nerve may be identified if exposed in the w o u n d or if readily accessible, but is typically spared by avoidance. The dissection usually proceeds from the inferior aspect
Laryngeal Eleclrode FIGURE 4. Postcricoid electrode in place, viewed through a suspension laryngoscope under general anesthesia.
94
REA AND KHAN
Tongue
Epiglottis / Thyroid cartilage Cricoid cartilage
Hard & palate
Electrode Vocal cord
FIGURE 6. Neurovision SE nerve locator as provided by RLN Systems, Inc. of the gland superiorly until the suspensory ligament of the thyroid gland to the trachea is isolated. This ligament is dissected with use of a "stimulating hemostat" (RLN Systems) as a dissector (Fig 7), the nerve warning tone being activated usually w h e n the ligament is first bluntly elevated. The Neurovision SE unit provides a simple warning tone to the surgeon when the nerve is approached. Severing of the ligament usually exposes the nerve immediately below the level of the ligament. Verification of nerve location and function can be done by resorting to further nerve stimulation. Occasionally the nerve is never seen. In cases with 30 nerves at risk done by Rea since May 1991 with the Neurovision SE nerve locator and laryngeal surface electrode, there were 26 positive identifications of the nerve. Four nerves were not visually identified but three of them were stimulated deep in the wound. One nerve was neither identified nor stimulated. These
RLN LOCATION BY EMG: A CLINICAL TECHNIQUE
FIGURE 5. Placement of the LSE using a modified Mackintosh laryngoscope.
cases represented the typical problems encountered in a private otolaryngology practice consisting of cold nodules, goiters, and a rare thyroid cancer. There were no postoperative nerve dysfunctions. A second accepted technique (Khan) of recurrent laryngeal nerve sparing in thyroidectomy, as described by Lor6,17 uses initial identification and anatomic exposure of the main trunk of the recurrent laryngeal nerve as it emerges from the superior thoracic outlet. Following the identification, the recurrent nerve is kept in constant view as dissection proceeds to remove the ipsilateral lobe. However, because of enlarged substernal goiters or massive extra-thyroidal tumors, it may not be possible to identify the nerve in the tracheoesophageal groove. In such circumstances, usually division of the isthmus and superior pole will allow enough rotation to be able to identify the nerve in that area. Preferentially, smaller blood vessels are ligated nearer to the gland rather than to the main trunk of the inferior thyroid artery. The final dissection proceeds to include exposure of the suspensory ligament, which is subsequently transected while keeping the recurrent nerve in full view. A simple nerve stimulation probe is used intermittently to verify the anatomic identification of the nerve on a Nicolet EMG with a monitoring assistant in attendance. The usefulness of nerve stimulation with a probe in the face of an already identified and exposed nerve is recognized in situations such as: (1) displaced recurrent nerve; (2) recurrent nerve surrounded by mass or tumor; (3) tethered nerve by adherent tissues as seen in thyroiditis; (4) nerve s u r r o u n d e d by scar tissue; and (5) extra-laryngeal branched recurrent nerve (seen in 39% of cases in a study by Nemiroff and Katz17). Following extubation, laryngeal nerve function is routinely assessed in the operating room by the operating surgeon with the use of a flexible scope.
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ryngeal n e r v e paralysis; one r e c o v e r e d at 3 m o n t h s , the other has still not r e c o v e r e d at 8 m o n t h s despite the verification of n e r v e continuity at the time of s u r g e r y a n d the cause of paralysis unclear.
SUMMARY A l a r y n g e a l s u r f a c e e l e c t r o d e c o n f i g u r e d to p e r f o r m e v o k e d m o n i t o r i n g f r o m the postcricoid h y p o p h a r y n x has b e e n d o c u m e n t e d . T h e m e t h o d of m o n i t o r i n g has b e e n described a n d case results o v e r several y e a r s h a s b e e n p r e s e n t e d . This technique m a k e s the identification of the recurrent laryngeal n e r v e b y e v o k e d E M G m o n i toring a useful clinical tool.
REFERENCES
FIGURE 7. Dissecting stimulating hemostat as provided by RLN Systems, Inc. In 31 cases b y K h a n d o n e since S e p t e m b e r 1992, intermittent stimulating w a s u s e d w i t h the Nicolet Viking 2E monitor. T h e s e cases w e r e f r o m an academic, referralb a s e d practice a n d consisted m a i n l y of v e r y large or complicated m a s s e s . T h e r e w e r e 39 n e r v e s at risk. T h r e e recurrent laryngeal n e r v e s did not stimulate at surgery. O n e w a s a n e r v e that w a s w i d e n e d b y c o m p r e s s i o n b y a v e r y large p a r a t h y r o i d a d e n o m a . The other t w o w e r e in the s a m e p a t i e n t a n d m a y h a v e r e p r e s e n t e d an unrecognized electrode d i s p l a c e m e n t . Earlier configurations of the LSE w e r e smaller a n d occasionally w o u l d displace. There w e r e t w o patients w i t h p o s t o p e r a t i v e recurrent la-
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1. Parnes SM: Laryngeal Electromyography, in ENTechnology, Boston, MA, Little, Brown, 1988 2. Lipton RJ, McCaffrey TV, Litchy WJ: Interoperative electrophysiologic monitoring of laryngeal muscle during thyroid surgery. Laryngoscope 98:1292-1296, 1988 3. Payne J, Higenbottam T, Guinde G: Respiratory activity of the vocal cords in normal subjects and patients with airflow obstruction; an electromyographic study. Clin Sci 61:163-167, 1981 4. Mu L, Yang S: A new method of needle-electrode placement in the posterior eircoarytenoid muscle for electromyography. Laryngoscope 100:1127-1131, 1990 5. Rea JL, Davis WE, Templer JW: United States Patent No. 4,155,353: Electrode and Method for Laryngeal Electromyography. May 22, 1979 6. Rea JL, Davis WE, Templer JW: Recurrent nerve locating system. Ann Otol Rhinol Laryngol 88:92-94, 1979 7. Rea JL, Davis WE, Templer JW: Design and testing of a new electrode for laryngeal electromyography. Arch Otolaryngol 104:685686, 1978 8. Davis WE, Rea JL, Templer JW: Recurrent laryngeal nerve location using a microlaryngeal electrode. Otolaryngol Head Neck Surg 87: 330-333, 1979 9. Kimura J: Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. Philadelphia, PA, Davis, 1983, p 86 10. Woodson G, Gayle E: Respiratory activity of the cricothyroid muscle in conscious humans. Laryngoscope 100:49-53, 1990 11. Payne JK, Higenbottam T, Guindi GM: Surface electrode for laryngeal electromyography. J Neurol Neurosurg Psych 43:853-860, 1980 (letter) 12. Rea JL: Postcricoid surface laryngeal electrode. Ear Nose Throat J 71:267-269, 1992 13. Ajmani ML: A metrical study of the laryngeal skeleton in adult Nigerians. J Anat 171:187-191, 1990 14. Woodburne RT: Essentials of human anatomy. Toronto, Ont, Oxford, 1969, p 164 16. Tucker GF: Human Larynx Coronal Section Atlas. Washington, DC, Armed Forced Institute of Pathology, 1971 16. Licht S: Electrodiagnosis and Electromyography. Baltimore, MD, Wavefly, 1971, p 384 17. Lor6JM Jr: An Atlas of Head and Neck Surgery (ed 3). Philadelphia, PA, Saunders, 1988, p 726-736
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