The effect of unilateral superior laryngeal nerve lesion on swallowing threshold volume

The LaryngoscopeVC 2013 The American Laryngological,Rhinological and Otological Society, Inc.

The Effect of Unilateral Superior Laryngeal Nerve Lesion

on Swallowing Threshold Volume

Peng Ding, MD, PhD; Regina Campbell-Malone, PhD; Shaina D. Holman, BS; Stacey L. Lukasik, BA;

Allan J. Thexton, PhD; Rebecca Z. German, PhD

Objectives/Hypothesis: The superior laryngeal nerve (SLN) is the major sensory nerve for the upper larynx. Damage tothis nerve impacts successful swallowing. The first aim of the study was to assess the effect of unilateral SLN lesion on thethreshold volume sufficient to elicit swallowing in an intact pig model; this volume was defined radiographically as the maxi-mum bolus area visible in lateral view. The second aim was to determine if a difference existed between ipsilateral and con-tralateral function as a result of unilateral sensory loss, measured as the radiologic density of fluid seen in the valleculae.Finally, we determined whether there was a relationship between the threshold volume and the occurrence of aspiration aftera unilateral SLN lesion.

Study Design: Repeated measures animal study.Methods: Four female infant pigs underwent unilateral SLN lesion surgery. The maximum vallecular bolus area in lateral

view and the relative vallecular density on each side in the dorsoventral view were obtained from videofluoroscopic record-ings in both the prelesion control and postlesion experimental states.

Results: In lateral view, the lesioned group had a larger maximum bolus area than the control group (P<.001).Although occasional left–right asymmetry in the dorsoventral view was observed, the vallecular densities were, on average,equal on both the left (intact) and right (lesioned) sides (P>.05). A bigger maximum bolus area did not predict aspiration inthe lesioned group (P>.05).

Conclusions: Unilateral SLN lesions increased the swallowing threshold volume symmetrically in right and left vallecu-lae, but the increased threshold may not be the main mechanism for the occurrence of aspiration.

Key Words: Superior laryngeal nerve, swallowing threshold, bolus area, aspiration, videofluoroscopy, deglutition/degluti-tion disorders, suck-swallow ratio, animal models, nerve lesion.

Laryngoscope, 123:1942–1947, 2013

INTRODUCTIONThe internal branch of the superior laryngeal nerve

(SLN) is a sensory nerve that supplies the hypopharyng-eal and supraglottic areas. Sensory deficits in theseareas correlate with the occurrence of swallowing dys-function and the development of aspiration pneumoniain selected patient populations.1 Using an animal model,we have previously shown that a unilateral SLN lesionresults in an increased incidence of aspiration;2 however,the mechanism for this aspiration is unclear. Studies ofthe threshold bolus volume required to elicit aerodiges-

tive reflexes show that a normal swallowing threshold isan important factor in preventing aspiration.3–5 Thethreshold liquid volume required to trigger the reflexivepharyngeal swallow in man is less than the maximumvolume of liquid that can be contained within the hypo-pharynx without it spilling into the airway.5 When thelocal reflexes are abolished with pharyngeal anesthe-sia,4,5 this threshold volume can be exceeded, which mayresult in aspiration. However, whether a unilateral SLNlesion results in an increased threshold and whether ahigher threshold volume in the valleculae is responsiblefor the occurrence of aspiration have not beeninvestigated.

Various methods can be used to study the swallow-ing threshold. In both human and animal studies, thethreshold volume of liquid required to trigger the pha-ryngeal swallow is a standard parameter used in manystudies.3–6 As the fluid is delivered by a syringe or aninfusion pump, the threshold volume is easily measured.However, for a solid bolus in human trials, the numberof chewing cycles preceding the swallow has been widelyused as an indirect measure of the swallowing thresh-old.7 Although it is not possible to measure bolus volumedirectly in a free-feeding subject, the bolus area of bar-ium liquid in the valleculae is a potential parameter toexpress threshold volume. Bolus area is easily measuredand steadily expands between swallows during feeding

From the Department of Physical Medicine and Rehabilitation,Johns Hopkins University School of Medicine (P.D., R.C.-M., S.D.H., S.L.L.,R.Z.G.), Baltimore, Maryland; Department of Neural and Pain Sciences,University of Maryland School of Dentistry (S.D.H., R.Z.G.), Baltimore,Maryland, U.S.A.; and Division of Physiology, King’s College (A.J.T.), Lon-don, United Kingdom.

Editor’s Note: This Manuscript was accepted for publicationJanuary 22, 2013.

The work was done at the Johns Hopkins University School ofMedicine.

This work was funded by NIH grant DC9980 to Rebecca Z.German, PhD.

The authors have no other funding, financial relationships, orconflicts of interest to disclose.

Send correspondence to Peng Ding, MD, 98 N. Broadway Suite409, Baltimore, MD 21231. E-mail: [email protected]

DOI: 10.1002/lary.24051

Laryngoscope 123: August 2013 Ding et al.: Unilateral SLN Lesion and Swallowing

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in the lateral view of the videofluoroscopic swallowingstudy (VFSS) in infant pigs.8 Infant pigs store milk intheir valleculae over several transport/suck cycles beforea swallow. The volume stored in the valleculae by therhythmic tongue movements of suckling is considered theprimary factor to initiate pharyngeal swallow.9 In thepresent study, we used this videofluoroscopic value at theonset of a swallow as a measure of the volume thresholdfor swallowing liquid containing barium; this is referredto as the maximal bolus area. In the dorsoventral view,the videofluoroscopically determined relative densities ofthe left and right valleculae were used as an index of theequality of filling of the two sides.

The purpose of this investigation was to evaluatethe association between a unilateral SLN lesion and theswallowing threshold as expressed by maximum bolusarea. We asked if unilateral SLN lesion has the potentialto change the swallowing threshold. If this lesion doesmodify the threshold of bolus volume in the valleculae,is bolus accumulation symmetric? That is, does a unilat-eral lesion produce changes only on one side, or is it achange that impacts on both sides. Finally, if there is anincrease in bolus threshold in the valleculae, does thisincrease cause aspiration? To answer all these questions,we tested three hypotheses in this study: 1) After a uni-lateral SLN lesion, the maximum bolus area is largerthan that of the control group; 2) the unilateral SLNlesion does not affect the symmetrical bolus dispersal inthe vallecula (i.e., both left and right sides contain equalvolume when measured by vallecular radiographic den-sity); and 3) a higher maximum bolus area correlateswith a higher incidence of aspiration in the unilateralSLN lesioned group (Fig. 1).

MATERIALS AND METHODSFour female pigs, 2 to 3 weeks old, were obtained from

Tom Morris Farms (Reisterstown, MD). They weighed 4 to 5 kgat the time of arrival at the vivarium of the Johns Hopkins Uni-versity. All pigs were fed on milk-replacement formula (LandO’Lakes Solustart pig milk replacer; Land O’Lakes, St. Paul,MN). Each pig was examined and determined to be healthy bya veterinarian before surgery. All procedures on living animalswere approved by the Johns Hopkins Institutional Animal Careand Use Committee (Protocol Number: SW10M212) and havebeen described in detail in a previous paper (Ding et al., perso-nal communication).

During the first 2 days at the facility, we trained the pigsto suckle milk from a bottle fitted with a commercial pig nipple

(Nasco Inc., Fort Atkinson, WI). On day 3, the first surgery was

performed to mark the right side SLN and implant markers

into the epiglottis and tongue. At this time, recording bipolar

electromyography electrodes were placed in the hyoid muscula-

ture, but the results from those measurements will be

addressed in a separate paper. After animals were entirely alert

and awake, approximately 3 to 4 hours postsurgery, we col-

lected control data using videofluoroscopy, as described later. A

second surgery was performed on day 4 to cut the marked SLN.

After this procedure, data were collected on the lesioned ani-

mals. Fluoroscopy was performed using a Toshiba Infinix

C-Arm imaging system (Infinix -I, Toshiba Corp., Tokyo, Japan).

The video images were recorded at a rate of 30 frames per sec-

ond and exported in AVI format for later analysis in ImageJ

(National Institutes of Health, Bethesda, MD). All subjects were

fed with milk mixed with barium powder in a fixed ratio of 8

ounces milk with one-third cup powdered barium (E-Z-HD, E-Z-

EM, Inc., Westbury, NY). During feeding, each pig was con-

tained in a Plexiglas feeding box, where it naturally held a

steady position on the teat. We collected the VFSS imaging data

separately in both dorsoventral and lateral projections, both

before and after a unilateral SLN lesion.

In this study, we used the new videofluoroscopic measure

of swallowing threshold referred to as the maximum bolus area.

This was defined as the maximum area of the liquid containing

barium in the valleculae in the lateral view of the VFSS, meas-

ured in the video frame recorded immediately before the epi-

glottis flipped during the swallow.9 A large maximum bolus

area would suggest an increased swallowing threshold volume,

which may predict the likelihood of aspiration. To establish the

utility of the maximum bolus area as a parameter representing

the threshold volume, we randomly sampled 30 swallows from

the control feedings and 30 from the lesioned feedings. These

were matched by individual.

There were one to five suck cycles before a swallow to

transport milk from the nipple into valleculae. At the end of

each suck cycle, we measured the vallecular bolus area by using

the “Polygon Selections” tool in ImageJ. This tool enables the

user to outline an area on an image. The program then calcu-

lates the area inside the outline. The end of a suck cycle was

defined as one video frame before the jaw started to close. Then

we performed linear regression analysis using SYSTAT 13

(Systat Software, Inc., Chicago, IL). If the bolus area in the val-

leculae has a positive relationship with the accumulation of

bolus volume by each suck, we can say that the bolus area

increases with and reflects the increase of bolus volume.

All videos selected for movement analysis were chosen onthe basis of minimum rotation or yaw of the head as deter-mined by the overlap of the images of the teeth from the twosides of the head. To examine hypotheses 1 and 3, we selected86 swallows from the control feedings (24, 22, 27, 13 swallowsin each subject) and 89 swallows from the lesioned feedings (25,13, 20, 31 swallows in each subject) from the four animals,recorded in the lateral view. The reduction in numbers from theinitial 35 randomly chosen swallows reflected swallows withhead rotation. In these cases, we used the “Polygon Selections”tool to digitize the maximum bolus area, the area of the boluscontained in the valleculae, one frame before the epiglottismoved posteriorly/inferiorly (the epiglottal “flip”) (Fig. 2). Weused this measure as a two-dimensional approximation of thevolume of the barium milk contained in the valleculae. Thelevel of radiologic image magnification was established from theknown nipple diameter and its uncompressed diameter fromvideos when suckling was not occurring. Aspiration was identi-fied following frame-by-frame examination of each swallow. Inthis paper, any barium milk passing the vocal folds into the

Fig. 1. The schematics of the three hypotheses (H1, H2, H3).BAmax 5 the maximum bolus area in valleculae; SLN 5 superiorlaryngeal nerve.


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trachea was considered aspiration (IMPAS score 7) (Holmanet al., personal observations).

In dorsoventral view, with the delivery of milk containingbarium from oral cavity to the valleculae, the optical density ofthe accumulated liquid in the valleculae changed more obvi-ously than the area. Therefore, for the dorsoventral data, weused the vallecular density on each side to express the relativeamounts of barium milk contained in each vallecula. To test hy-pothesis 2, we included 58 swallows from the three-animal con-trol group (no dorsoventral control data was available foranimal C) (i.e., respectively, 11, 20, and 27 swallows in the three

subjects and 90 swallows from the lesioned group of four ani-mals; and, respectively, 21, 17, 25, and 27 swallows from thesubjects). These were examined in the dorsoventral plane, andthe vallecular density of each side was measured one framebefore the epiglottis inversion in the swallow. Within each poly-gon outlining the valleculae, we used the “Mean Gray Value”tool in ImageJ to calculate the density of barium in that space.We used the same-sized area of the vallecula of each side(Fig. 3). The Mean Gray Value was the average gray valuewithin the selection. This was the sum of the gray values of allthe pixels in the selection divided by the number of pixels.Because we were interested in comparing left to right, variationfrom frame to frame or session to session in density was not aproblem.

For hypothesis 1, we used the linear mixed-model (withindividual included as a random factor) to test whether themaximum bolus area increased after a unilateral SLN lesion. Totest hypothesis 2, concerning vallecular density differencesbetween the two sides, we used the repeated measures analysisof variance to compare the radiographic vallecular density ofthe left and right sides in dorsoventral image data. Finally weused binary logistic regression to analyze the relationshipbetween the maximum bolus area and aspiration incidence withaspiration as a dependent variable, separately in the controlgroup and in the lesioned group.10 For all of these analyses, theunit of analysis was a swallow.

RESULTS

Validation for Using the Maximum Bolus AreaThere was a total of 75 sucks proceeding the 30

swallow cycles in the control group and 89 sucks preced-ing the 30 swallow cycles in the lesioned group. Of the60 swallows, the bolus areas in 59 increased monotoni-cally over the sucks proceeding the swallow. The onethat decreased in volume did so by only 2%. In this case,aspiration did not occur, but more liquid was transportedinto the piriform sinuses. Bolus area and the number of

Fig. 2. The dotted line showed the area of the bolus in the vallecu-lae as measured in the lateral videofluoroscopic swallowing studyimage.

Fig. 3. The yellow line in the right picture showed the areas in which the vallecular density on each side was measured in the dorsoventralvideofluoroscopic swallowing study image. The left picture is the original image. L 5 left; R 5 right.


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sucks per unit time were positively correlated in boththe control and lesioned groups.

Results for Hypothesis 1After correction for image magnification, the mean

maximum bolus area was 1.76 cm2 (standard deviation[SD], 0.65) in the control group and 2.43 cm2 (SD, 0.75)in the lesioned group. The maximum bolus area was sig-nificantly larger in the lesioned group than in the con-trol group (P<.001). There was variation among theindividuals, but the area in the lesioned state was larger(Fig. 4).

Results for Hypothesis 2However, in the lesioned group viewed dorsoven-

trally, the differences in left to right vallecular densitieswere not significant (P 5.224) (Fig. 5), except for pig C(P 5.002), where the right side was denser. Similarly, inthe control group, the vallecular densities were also notsignificantly different between the two sides (P 5.171).

Results for Hypothesis 3Among the 86 swallows in the control group, viewed

laterally, three involved aspiration, which would be anincidence of 3.5%. The difference in the maximum bolusarea between the cases with aspiration and those with-out aspiration was not significant in the control group(P 5.619) (Fig. 6). In contrast, the incidence of aspirationin the lesioned group was 67.4% (60 of 89). There wasno significant difference in the maximum bolus areabetween cases with aspiration and without aspiration inthe lesioned group (P 5.602) (Fig. 6). All values for maxi-

mum bolus area in the control and the lesioned groupsare shown in Table I.

DISCUSSIONSwallowing is a reflex that is critical for the protec-

tion of the respiratory tract.10–12 Many pathologic lesionscan result in swallowing disorders by depressing theaerodigestive reflexes so that there is a higher swallow

Fig. 4. The difference in maximum bolus area between the controland the unilateral superior laryngeal nerve lesioned groups. Datafrom the lateral videofluoroscopic swallowing study images in foursubjects (P<.001, 175 cases, linear mixed-model).

Fig. 5. The difference of vallecular density between left (L) andright (R) side in the dorsoventral videofluoroscopic swallowingstudy images of the unilateral superior laryngeal nerve lesionedgroup among four subjects (P 5.224, 90 cases, analysis of var-iance repeated measures).

Fig. 6. The comparison of mean maximum bolus area betweennonaspirating and aspirating cases in the control group (P 5.619,86 cases, binary logistic regression test) and the unilateral supe-rior laryngeal nerve lesioned group (P 5.602, 89 cases, binarylogistic regression test). Data from lateral videofluoroscopic swal-lowing study image in four subjects.


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threshold volume.13–16 We have previously reported,using a pig model, that unilateral SLN lesions result ina higher risk of aspiration.2 In the present study, we fur-ther demonstrate that there is an increased maximumbolus area after a unilateral SLN lesion. This impliesthat additional sensory input, arising from a larger accu-mulation of fluid, is required to reach the reflex thresh-old before a swallow can be elicited. This finding alsocorrelates with the anatomy of sensory innervation bythe SLN. The highest density of sensory receptors islocated in the posterior tonsillar pillars, the laryngealsurface of the epiglottis, and the postcricoid and aryte-noid regions.17 These areas of the SLN innervations arethe same areas that are known to be the most sensitivefor triggering reflexive swallowing or glottal protec-tion.17 Hence, even a unilateral SLN lesion could resultin an increased threshold volume for swallowing.

As hypothesized, there was no asymmetry when val-lecular density was compared between two sides in thedorsoventral plane after a unilateral SLN lesion. Both ip-silateral and contralateral valleculae held equal amountsof milk. A possible explanation is that the pharyngealphase is relatively isolated from the oral phase, which isresponsible for the transport of milk into the vallecula;this is consistent with Lang’s view that the oral, pharyn-geal, and esophageal phases of swallowing can be inde-pendent of each other.18 Consequently, the lack ofsensory feedback from one side of the hypopharyngealand supraglottic areas would not affect the milk deliveryinto the vallecula; in the present experimental situation,milk delivery to the valleculae was a function of the cen-tral pattern for sucking, preceding any pharyngeal swal-low. In humans, unilateral vallecular pooling is seldomseen at all. This could be because, unlike the pyriformsinuses, a liquid bolus is unlikely to occupy one side ofthe vallecula even in the presence of a significant unilat-eral motor or sensory impairment. The median glossoepi-glottic fold separates the vallecula into right and leftsides. It may provide an insufficient barrier to contain abolus of liquid in one “side” of the vallecula or the other.In infant pig anatomy, however, the median glossoepi-glottic fold separates the vallecula into two sides based

on our observation by the direct laryngoscope and thevallecular impression made on the fresh pig cadavers.

In the lesioned group, a higher maximum bolusarea did not predict the occurrence of aspiration. Wefound no relationship between bolus volume and inci-dence of aspiration, although a unilateral SLN lesion didincrease the maximum bolus area. Although the volumeof the bolus may be a factor in aspiration, it is clearlynot the major one, and the SLN plays a critical role inairway protection by eliciting the laryngeal adductorreflex. The main reason for aspiration could be relatedto incomplete closure of the larynx during the pharyn-geal phase of swallowing, which has been reported afterbilateral SLN lesions.19 In a diverse cohort of patientswith dysphagia, absence of the laryngeal adductor reflexwas significantly associated with laryngeal penetrationand aspiration during swallowing.20 Hence, even oneside sensory loss of the SLN could compromise the pro-tective function. In the control group, cases of smallamounts of aspiration may contribute to the nonsignifi-cant statistic result. Hence we can’t exclude the possibil-ity that a higher bolus volume could be a risk foraspiration in normal swallows.

In this study, a positive relationship was foundbetween the number of sucks and the results, radio-graphically defined, for area of the bolus in the valleculae(seen in the lateral view VFSS). This demonstrated thatthe maximum bolus area was an appropriate surrogatefor the volume of liquid in the valleculae and also thatthe area recorded immediately before epiglottal flexionwas an expression of the threshold volume required toelicit the swallow. In conscious, active infants, it is rarelyfeasible to test the liquid threshold volume for the pha-ryngeal swallow by direct liquid infusion into the phar-ynx. Therefore, the method of this study may be ofparticular benefit in the study of dysphagic infants.

CONCLUSIONOur study demonstrated that the unilateral SLN

lesion increases the threshold volume of liquid for swal-lowing using a new videofluoroscopic parameter for val-lecular volume, the maximum bolus area. The unilateralsensory loss following SLN lesion does not affect thesymmetry of bolus delivery into the valleculae, which isa function of the central pattern generator for sucking.However, an increased threshold volume is not the mainfactor contributing to aspiration in the unilateral SLNlesioned group.

ACKNOWLEDGMENTSThe authors appreciate Laurie Pipitone for her support inradiologic data collection. The authors also thank TakakoFukuhara and Wallace Feng for their assistance in datacollection.

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TABLE I.The Values of the Maximum Bolus Area in the Control and

Lesioned Groups.

Swallow No. BAmax, cm2 SD, cm2

SLN

Intact–control 86 1.76 0.65

Lesioned 89 2.43 0.75

Control group

No aspiration 83 1.75 0.66

With aspiration 3 1.94 0.79

Lesioned group

No aspiration 29 2.37 0.73

With aspiration 60 2.46 0.77

BAmax 5 the maximum bolus area in valleculae; SD 5 standard devia-tion; SLN 5 superior laryngeal nerve.


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The effect of unilateral superior laryngeal nerve lesion on swallowing threshold volume

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