Introduction

Laryngeal paralysis is a syndrome in which there is failure of normal arytenoid movement.¹ Most importantly, the arytenoid cartilages fail to abduct during inspiration, thereby obstructing airflow at the rima glottidis (RG), the narrowest part of the laryngeal airway.^1,2 The idiopathic form of this condition is commonly seen in older, large-breed dogs and is the result of a polyneuropathy, leading to degeneration of the recurrent laryngeal nerve. Degeneration of the nerve results in neurogenic atrophy of the dorsal cricoarytenoid (CA) muscle, leading to its failure to contract during inspiration.^1–4

Many surgical procedures have been developed to treat laryngeal paralysis. Unilateral arytenoid lateralization (UAL) is the most commonly used procedure performed to treat laryngeal paralysis in dogs because it is successful at increasing the area of the RG and has a relatively low complication rate.^2,3,5–9 Ideally, UAL will mimic the action of the dorsal CA muscle by pulling the muscular process of the arytenoid cartilage in a caudal direction.^1,10 Most commonly, the CA joint is disarticulated, and a suture is placed from the arytenoid to the cricoid cartilage.^2,11–15,17

Arytenoid lateralization without disarticulating the CA joint has been previously referred to as the abductor muscle prosthesis technique.¹⁶ Purported benefits of this technique include its ease of performance and minimal trauma to the laryngeal structures.¹⁶ Because the CA joint and medial attachments of the arytenoid remain intact, the arytenoid cartilage is pulled caudomedially by the suture in its natural, arch-like fashion.^16,17 It is unknown if the desired amount of abduction or the proper suture placement is achievable with this technique.

When performing any variation of the arytenoid lateralization procedure, the potential complications have to be considered. The postoperative complication rate ranges from 28% to 58%, with aspiration pneumonia being the most important complication.^3,4,7 Postoperative pneumonia has been attributed to the surgical procedure, muscle disruption during the surgical approach, and to concurrent esophageal dysfunction.¹⁸ In one study, all dogs affected with idiopathic laryngeal paralysis had some degree of esophageal dysfunction, and postoperative aspiration pneumonia rates were higher in dogs with greater esophageal dysfunction.¹⁹ Evaluating esophageal and neurologic function prior to surgery and achieving the desired amount of arytenoid abduction are therefore important in ensuring surgical success.^1,20,21

Although the abductor muscle prosthesis technique has been described, it is unknown how arytenoid lateralization without disarticulation of the CA joint compares to the current standard technique in terms of increasing RG area. While the intent of UAL is to increase the area of the RG with the epiglottis open, it is equally important to prevent a significant increase in the aperture not covered when the epiglottis is closed, as this may increase the risk of aspiration pneumonia.²¹ Another important variable in the UAL technique is that of suture tension. The effect of suture tension on overall outcome of the surgical technique has been evaluated in recent studies.^20–22

The purpose of this study was to evaluate how each of these variables affects the outcome of the UAL procedure by comparing the effects of CA joint preservation versus disarticulation on RG area with the epiglottis open and closed under both high and low suture tension.

Materials and Methods

Eleven larynges were harvested from cadavers of adult, medium- and large-breed dogs, euthanized for reasons unrelated to this project. All larynges appeared to be anatomically normal. Specimens were kept moist and refrigerated until used; storage time was approximately 2 wk. Needles were placed through peripheral laryngeal tissues in a manner that would not impede laryngeal motion and secured to a corkboard so that the larynx was stabilized in a dorsoventral position. This resulted in a view similar to laryngeal examination in a live dog.

A single polypropylene (2-0) suture was placed through the mucosa at the rostral point of the epiglottis and then passed through the tracheal lumen so that the epiglottis could be moved from a resting (open) position to a closed position.²² Arytenoid lateralization was performed on the left side of each larynx. The muscular process of the arytenoid was palpated under the junction of the dorsal CA muscle and thyroarytenoid muscle to localize the CA articulation. A simple interrupted 2-0 polypropylene suture was placed through the caudodorsal edge of the cricoid cartilage and then through the muscular process of the arytenoid cartilage, leaving the CA articulation intact. Tension was measured using a hanging scale^a tied to one end of the suture line. Low tension was achieved by pulling until tension was felt. This tension was then measured and set as our “low” tension value, measuring 0.06 kg. High tension was achieved by pulling until the suture could not be tightened further, at which point the tension was measured and set as our “high” tension value, measuring 0.24 kg. Digital images were taken between each step.

After images were recorded, the suture was removed, and the dorsal CA muscle was transected midbody with scissors after blunt dissection, being careful not to damage the muscular process.^20,22 A simple interrupted 2-0 polypropylene suture was placed through the caudodorsal edge of the cricoid cartilage and through the center of the articular surface of the muscular process of the arytenoid cartilage. The suture was placed to achieve both low and high tension, and images were obtained with the epiglottis open and closed. All surgical procedures were performed by the same investigator (BS).

Images of the glottic opening for each specimen were obtained at each step of the experiment with a digital camera^b positioned in a plane parallel to the glottic opening with the lens 15 cm from the RG. A ruler was placed next to the larynx for image scaling. Computerized planimetric analysis^c was used to measure RG cross-sectional area on each image. Rima glottidis was defined as the medial side of the corniculate process, vocal folds, and the base of the epiglottis ventrally with the epiglottis in an open position. With the epiglottis closed, RG was defined as the aperture uncovered by the epiglottis.²⁵ Data were evaluated using commercially available software^d. The continuous variable RG area was evaluated for normality by visual inspection of a histogram, P-P plots, and Q-Q plots. Data were displayed as group median (range). Related-samples Friedman’s two-way analysis of variance by ranks was performed in a pairwise fashion to evaluate the difference in RG area between groups (CA intact versus disarticulated, high and low suture tension both with the epiglottis open and closed). Additionally, the percentage change in RG area over baseline for each larynx under each condition was calculated as RG area at baseline (epiglottis open or closed) / RG area following UAL with specific technique (epiglottis open or closed) x 100. This was done so as to account for inherent differences in the RG size between cadavers. Again, data were compared between groups using related-samples Friedman’s two-way analysis of variance by ranks.

Results

Unilateral Arytenoid Lateralization with Epiglottis in an Open Position

With the epiglottis in an open position, the RG area was significantly greater following UAL than the RG area at baseline, for all techniques used (i.e., CA intact and disarticulated, high tension and low tension) (P = .001) (Table 1, Figure 1). With the epiglottis open and the CA joint intact, high suture tension resulted in a greater RG area (P = .001) and greater increase in RG area compared to low suture tension (P = .001). Similarly, with the epiglottis open and the CA joint disarticulated, high tension resulted in a greater RG area (P = .007) and a greater increase in area relative to baseline when compared to low suture tension (P = .005). However, there was no difference in the RG area or percentage increase from baseline with the epiglottis in the open position when comparing UAL with the CA intact versus CA disarticulated at low or high tension (P = .366 for all comparisons). Percentage increases in RG area relative to baseline are shown in Table 2. Thus, with the epiglottis open, high tension resulted in a significantly greater increase in RG area than low tension, while disarticulation did not affect RG area.

TABLE 1 Rima Glottis Area Displayed as Group Median (Range) for the Epiglottis in the Open Position Using Various UAL Techniques Compared to Baseline

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TABLE 2 The Percentage Increase in RG Area Over Baseline Displayed for Different Conditions, Notably, CA Intact Versus Disarticulated and UAL Under Low Tension Versus High Tension with the Epiglottis in the Open Position

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With the epiglottis in a closed position, there was no significant difference in the absolute area of the RG following UAL with the CA intact at low suture tension and the CA disarticulated at low or high suture tension (Table 3). However, UAL with the CA intact and high suture tension did increase the RG area (P = .003) with the epiglottis closed and resulted in a significantly greater increase in area than low suture tension relative to baseline (P = .0001). With the epiglottis closed and the CA intact, high suture tension resulted in a higher absolute RG area (P = .003) and greater percentage increase over baseline (P = .005) than low suture tension. Similarly, with the epiglottis closed but the CA disarticulated, high suture tension resulted in a higher absolute RG area (P = .003) and greater percentage increase over baseline (P = .008) than low suture tension. Again, there were no differences in the RG area or percentage increase from baseline with the epiglottis in the closed position when comparing UAL with the CA intact versus CA disarticulated at low (P = .705) or high tension (P = .480). Percentage increases in RG area relative to baseline are shown in Table 4.

TABLE 3 Rima Glottis Area Displayed as Group Median (Range) for the Epiglottis in the Closed Position Using Various UAL Techniques Compared to Baseline

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TABLE 4 The Percentage Increase in RG Area Over Baseline Displayed for Different Conditions, Notably, CA Intact Versus Disarticulated, and UAL Under Low Tension Versus High Tension with the Epiglottis in the Closed Position

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Discussion

In this study, when measured with the epiglottis open, there was no significant difference in the RG area following UAL with the CA joint intact compared to the CA joint disarticulated at both low and high suture tension. The intent of arytenoid lateralization or rotation surgery is to increase the area of the RG with the epiglottis open to reduce respiratory distress by decreasing laryngeal resistance.^20,26,27 Despite not knowing the exact amount of RG area increase necessary to relieve clinical signs, numerous studies have shown that UAL performed with a disarticulated CA joint is capable of eliminating clinical signs in vivo and of increasing the RG area with an open epiglottis in cadaver specimens.^{3,9,13,20,22,29} Performing UAL with the CA joint intact should therefore increase the area of the RG enough to diminish or eliminate clinical signs, with the purported benefits ease of performance and minimal trauma to the laryngeal structures.¹⁶

While the intent of UAL is to increase the area of the RG with the epiglottis open, it is equally important to prevent a significant increase in the aperture not covered when the epiglottis is closed, as this may increase the risk of aspiration pneumonia.²² In this study, the aperture not covered by the closed epiglottis was comparable when low-tension UAL was performed with an intact CA joint and when low- and high-tension UAL were performed with a disarticulated CA joint. High-suture-tension UAL with the CA joint intact did result in a significant increase in the RG aperture relative to baseline.

Based on previous studies of UAL in the canine cadaver with low and high suture tension, it has been proposed that low tension, not high, be used when performing UAL. Low tension creates a desirable increase in the RG area, which leads to reduction in laryngeal resistance during inspiration with the epiglottis open but does not increase the RG area excessively with the epiglottis closed, as high suture tension does.^20,22 Clinically, it is difficult to objectively measure tension; however, in this study, tension was objectively measured. While the values used for low and high tension in this study were lower than those used in a previous study, our tension measurements were dictated by what might be performed in a clinical setting.²¹ Low tension in this study was achieved by pulling until tension was felt, and this tension was then measured and set as our low-tension value. High tension was achieved by pulling until the suture could not be tightened further, at which point then tension was measured and set as our high-tension value. Surgeon experience plays a role in suture tension and therefore arytenoid lateralization. The endotracheal tube diameter also plays a role in determining the extent of arytenoid lateralization in the clinical setting. If a large tube is used, low tension may be capable of producing excessive lateralization.

The effect of CA disarticulation in conjunction with arytenoid lateralization has been examined previously in canine cadaveric larynges.^27,29 In previous studies, disarticulation resulted in a larger area of the RG than did preservation of the CA joint; however, the RG area was only measured with the epiglottis open, with high suture tension, and with the sesamoid band severed, which has been shown to distort the dorsal margin of the RG.²⁹ In this study, UAL with disarticulation did not increase RA area relative to UAL with the CA joint intact, at either low or high suture tension, with the epiglottis open or closed.

Suture placement through the arytenoid cartilage has been shown to be very important in a previous study.²³ It has been shown that sutures placed through the arcuate crest of the muscular process have greater pullout strength.²³ Improper placement may increase the risk of cartilage breakage and failure of this technique. It is possible to place sutures in this location without opening the CA joint, although visibility is lower. A limitation of this study was that correct suture placement was not verified; however, this is often the case in the clinical setting. Surgeons often verify that the suture has been placed correctly by extubating the patient and looking at the larynx itself to verify abduction of the arytenoid. As in the clinical setting, in this study, the larynx itself was examined after suture placement in order to verify arytenoid abduction. It is also important to note that passage of subsequent sutures was unique, which may have affected the results.

There are limitations to any study using cadaver larynges to evaluate laryngeal function. These limitations include loss of supporting perilaryngeal tissues, lack of response to tissue handling, and loss of muscular tone.²¹ Despite these drawbacks, studies have shown that functional laryngeal structure is preserved, and the effects associated with various surgical techniques can be evaluated.^20,27,30

A further limitation of this study is the lack of randomization and a relatively small sample size. In total, 11 cadaver larynges were used, and each larynx had the entire set of measurements performed on it. Thus, the procedures with the intact CA joint were performed on each larynx first, followed by the same measurements with the CA joint disarticulated. Ideally, there should have been two separate sets of larynges, one set for performing the measurements with the CA joint intact and another for performing measurements with the CA joint disarticulated. Because limited numbers of cadaver larynges were available, this was not possible. While performing the procedures sequentially on each larynx was not ideal, because the initial procedure required little dissection, it is unlikely that major anatomical changes occurred that could have altered the data.

Conclusion

In conclusion, UAL with both preservation and disarticulation of the CA joint resulted in an increase in RG area with the epiglottis open. High suture tension with either UAL technique further increases the RG area compared to low suture tension with the epiglottis open. High suture tension also increased the RG area relative to baseline with the epiglottis closed and the CA intact, which could translate to adverse clinical consequences. While there were no differences between RG area noted with preservation or disarticulation of the joint with low or high tension, it has been shown previously that proper suture placement is an important aspect of this surgical procedure. Since exact suture placement cannot be determined, further studies are warranted before UAL without CA disarticulation can be recommended for clinical use.