A functional evaluation of ansa cervicalis nervetransfer for unilateral vocal cord paralysis: Futuredirections for laryngeal reinnervationDAVID C. GREEN, MD, GERALD S. BERKE, MD, and MICHAEL C. GRAVES, MD, Los Angeles, California

There are a variety of methods for treating unilateral vocal cord paralysis. but to datethere are few objective studies that evaluate the functional results of nerve transferfrom the ansa cervlcalls. Six dogs underwent unilateral recurrent laryngeal nerve sec.tlon with Immediate reanastamosls to the sternothyroid branch of the ansa cervlcalls.After 5 to 6 months. measurements of vocal efficiency and acoustic parameters. vi.deolaryngoscopy. vldeostroboscopy. and evoked electromyography were performed.Identical measurements were made In eight control dogs during normal electricallyInduced phonation and a simulated unilateral recurrent laryngeal nerve paralysis.Histologic analysis of both vocalls muscles. recurrent laryngeal nerves. ansa cervlcalls.and the ansa·recurrent laryngeal nerve anastamosls site was performed. Evidence ofreinnervation was found In all of the animals that underwent nerve transfer. The vocalefficiency and acoustic quality after ansa cervlcalls nerve transfer were dependenton the degree of electrical stimUlation from the transferred nerve to the relnnervatedcord during phonation. In the absence of electrical stimulation to the nerve transfer.physiologic vocal cord motion could not be elicited from the relnnervated cord. (OTO­LARYNGOL HEAD NECK SURG 1991;104:453.)

There are a variety of methods for treating unilateralvocal cord paralysis. These include Teflon injection, I

thyroplasty,' arytenoid adduction," and nerve" andnerve-muscle pedicle transfer.' A review of the litera­ture reveals a lack of consensus regarding the optimalprocedure for voice improvement.6

Crumley et al. 4 have recommended nerve transferfrom the ansa hypoglossi to the recurrent laryngealnerve (RLN) as a treatment for unilateral vocal cordparalysis. This procedure was performed on five pa­tients and the voice was thought to be superior to thatachieved with Teflon injection because of the restorationof normal stiffness, mass, and symmetry of the cord.Spectral analysis was improved postoperatively," andstroboscopic examination revealed synchronous mu­cosal waves. This technique required an open proce­dure, but did not expose or manipulate the larynx. Inaddition, Crumley et al." postulated that the denervated

From the Division of Head and Neck Surgery, UCLA School ofMedicine.

Supported by VA Merit Review Research Fund.Received for publication July 3D, 1990; accepted Aug. 29, 1990.Reprint requests: David C. Green, MD. 10992 Ashton Ave.,

Apt. 304, Los Angeles, CA 90024.23/1125135

sternothyroid muscle medialized the thyroid lamina andvocal cord. Tucker has also described a technique forlaryngeal reinnervation that uses an ansa hypoglossineuromuscular pedicle passed through a window in thethyroid cartilage.

The literature on laryngeal reinnervation is extensive.I An excellent review of the subject was recently pub­

lished by Rice." The idea goes back to 1909, whenHorsley" first reported the repair of a recurrent laryngealnerve after transection by a gunshot wound. He reportedcomplete return of function in 3 months. In 1926, Bla­lock and Crowe" described some return of motion with­out atrophy of the cord after anastamosis of the distalsegment of the RLN to either the ansa hypoglossi (end­to-end), the phrenic, or into a slit in the opposite RLN.Histologic studies demonstrated regrowth of axons afteranastarnosis in all three cases. Frazier and Mosser'!anastomosed the RLN to the ansa hypoglossi in 10patients with paralysis present from 8 months to 11years. They found improvement in 60% overall but littleimprovement in patients with paralysis for more than6 years,

In contrast to these early encouraging results, Hoo­ver," in 1953, stated that attempts at restoration ofrecurrent laryngeal function by nerve anastamosis hadbeen unsuccessful. This situation was clarified in 1963

453

... GREENeta!.

by Siribodi et al.," who found a return of electromy­ographic activity after repair of the laryngeal nerve, butno functional movement. They proposed that aberrantregeneration of adductor and abductor fibers accountedfor the lack of function. This finding is now termedlaryngeal synkinesis."

Although Doyle et al. IS reported almost completereturn of function after anastamosis of the RLN in bothdogs and human beings, most reports have been con­sistent with the findings of Siribodi et al." Gordon andMcCabel6 found that after section and immediate repairof the RLN, only adductor function returned. Theyattributed this to the fact that adductor muscles out­numbered abductor muscles. Boles and Fritzell 17 foundpolyphasic potentials by means of electromyography(EMG) 3 to 6 months after RLN section and anasta­mosis in dogs with little motion in the reinnervatedcord. In contrast, in the crush injuries they studied,recovery took place in approximately 2 months withnormal cord movement. They concluded that surgicalrepair of the RLN could possibly return tone and someadduction to the cord and improve the glottis as a pho­nator and a protective sphincter. Tashiro" found EMOevidence of recovery but no definite motion after repairof the RLN. Murakami and Kirchner" confirmed thepresence of simultaneous activation of the adductorsand abductors after repair of the RLN by EMO. Thissupported the idea of misdirection in axon regrowth.Satoh et al. 1O performed direct muscle stimulation andEMO studies and thought that impaired muscle func­tion, in addition to axon misdirection, contributed topoor function of the reinnervated laryngeal muscles.Ded021 found a small degree of adduction in the rein­nervated cord after RLN anastamosis. He believed theadduction resulted from the action of the cricothyroidmuscle.

As reinnervation of the larynx began to emerge as apotential treatment for unilateral vocal cord paralysis,controversy arose as to which nerve transfer would givethe best results. Crumley et al." recommended theansa cervicalis branch to the sternothyroid branch forseveral reasons. This branch to the sternothyroid is lo­cated very near the RLN. Although the pattern of ster­nothyroid activation is predominantly inspiratory, itfires throughout the respiratory cycle, giving a certaindegree of continuous tone to the vocalis. Also, it isthought that the subsequent paralysis of the sterno­thyroid branch relaxes tension on the thyroid cartilageand further medializes the paralyzed cord. However,some investigators have argued against the use of thesternothyroid branch because the pattern of activation

OtoIaryngoJogy­Head and Neck SUrgery

from this nerve may result in cord adduction duringinspiration rather than phonation." Marie et al. 23 found,after ansa-adductor branch transfer in the canine model,that during extended obstructive dyspnea, there wasadduction of the cord, which was abolished after sec­tioning of the ansa. In this same canine model, thereinnervated cord failed to reach midline during whin­ing, demonstrating a lack of ansa stimulation duringphonation.

Crumley" has advised against using the ansa'sbranch to the thyrohyoid despite its being the mostactive strap muscle during phonation, with peak activityduring expiration. His reluctance stems from the re­quirement of a cable graft to reach the RLN stump andthe resulting paralysis of the thyrohyoid, eliminatingthe adductor function of this muscle.

The superior laryngeal nerve (SLN) has been pro­posed by Rice" to be ideal for nerve transfer to improvephonation after a unilateral vocal fold paralysis. Thisnerve is active during phonation and anatomically closeto the RLN. Woodsen2S found that during normal res­piration in human beings, the cricothyroid muscle isactive predominantly during inspiration. This patternofactivation may be ofconcern because it is conjecturedthat inspiratory activation of the thyroarytenoid maycause airway obstruction. Rice" and Crumley" haveperformed a SLN-adductor branch anastamosis in dogs,which demonstrated spontaneous adduction of the cordwith coughing or whining. However, Crumley" be­lieves that use of the superior laryngeal nerve is "rob­bing Peter to pay Paul" in that this would eliminate thenormal adductive effect of the cricothyroid muscle.

Crumley" has also counseled against the use of theproximal RLN in RLN-RLN anastamosis. His concernsare again related to the possibility ofairway obstructionduring inspiration as a result of hypertrophy and medialbulging of the thyroarytenoid from RLN synkinesis. 13

Although Tashiro," Sirbodhi et al., 13 Murakami andKirchner," Sato et al.,20 Veda et al.," and Iwamura"have shown activation of the adductors during inspi­ration, none of these studies has demonstrated respi­ratory obstruction with dyspnea from a unilateral RLN­RLN anastamosis in the presence of a normal oppositecord. Iwamura" stated "such minor paradoxical ad­duction of the reinnervated vocal cord appears to be ofno great importance, since the inspiratory adductionlasts for only a fraction of a second." As mentionedpreviously. the RLN·RLN anastamosis may not be theonly "offender" in this regard. Marie et aI.23 reportedparadoxical inspiratory adduction of the reinnervatedcord by means of the ansa branch to the sternothyroid.

Volume 104 Number 4April 1991

A recent" clinical case report of a unilateral RLN-RLNanastamosis resulted in an excellent voice without air­way obstruction.

Another issue is the duration of time that a nervetransfer can still be used after vocal cord paralysis.Janecka," by means of the rat siatic nerve, found thatsevered axons will not regrow when the nerve segmenthas lost contact with the target muscle. How long de­nervated muscle can provide this trophic response isnot known. Some authors obtain an EMG from anypatient who has been paralyzed for more than 2 years.Patients with absence of muscle activity on a thyroary­tenoid EMG are then treated with Teflon injection.

Although the voice after nerve transfer has been re­ported as excellent, there are some disadvantages. Thepatient must undergo general anesthesia and an opensurgical procedure. An intact distal stump of the RLNmust be present. The patient must wait approximately3 to 4 months before there is improvement from theprocedure. Gelfoam injection into the paralyzed cordmay improve glottal closure during the reinnervationperiod.

Few reports have used objective voice measurementsto study the results of nerve transfer for unilateral pa­ralysis of the vocal folds. Vocal efficiency is an objec­tive measure of the voice that was first studied by vanden Berg" in 1956. He defined the efficiency of voiceas the ratio of the acoustic power of the voice to theSUbglottic power. The subglottic air power can be es­timated as the product of the mean glottic air flow rateand the subglottic pressure. Clinically, vocal efficiencyhas been shown to decrease with some forms of laryn­geal disease, such as invasive carcinoma and vocal cordparalysis.F-" Ueda et al. 27 studied acoustic intensity.subglottic pressure, and flow rate during induced whin­ing in dogs after a combination SLN-SLN and RLN­RLN anastamosis. The vocal efficiency after reinner­vation in this canine study was significantly less thannormal.

Although vocal efficiency is useful as a functionalmeasure of voice, it may not correspond with vocalquality. The voice may be quite harsh with a normalvocal efficiency. Also. vocal efficiency does not indi­cate the degree of control the patient has over theglottis. 34

To measure voice quality, a number of acoustic mea­sures have been used clinically. These include jitter.shimmer. and signal-to-noise ratio. Jitter is defined asthe fluctuation in the time interval between successivepeaks of the fundamental frequency. Shimmer is thecycle-to-cycle variation in the amplitudes of the peaks.

Future directions for laryngeal reinnervation 411

Signal-to-noise ratio is the ratio of the sound energyin the acoustic signal to the background noise.35.36

Lieberman" was the first to report an increased jitterin pathologic phonation, and Zyski et al.38 found in­creased jitter and shimmer in patients with laryngealtumors and unilateral vocal cord paralysis. Efforts havebeen made to use these measures as screening devicesfor pathologic conditions of the larynx" and to docu­ment the results of laryngeal surgery."

This study used an in vivo canine model to evaluatethe functional results of nerve transfer from the ansacervicalis to the RLN as a treatment for unilateralvocal cord paralysis. Measurements of vocal effi­ciency, acoustic analysis, videolaryngoscopy, video­stroboscopy, and evoked EMG were performed afteransa cervicalis nerve transfer and compared with a con­trol group of animals during normal phonation and asimulated RLN paralysis. Tissue samples were takenfrom both vocalis muscles, RLNs, ansa cervicalis, andthe ansa-RLN anastamosis site for histologic analysis.

METHODS AND MATERIALSExperimental Design

The experimental group consisted of six mongreldogs (25 kg each) that underwent a unilateral RLNsection with immediate reanastamosis to the sternothy­roid branch of the ansa cervicalis under general anes­thesia. One dog had a reaction to the anesthetic anddied before the evaluation. After 5 to 6 months, mea­surements for vocal efficiency and acoustic analysis(jitter, shimmer, and signal-to-noise ratio) were madeon the remaining five experimental dogs, in addition tovideolaryngoscopy and videostroboscopy. In the ex­perimental group, vocal efficiency and acoustic mea­sures were made, with and without electrical stimula­tion of the ansa cervicalis nerve transfer. In addition,measurements of vocal efficiency and acoustic analysis(jitter, shimmer, and signal-to-noise ratio) were madeon a group of eight control dogs during "normal" elec­trically stimulated phonation and a simulated RLN pa­ralysis. In the experimental group, evoked EMG wasperformed to measure the nerve conduction velocitiesand the response amplitudes of the normal and rein­nervated vocal folds. Tissue samples were taken of bothvocalis muscles, RLNs, ansa cervicalis, and the ansa­RLN anastamosis site for histologic comparison.

Surgical Technique In the Experimental GroupEach animal was premedicated with acepromazine

and then underwent general anesthesia with endotra­cheal intubation. The ventral neck was shaved and pre-

.Q6 GREEN at oJ.

Vent ,lotor~

"'IICROPHONE

SOUND LEVEL METER

"'IULTICHANNElSTORAGEOSCILLOSCOPE

PRESSURETRANSDUCER

Otolaryngology­Head and Neck Surgery

DIREC TCOMPUTER IZEDDIGITIZ ATION

.r-:-J\-

Fig. 1. Schematic representation of experimental setup for In vivo canine model of phonatlon otteransa cervlcalls nerve transfer.

pared with povidone iodine. The animal was placed inthe supine position on a table in the operating room,and a midline neck incision was made from the mid­thyroid cartilage to 4 cm below the cricoid. The dis­section was carried down to the trachea, and theRLN and the ansa cervicalis (ansa) were identified bi­laterally. On the basis of the proximity of the ansa andthe RLN, one side was chosen for reanastamosis . Atapproximately 5 cm from the cricothyroid joint, theRLN was sectioned sharply with a no. 15 blade. Theproximal end of the RLN was ligated with 2-0 silksutures. The distal end was then anastamosed end-to­end to the ansa cervicalis. The anastamosis was per­formed with four 10-0 nylon sutures according to stan­dard microsurgical technique . The site of the ansa-RLNanastomosis was marked with a loose 2-0 silk sutureplaced I em away from the anastamosis . The woundwas closed in three layers with 3-0 VicryI. Six monthspostoperatively, the laryngeal phonatory characteristicswere evaluated by means of the in vivo canine modelof phonation ." Humane animal care was assured incompliance with The Principles of Laboratory AnimalCare. formulated by the National Society for MedicalResearch, and the Guide for the Care and Use of Lab-

oratory Animals. prepared by the National Academy ofSciences and published by the National Institutes ofHealth (NIH publication No. 80-23, revised 1978).

In Vivo Canine Phonation Model

Each dog in the control and experimental groups waspremedicated with intramuscular acepromazine. Intra­venous thiopental was administered to a level of cornealanesthesia , and additional thiopental was used tomaintain this level of anesthes ia throughout the ex­periment.

The animal was placed in the supine position on theoperating table, and a midline incision was made toexpose the trachea from the hyoid to the sternal notch(Fig . I). In the animals undergoing experimental nervetransfer, the RLN on one side and the RLN-ansa an­astamosis on the opposite side were identified and pre­served. In the control group of animals, both RLNswere identified. For both control and experimentalgroups, the superior laryngeal nerves were identifiedalong their course to the cricothyroid muscles . A lowtracheotomy was performed at the level of the supra­sternal notch, through which an endotracheal tube waspassed to allow ventilator-assisted respiration. A second

Volume 104 Number 4April 1991

tracheotomy was performed in a more superior location,through which a cuffed endotracheal tube was passedin a rostral direction and positioned with the tip 10 embelow the vocal folds. The cuff was inflated to just sealthe trachea. Humidified heated air was passed throughthis rostral endotracheal tube from a compressed airtank. Flow was controlled with a valve and measuredwith a Gilmont flowmeter (model FI500, Gilmont In­struments, Great Neck, N.Y.). The air flow was hu­midified and heated by bubbling it through 5 em ofheated water so that the temperature of the air was37° C when measured at the glottic outlet. A l-cmbutton was used to suspend the epiglottis from a fixedpoint to provide direct visualization of the larynxthrough the oral cavity.

A l-cm segment of each superior laryngeal nerve(external branch) was isolated, and Harvard miniatureelectrodes (Harvard Apparatus Inc., Millis, Mass.)were applied around each nerve. Harvard electrodeswere also applied to the RLN on one side and the ansacervicalis on the opposite side in the animals under­going nerve transfer or both RLNs in the control ani­mals. Care was taken to place the electrode on the ansacervicalis 2 em proximal to the anastamosis site in theanimals undergoing nerve transfer, to avoid currentspread to the distal RLN. The electrodes were theninsulated from surrounding tissue. The cut proximalRLN on the side of the anastamosis in the animalsundergoing nerve transfer was also dissected out andstimulated with a supramaximal current of 5 mA toverify an absence of vocal cord movement or EMGstimulation. Two Model S2LH constant-current nervestimulators (WR Medical Electronics Co., St. Paul,Minn.) were used to stimulate the RLN, ansa cervicalis,and superior laryngeal nerves independently. Thesenerves were stimulated at 70 to 80 Hz stimulus fre­quency, with 0.5 to 2.0 mA intensity for 1.5-msecduration. Phonation was produced with an air flow of318 to 523 cc/sec, applied through the larynx by therostral endotracheal tube.

Acoustic MeasuresAcoustic measurements were obtained for the ex­

perimental nerve transfer group, both with and withoutstimulation of the ansa nerve transfer. Acoustic mea­surements for the control group were obtained duringsimulated normal phonation and RLN paralysis. Nor­mal phonation required stimulation of all four laryngealnerves. A unilateral RLN paralysis was simulated bynot stimulating one RLN. The sound level measure­ments were made with a I-inch Quest condenser mi­crophone placed 30 em from and level with the glotticoutlet. The microphone was directed 90 degrees from

Future directions for laryngeal reinnervation ••7

the direction of the sound source. The sound level mea­surements were made in decibels, with a Quest soundlevel meter on the C scale. The acoustic signal was alsodigitized, after C scale filtering at 20 kHz, and storedon the hard disk of a personal computer.

Subglottic pressure was measured with a MillarMikro-Tip catheter pressure transducer (model No.SPC-330, size 3F, Millar Instruments, Inc., Houston,Texas) passed rostrally through the superior tracheot­omy. It was placed 5 em below the glottis. This signalwas low-pass filtered at 3 kHz, digitized at 20 kHz,and stored in a personal computer. Because of the vari­ation in subglottic pressure during phonation, the peakpressures attained during the glottic cycle were used.These peaks were identified by means of a commer­cially available software package for the PC system("C-Speech," Paul Milenkovic, University of Wiscon­sin, Madison, Wis.).36 The pressure transducer was cal­ibrated before each experiment against a mercury ma­nometer at 3r C.

Vocal efficiency was calculated as the ratio of theacoustic power of the voice to the subglottic power.The total acoustic power was calculated according tothe method of Koyama et al.," in which total soundpower = 2 rPc

2/Poc. This formula applies for a sound

power radiating with no known direction into ahemisphere of area (2 r'), a distance (r) away fromthe source. The product Po (the density of the medium)and c (the velocity of propagation) is the specificacoustic impedance of the medium, which is 41.1dynes/sec/em' in air at 200 C. The term P, is the rootmean square sound pressure in dynes/em' at the dis­tance r from the sound source. The subglottic powerwas calculated as the product of the flow rate times thepeak subglottic pressure.

Acoustic analysis of the digitized acoustic signal wasperformed with a commercial software program ("C­Speech," Paul Milenkovic, University of Wisconsin,Madison, Wis.). Jitter, shimmer, and signal-to-noiseratio were calculated on approximately 0.3 second ofstable phonation from each trial. The background noisein the experimental quarters was 35 dB lower than thevalues measured with the C scale. To normalize forvarying fundamental frequency, jitter was calculated asa fraction of the period of the fundamental frequency.

For each dog in either the experimental or the controlgroup, the jitter, shimmer, signal-to-noise ratio, andvocal efficiency for that dog were calculated from themean of five trials of phonation. A mean value for theentire experimental group, with and without ansa stim­ulation for jitter, shimmer, signal-to-noise ratio, andvocal efficiency, was calculated by taking the averageacross all five experimental dogs. A mean value for the

.51 GREEN et 01.

control group during simulated normal phonation andRLN paralysis for jitter, shimmer, signal-to-noise ratio,and vocal efficiency was calculated by taking the av­erage across all eight control dogs.

VldeoltrobolcoPY and VldeolaryngolcoPY

Videolaryngoscopy was performed during singlenerve stimulation of the RLN on the normal side oransa cervicaJis on the nerve transfer side. The currentstimulus was increased, starting from zero, to observethe effect of stimulation from each nerve separately.The image was detected by a Jed-Med CCD (charge­coupled device) video camera (Model 70-5110, Jed­Med, St. Louis, Mo.) and a Sony V-matic videocassetterecorder (VO-5850. Park Ridge. N.J.). Videolaryn­goscopy was also performed under a lighter plane ofanesthesia to detect the degree of spontaneous motionof the cords during laryngeal stimulation with a cottonswab and obstructive respiratory dyspnea.

Videostroboscopy was performed, both with andwithout stimulation of the ansa cervicalis, in the ani­mals undergoing nerve transfer, and with and withoutstimulation of one RLN in the normal control animals,while the other three laryngeal nerves were beingstimulated. For stroboscopic imaging of the larynx.a Bruel & Kjaer laryngostrobe unit (model 4914,Bruel & Kjaer, Orange. Calif.) was used. The strobo­scope was connected to a Storz zero-degree telescopevia a fluid-filled light cable. The video images wereanalyzed frame-by-frame with the videorecordingunit.

Evoked EMG

Conventional spontaneous EMG recordings were ini­tially made during anesthesia. to detect fibrillation po­tentials in the thyroarytenoid muscle. Evoked EMG wasthen performed in the nerve transfer experimental an­imals by stimulating the ansa cervicalis nerve transferor the normal RLN on the opposite side with a single0.5 msec pulse of sufficient voltage through a Harvardminiature electrode. to obtain an adequate response.The response was detected with a bipolar needle placedtransorally into the ipsilateral thyroarytenoid muscle.The signal was digitized at 20 kHz and stored in apersonal computer. A conduction velocity for eachnerve was calculated by measuring the latency of theresponse in milliseconds between two points along thenerve. The time difference in the latency between thestimulated points divided by the distance between thesepoints gave the conduction velocity between thesepoints. In the experimental animals, the conduction ve­locity was measured on the normal side between two

Otolaryngology­Head and Neck Surgery

points along the distal RLN. On the nerve transfer side,conduction velocity was measured between a pointproximal and distal to the ansa-Rl.N anastamosis. Inaddition, the amplitude of the evoked EMG was re­corded in millivolts .as a measure of the amount ofdepolarizing muscle.

Histologic AnaIY.'1

The larynx of each animal undergoing nerve transferwas removed. divided into right and left halves, andplaced in 10% .formalin. A 3-mm thick cross-sectionof each vocalis muscle was taken midway between theanterior commissure and the tip of the vocalis processfor staining with hematoxylin and eosin.

A l-cm segment of the RLN was removed at a siteI cm proximal to the cricothyroid joint bilaterally andat 5 cm proximal to the cricothyroid joint on the sidenot operated on. A l-cm segment of ansa cervicaliswas taken I em proximal to the anastamosis on the sideoperated on and at a similar site on the ansa of the sidenot operated on. The site of the ansa-RLN anastamosiswas also removed. All five nerve specimens were fixedin 4% paraformaldehyde-0.4% glutaraldehyde. Theanastamosis site was embedded in parlodion and sec­tioned longitudinally. The remaining specimens werestained with osmium. embedded in Eponl Araldite(Electron Microscopy Sciences, Fort Washington, Pa.)and thin sections (0.5 "",m) were cut transversely.

Statistical Ana'y."

A paired t test was used to compare mean changesfrom ansa stimulation to nonstimulation within the ex­perimental group (N = 5) for vocal efficiency, jitter,shimmer, and signal-to-noise ratio. Two group t testswere used to compare the mean difference in thesemeasurements between the experimental group and thecontrol group (N = 8). For the vocal efficiency mea­surement, the t test was computed on the log values.Between-dog comparisons by means of Mann-Whitneyand Wilcoxon rank-sum criteria instead of the t test didnot change any conclusions.

RESULTSNerve and Vocal Cord Histologic Findings

There was little sign of atrophy in the reinnervatedthyroarytenoid muscle when compared to the normalside. The average diameter of both the reinnervated andnormal thyroarytenoid muscles was identical at 7.3 mm.Figure 2 shows the lack of atrophy in the reinnervatedcord when compared to normal.

Examination of the RLN on the side of the nervetransfer showed regenerated axons as seen in Fig. 3.

Volume 104 Number 4April 1991

A

B

Future directions for laryngeal reinnervation 419

Fig. 2. Cross section of A, normal cord and B, reinnervated cord.

VldeolaryngolcoPY of Groll Movement

Increasing electrical stimulation of the normal RLNdemonstrated increasing ipsilateral vocalis musclebulging and arytenoid adduction in all of the dogs.The threshold for vocalis contraction on the normal sidewas between 0.05 and 0.19 rnA. With increasing stim­ulation of the ansa cervicalis on the side of the nervetransfer, increasing ipsilateral vocalis contraction wasseen in all dogs. In four of the dogs, this vocalis con­traction was associated with arytenoid adduction, but

in one animal there was distinct arytenoid abductionwith vocalis contraction. The threshold for vocalis con­traction from ansa stimulation was between 0.01 and0.2 rnA.

During obstructive respiratory dyspnea, under a lightplane of anesthesia, after sectioning both superior la­ryngeal nerves, the normal cord was found to abductwith inspiratory efforts in all dogs. The reinnervatedcord did not adduct or abduct to any degree during thistime in any subject.

<t6O GREEN at 01.

A

,

r

otolaryngology­Head and Neck Surgery

B

Fig. 3. Cross sectionof A, normal RLN and B, reinnervated RLN.

-

During stimulation of coughing under a light planeof anesthesia. the normal cord adducted but the rein­nervated cord showed no gross movement in any of theanimals.

VldeoltrobolCoPY

The results of videostroboscopy are summarized inTable I. Videostroboscopy of normal phonation in thecontrol animals showed glottal dynamics similar to

Volume 104 Number 4April1991 Future directions for laryngeal reinnervation 461

Normal Cord

Reinnervated Cord

25

20

15

mV10

5

o2 3 4

Dog Number5 Mean

Fig. 4. Amplitude of response to evoked EMG in millivolts.

those of phonation in the animals undergoing nervetransfer during stimulation of the ansa cervicalis nervetransfer. There was complete glottic closure, two-mass(upper and lower margin) motion of the mucosa on bothcords, and complete glottic symmetry. Videostrobo­scopy without stimulation of the nerve transfer in theanimals undergoing nerve transfer resulted in a picturesimilar to that of a simulated unilateral vocal cord pa­ralysis. There was a mild posterior glottic chink incom­petence and one-mass motion of the paralyzed cord.Without stimulation of the ansa cervicalis nerve trans­fer, the reinnervated cord stroboscopically appearedto track greater lateral excursions on opening thanthe normal cord, which remained primarily in themidline.

Evoked EMGand Conduction Velocity

At 5 to 6 months after RLN section and nerve trans­fer, no fibrillation potentials were seen in any of thereinnervated vocal cords, indicating some degree ofreinnervation in all subjects. Figure 4 shows the am­plitude in millivolts of the evoked EMG. The amplitudeof the response of the reinnervated side was between5% and 155% of the normal side with an average of66% for the five subjects. By measuring the delay inthe EMG response to the evoked stimuli at two separatestimulation points on the nerve, a conduction velocitywas obtained for the normal and nerve transfer side.Table 2 shows these conduction velocities. For the nor­mal side, the conduction velocity varied from 34.6 to50 m/sec, with an average of 42.2 m/sec. For thereinnervated side, the conduction velocity varied from

Table 1. Summary of stroboscopic analysis

Glottic Mucosal CordAnimal group closure motion motion

Control groupNormal Complete Two-mass Symmetric

bilaterallyParalysis Incomplete; One-mass Asymmetric

posterior paralyzedglottic cordleak

Nerve transfergroupAnsa stimu- Complete Two-mass Symmetric

lated bilaterallyAnsa not Incomplete; One-mass Asymmetric

stimu- posterior paralyzedlated glottic cord

leak

Table 2. Conduction veloclfy (m/sec):

Experimental group

Normal Ansa cervlcaUsDog RLN nerve tranafer

1 50.0 18.0

2 34.6 3563 42.9 13.04 37.6 37.5

5 45.8 50.0

MEAN 42.2 30.8

SD 6.2 151

RLN. Recurrent laryngeal nerve.

462 GREEN et al.

Tabl. 3. Vocal efficiency: Control group

Dog Normal Paralyal.

1 30 0.0982 14.0 1.53 2.8 0.0554 23.0 0,0655 11.0 0.26 43.0 1,47 70.0 0,0658 49.0 1.3

MEAN 27.0 0,58so 25,S 0.68

All values are x 10- 4

Tabl. 4. Vocal efficiency: Experimental group

Wllhansa Without ansaDog Illmulallon .tlmulatlon

1 15.2 1.22 24.5 19.23 4.7 0.0894 18.4 0.1025 4.5 0.056

MEAN 13.4 4.1so 8.8 8.4

All values are x 10- 4

13 to 50.0 m/sec, with an average of 30.8 m/sec. Thesurface temperature of the nerve tissue was 31° C.

YocallfftolenoyTable 3 shows the vocal efficiency of all eight control

animals during normal phonation and simulated RLNparalysis. For the normal canine larynx, the vocal ef­ficiency varied from 3 x 10- 4 to I x 10-3, with amean of2.7 x 10- 3

• For recurrent laryngeal paralysis,the vocal efficiency decreased in every instance by atleast a factor of 10. The values for paralysis variedfrom 5.5 x 10- 6 to 1.3 x 10- 4

, with a mean of5.8 x 10- s. The vocal efficiency for normal phonationwas significantly lower than that for unilateral RLNparalysis (p < 0.05).

Table 4 shows the vocal efficiency for all five rein­nervated dogs. With stimulation of the ansa cervicalisnerve transfer during phonation, the vocal efficiencyranged from 4.49 x 10- 4 to 2.45 X 10- 3

, with amean of 1.34 x 10- 3

• Without stimulation of the ansacervicalis, the vocal efficiency had a range of9.24 x 10- 7 to 1.93 X 10- 3

, with a mean of3.93 x 10- 4 • The vocal efficiency with ansa stimu­lation was significantly higher than that without stim­ulation (p = 0.02).

Otolaryngology­Head and Neck SUrgery

The vocal efficiency of 1.34 x 10-3 with ansa stim­ulation was not significantly different from the valueof 2.7 x 10-3 for normal phonation, but was signifi­cantly higher than the value of 5.8 x lO- s for unilat­eral RLN paralysis. The vocal efficiency of3.93 x 10-4 without ansa stimulation was not signif­icantly different from the value for unilateral RLN pa­ralysis but was significantly lower than the value fornormal phonation (p < 0.05).

Acoustic Analy."Figs. 5, 6, and 7 show the mean jitter, shimmer, and

signal-to-noise ratio, respectively, for normal phona­tion, unilateral RLN paralysis, and ansa cervicalisnerve transfer with and without stimulation. The meanjitter for normal phonation was 24.1% (standard error[SE] = 6.1), 38% (SE = 6.8) for paralysis, 6.7%(SE = 3.6) for nerve transfer with ansa stimulation,and 18.0% (SE = 6.8) without stimulation. Jitter val­ues for normal phonation and nerve transfer with ansastimulation were significantly lower (p < 0.05) thanthose for unilateral RLN paralysis.

The mean shimmer was 13.0% (SE = 4.9) for nor­mal phonation, 39.1 % (SE = 9.2) for paralysis, 5.7%(SE = 3.2) with ansa stimulation, and 16.7%(SE = 7.2) without stimulation. Shimmer values fornormal and ansa nerve transfer stimulation were sig­nificantly lower (p < 0.05) than those for unilateralRLN paralysis.

The mean signal-to-noise ratio was 10.9 dB(SE = 2.2) for normal phonation, 4.9 dB (SE = 1.5)for paralysis, 20.6 dB (SE = 3.0) with ansa stimula­tion, and 9.2 dB (SE = 3.8) without stimulation. Thesignal-to-noise ratio for normal phonation and ansanerve transfer stimulation was significantly higher(p < 0.001) than that for unilateral RLN paralysis. Inaddition, phonation with ansa stimulation had a signif­icantly higher signal-to-noise ratio than that withoutansa stimulation (p < 0.05).

DISCUSSION

In general, the vocal efficiency and acoustic qualityafter ansa cervicalis nerve transfer were dependent onthe degree of stimulation from the transferred nerve tothe reinnervated cord during phonation. Stimulation ofthe ansa cervicalis nerve transfer resulted in significantimprovement in vocal efficiency and the acoustic signal(signal-to-noise ratio) and a decrease in the frequencyand amplitude perturbation (jitter and shimmer) com­pared to unilateral RLN paralysis. The degree of im­provement over the unilateral paralysis state after thenerve transfer was dependent on the amount of stim­ulation provided by the ansa cervicalis.

Volume 104 Number 4April 1991 Future directions for laryngeal reinnervation 463

--i

Without AnsaStimulation

With AnsaStimulation

40

35

30

25

% 20

15

10

5

0 --+-

Normal Paralysis

Fig . 5. Mean jitter for norma l phonation and unilatera l RLN paralysis in control group (N = 8). andwith and without stimulation of ansa cervica lis nerve transfer in experimental group (N = 5).

Normal ParalysisWith AnsaStimulation

Without AnsaStimulatio n

Fig. 6. Mean shimmer for normal phonation and unilaterol RLN paralysis In control group (N = 8).and with and without stimulation of ansa cervlcous nerve transfer In experimental group (N = 5).

It is generally accepted that the bulk of the vocalcord adductor muscles outweighs that of the abductors.For this reason , random synkinesis usually results inadduction during stimulation of the reinnervated nerve.In four of the reinnervated dogs, stimulation of the ansacervicalis nerve transfer produced vocalis contractionand arytenoid adduction ; however, one dog demon­strated vocalis contraction and arytenoid abduct ion.During phonation in the dog with the arytenoid abduc­tion, the normal cord and arytenoid crossed the midlineto meet the reinnervated cord, thereby completing glot­tic closure during laryngeal nerve stimulation . AI-

though the posterior glottis was shifted to the reinner­vated side in this dog, there was symmetric vocal iscontraction and two-mass mucosal motion in bothcords. It is interesting to speculate what pattern of rein­nervation led to this abduction of the cord with vocaliscontraction . The evoked thyroarytenoid EMG on thereinnervated side in this dog was among the lowest. at30% of the normal side. This finding lends support tothe notion that there was a relative lack of reinnervationof the lateral cricoarytenoid and vocalis and strong rein­nervation of the posterior cricoarytenoid. This findingof cord abduction with reinnervation of the RLN stump

... GREEN at al.

25

20

15

db10

5

0

Normal ParalysisWith AnsaStimulation

Ofolaryngology­Head and Neck SUrgery

--i

Without AnsaStimulation

Fig. 7. Mean signal-tD-noise ratio for normal phonation. unilateral RLN paralysis in control group(N = 8). and with and without stimulation of ansa cervicalis nerve transfer in experimental group(N = 5).

by the ansa cervicalis has not been previously describedin human beings.

During stimulation of all the laryngeal nerves­including the ansa cervicalis nerve transfer­videostroboscopy appeared normal in all five of thereinnervated dogs with complete glottic closure, two­mass mucosal motion, and symmetry. Absence of nervetransfer stimulation during stimulation of the other threelaryngeal nerves provided the stroboscopic picture ofan acute unilateral RLN paralysis with a mild posteriorglottic chink incompetence, one-mass motion of theparalyzed cord, and greater lateral excursions of thereinnervated cord than the normal cord.

In the past, other studies have relied on the isolatedpresence of vocal cord mobility on laryngoscopy orEMG evidence to prove reinnervation. However, theseprocedures may not be sufficient evidence of reinner­vation because of-the action of the other intrinsic andextrinsic laryngeal muscles and the ability of neigh­boring nerves to reinnervate adjacent paralyzed mus­cles." This study used the combination of evoked EMGfrom direct stimulation of the transferred nerve, ob­served motion from direct nerve stimulation, histologicevidence of preserved vocalis muscle bulk, and axonalregrowth into the RLN stump as evidence of a suc­cessful nerve transfer.

The amount of reinnervated muscle in the thyroary­tenoid was estimated by the amplitude of depolar­ization on the evoked EMG. In human beings, Peytzet al.44 found the peak-to-peak amplitude of the evokedthyroarytenoid muscle action potential to be 6.8 ± 0.7

mY, and Steiss and Marshall" obtained values of 1.2to 26.0 mV in the dog. The values of 5.5 to 20.0 mVobtained in this study are in agreement with the findingsof Steiss and Marshall." Of the five dogs tested in thisstudy, all showed some degree of reinnervation byevoked EMG, with an average amplitude of 66% of thenormal side. Possible reasons for partial reinnervationafter nerve transfer include axonal escape from theanastamosis, tension or infection of the anastamosis,and devascularization of the nerves after dissection.One of the dogs had an evoked EMG in the reinnervatedthyroarytenoid that was 156% of the normal side. Thismay have been a result of synkinesis within the rein­nervated thyroarytenoid. Intramuscular synkinesis maytend to simultaneously depolarize a greater muscle bulkthan the normal side, which would depolarize the samemuscle bulk, but over a longer period of time. Althoughthe absolute bulk of depolarized muscle, as calculatedby the integral under the depolarization curve, may bethe same on both sides, the peak-to-peak amplitude forthe reinnervated cord may be higher.

Motor nerve conduction velocities are closely relatedto the fastest conducting fibers." These fibers are thosewith the largest diameter. The maximal conduction ve­locity increases by approximately 6 m/sec/ umincreasein fiber diameter. Braund et al." calculated the con­duction velocity of the RLN to be 54 to 60 m/sec basedon the largest fiber diameter size of 9 to 10 urn in theRLN. Several studies have measured the conductionvelocity of the RLN directly with evoked EMG.Atkins" found a symmetric conduction velocity of 65

Volume 104 Number 4Aprt11991

m/sec in the proximal RLN and 30 m/sec in the distalRLN. This slowing of conduction in the distal portionof the nerve has also been found by Shin and Rabuzzi.49

Their study also found a faster conduction on the leftin the proximal portion of the RLN. Steiss andMarshall" found the mean normal canine conductionvelocity to be approximately 56 m/sec bilaterally.These values are in agreement with the conductionvelocity of 42.2 m/ sec found in this study for the nor­mal RLN. The RLN conduction velocity has been mea­sured in human beings and found to be 29 m/sec bi­laterally."

In this study, the reinnervated side had a conductionvelocity of 30.8 m/sec on average. This slowing of theconduction velocity by approximately 10 m/sec couldbe the result of either a decrease in the axon fiber di­ameter by 1 to 2 ....m or a relative absence of myelinafter 6 months of regrowth.

The thyroarytenoid muscle is composed of 95% fast­twitch type 2 muscle fiberscompared to 5% slow-twitchtype I fibers." Rice and Cooper" studied the contractileproperties of the canine thyroarytenoid muscle rein­nervated from the ansa cervicalis. They found that thetwitch contraction times increased by 23% to 60% overthe normal side. They attributed this change to adap­tation of the muscle to the different nature of the motorneuron innervating it with a change in the fiber type.The thyrohyoid and sternothyroid have contractiontimes of 50 msec compared to 12.5 msec for the thy­roarytenoid. In two of the three dogs studied by Riceand Copper.50 the twitch was weaker than on the op­erated side on. However, they did not study fatigability.

In this study, the vocal efficiency (the ratio of acous­tic to subglottic power) and the resulting acoustics afteransa nerve transfer were much improved over the pa­ralysis state if there was stimulation of the ansa cer­vicalis nerve transfer. To what degree the ansa-RLNtransfer is capable of providing this necessary degreeof stimulation during phonation in the human beingsis unclear. Fata et al." found in the canine that asternothyroid nerve-muscle pedicle, when implantedinto the posterior cricoarytenoid, could reinnervatebut not spontaneously activate this muscle, evenduring prolonged occlusion of the airway. Grundfest­Broniatowski et al." found that reinnervated strap mus­cles do not re-establish proprioception. Loss of pro­prioception may have an effect on the ability of thecentral nervous system to control the fine motor move­ments of a reinnervated muscle. To obtain muscle con­traction, Broniatowski et al." used direct electronicstimulation of the ansa cervicalis nerve-muscle pedicle.This was done to contract an implanted posterior cri-

Futuredlrecttons for laryngeal reinnervation 461

coarytenoid muscle as treatment for bilateral vocal cordparalysis.

What future role the nerve transfer procedure willplay in the treatment of unilateral vocal cord paralysisis evolving. Without physiologic arytenoid adductionand thyroarytenoid contraction, a nerve transfer mayserve to merely maintain the bulk and tone of the par­alyzed cord. On the other hand, the choice of appro­priate nerve for transfer may produce physiologic ary­tenoid adduction with vocalis contraction. This wouldresult in improved vocal efficiency and acoustics duringphonation. The optimal nerve for transfer in the humanto provide this necessary degree of stimulation shouldbe the subject of future research.

CONCLUSIONIn this study, the vocal efficiency (the ratio of acous­

tic to subglottic power) and acoustics (jitter, shimmer,and signal-to-noise ratio) after ansa-RLN nerve transferfor unilateral recurrent laryngeal paralysis were signif­icantly improved with stimulation of the ansa-RLNanastamosis (as opposed to no stimulation). In the ab­sence of stimulation. physiologic movement could notbe elicited from the reinnervated cord. To what degreethe ansa-RLN transfer is capable of providing this nec­essary degree of stimulation during phonation in humanbeings is unclear. The optimal nerve for transfer to theRLN. to rehabilitate laryngeal function, should be thesubject of future studies.

We wish to thank Jeff Gombein, DrPH. for his assistancein statistical analysis.

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