Abstract
Objectives
This study aimed to document the percentage increase in the rima glottidis volume after unilateral cricoarytenoid lateralisation (CAL) in feline cadavers and establish categories and ranges to evaluate the degree of arytenoid abduction.
Methods
Left CAL was performed in nine cadavers. Pre- and postoperative video endoscopy was performed on each resting and lateralised larynx. The images were analysed using an imaging program. Based on reference markings, the increase in the volume of each rima glottidis was measured, and the penetration of the mucosa by the suture material was documented. Measurements were evaluated using Wilcoxon’s signed-rank test and objectively graded into four categories based on percentile volume increase, which were defined as unchanged (<103%), moderately lateralised (207%), sufficiently lateralised (311%) and over abducted (>312%).
Results
During laryngoscopic examinations, none of the nine arytenoid cartilages was considered unchanged; one case was considered to be moderate, five were sufficiently lateralised and three were categorised as over abducted. Through objective videoendoscopic evaluations, we found that the mean increase in the rima glottidis volume was 264%. None of the larynxes were categorised as unchanged; two were categorised as moderately lateralised, with volume increases of 169% and 195%, respectively; four were categorised as sufficiently lateralised, with values in the range of 229–303%; and three were categorised as over abducted, with volume increases in the range of 324–335%. There was strong agreement between the objective and subjective evaluations. There was no evidence of mucosal perforation caused by the suture material.
Conclusions and relevance
Unilateral CAL in cats is a feasible option for increasing the rima glottidis volume. Postoperative laryngoscopic values correlated well with objective measurements; therefore, it is a good method to assess the degree of arytenoid abduction. Moderate lateralisation results in a significant volume increase, but values above 324% cause deformation of the larynx.
Keywords
Introduction
Laryngeal paralysis has occasionally been reported in cats. This disorder is characterised by impaired abduction of one or both arytenoid cartilages, leading to the obstruction of the upper respiratory tract during respiration owing to the narrowing of the glottic lumen.1,2
The management of laryngeal paralysis in cats involves medical treatment, mostly aimed at treating acute respiratory distress; however, most publications have focused on dogs. 3 Positive pressure ventilation has been described for the treatment of cats; 4 nevertheless, surgical intervention – depending on the severity of the condition and the underlying cause of the disease – has been described in most publications concerning cats.5 –9 In moderate-to-severe cases, surgical intervention is the treatment of choice, with arytenoid lateralisation being the most frequently performed procedure. However, cats show severe clinical signs of unilateral laryngeal paralysis, and surgical treatment is recommended in these patients at an early stage of the disease.8,10 –12
This study aimed to document the percentage of increase in the rima glottidis volume after unilateral arytenoid lateralisation in intact feline cadavers and to establish categories and ranges for evaluating the degree of arytenoid abduction. We hypothesised that the rima glottidis volume would significantly increase by a minimum of 200% after unilateral cricoarytenoid lateralisation and that no penetration of the mucosa would occur by the suture material. Furthermore, postoperative subjective laryngoscopy was tested as a reliable method to evaluate the degree of arytenoid cartilage abduction. To the authors’ knowledge, no previous study has investigated any of the above in whole feline cadavers.
Materials and methods
Cadaver preparation
Nine client-owned domestic shorthair cat cadavers were used in this study. The animals were euthanased for reasons unrelated to this study, and none of the animals had conditions that affected the laryngeal region. The specimens were stored at −20°C and warmed to 18°C before the procedure. The cadavers were utilised within 1 day of thawing and were periodically moistened throughout the experiments.
Preoperative imaging
Preoperative video endoscopic examination
Pre- and postoperative video endoscopic examinations were performed in each cadaver using a 0°, rigid, 30 cm endoscope with a diameter of 2.8 mm (Karl Storz). The examination was performed in sternal recumbency with the head elevated by a positioning bag; the tip of the endoscope was placed rostrally to the caudal border of the soft palate to allow repeat endoscopy at a defined distance from the larynx. A distance of 1 cm was measured using a Castroviejo surgical caliper, and two marking points were made on the soft palate using monopolar electrocautery. These markings were later used to evaluate the increase in the rima glottidis volume. Photographs of the rima were taken pre- and postoperatively, and the volume of each rima glottidis was measured using the National Institutes of Health image program, based on the marking points as a reference for abduction and increase in volume.
Surgical technique
Unilateral cricoarytenoid lateralisation
All procedures were performed by the same surgeon. The cats were placed in right lateral recumbency with a positioning bag placed under their neck, resulting in an elevated larynx;10,13 no endotracheal tube was used. A skin incision was made on the left side, ventral to the jugular vein, located just over the larynx and caudal to the vertical ramus of the mandible. To expose the thyropharyngeus muscle, the subcutaneous tissues and the platysma muscle were dissected. The dorsal edge of the thyroid cartilage was palpated and laterally retracted. The thyropharyngeus muscle was incised along the dorsal edge of the wing of the thyroid cartilage. To obtain a better exposure, a stay suture was placed on the wings of the thyroid cartilage. The cricothyroid junction was left intact to prevent laryngeal destabilisation. The external surface of the mucosal lining of the laryngopharynx was exposed by retracting the thyroid cartilage and incising the thyropharyngeus muscle. The cricoarytenoideus dorsalis muscle, which is attached to the muscular process of the arytenoid cartilage, was identified by blunt dissection using cotton tips. The muscular process was palpated and the muscle was transected. To facilitate the handling of the muscular process, the muscle was cut further away from the insertion. Cricoarytenoid articulation was completely separated by incising the joint capsule, and the muscular process was mobilised from the cricoid cartilage by gentle dissection with iris scissors. The joint cartilage was visualised, and a single non-absorbable 3-0 polypropylene suture with a taper needle was passed through the caudal aspect of the cricoid cartilage. Once the suture was passed around the caudal border of the cricoid cartilage from the medial to lateral direction, it was inserted through the arytenoid joint surface of the muscular process. The suture was tied under tension. The thyropharyngeus muscle was re-apposed in a simple continuous pattern. The subcutaneous tissues and skin were closed routinely.
Postoperative examination and imaging
Postoperative laryngoscopic examination: subjective evaluation
After completing the procedure, the cadaver was placed in sternal recumbency. Each larynx was examined by the surgeon using a laryngoscope, subjectively evaluating the shape of the larynx and position of the arytenoid cartilage, and graded into one of four categories.
These four grades were defined as unchanged, moderately lateralised, sufficiently lateralised or over abducted. In each case, the position of the arytenoid cartilage was assessed, and the lateralised side was visually compared with the resting larynx on the contralateral side (Table 1). Although the cuneiform and corniculate processes of the feline arytenoid cartilage do not form a prominent structure, their positions were evaluated. In the unchanged category, the lateralised larynx showed no to minimal difference in the position of the arytenoid cartilage compared with the resting larynx, accounting for an opening increase of the rima glottidis by one-quarter of the operated side. In the moderately lateralised category, the corniculate process was displaced laterally, but the cuneiform process position was unchanged; the opening increase reached half compared with the resting larynx. In the sufficiently lateralised category, the opening increase of the rima glottidis was three-quarters that of the resting larynx, and the corniculate and cuneiform processes were markedly displaced laterally without causing any deformation of the contralateral side. An increase in the opening above three-quarters was categorised as over abduction, and the arytenoid cartilage was suspected to be lateralised, causing deformation of the contralateral arytenoid cartilage (Figure 1).
Detailed description of the categories criteria with ranges showing the increase in the rima glottidis opening
Video endoscopic images: (a) preoperative state, unchanged category; (b) moderately lateralised; (c) sufficiently lateralised; and (d) over abducted
Postoperative video endoscopic examination: objective evaluation
The volume of each rima glottidis was calculated, and the increase in volume was expressed as a percentage. The grades unchanged, moderately lateralised, sufficiently lateralised and over abducted represent less than 103%, 207%, 311% and more than 312%, respectively.
These values were established based on a study by Bonanno and White, 13 where the maximum increase in the rima glottidis volume was 311.5% using the same surgical technique as in this study, with the complete disarticulation of the cricoarytenoid joint. As they did not report any deformation of the contralateral side, in the resting larynx, with a volume increase of 311.5%, over abduction was classified as a value higher than 312%. Each larynx was assessed for mucosal penetration by the suture.
Statistical analysis
The increase in the rima glottidis was described in terms of a mean and median (point estimates and 95% confidence intervals [CIs]) as a substantiated normality assessment of the distribution of observed values was not possible. The non-parametric Wilcoxon signed-rank test was used to test the null hypothesis of no increase. The agreement between lateralisation categories based on objective and subjective assessment approaches was a point and interval estimated using Cohen’s kappa. The one-sided 95% CI for the probability of suture penetration was based on exact binomial calculations. The statistical package Stata (StataCorp) was used for data analysis. The threshold of significance was set at alpha (α) = 0.05.
Results
Postoperative examination and imaging
Postoperative laryngoscopic examination: subjective evaluation
None of the arytenoid cartilages were considered unchanged, whereas, in one case, the surgeon concluded that the amount of lateralisation was moderate. In 5/9 cadavers, the outcome of the procedure was considered to be sufficiently lateralised; however, in three other cases, the outcome was categorised as over abducted. In cases of over abduction, the displaced arytenoid cartilage of the resting larynx was not considered when assessing the increase in the opening of the rima glottidis. Once the arytenoid cartilage of the resting larynx was displaced towards the median, the overall opening of the rima decreased.
Postoperative video endoscopic examination: objective evaluation
In 7/9 cadavers, the increase in the rima glottidis volume reached 200%. The minimum amount of volume increase was 169%, whereas the largest value was 335%. The mean and median increases were estimated to be 264% (95% CI 217–311) and 250% (95% CI 198–330), respectively. The P value from Wilcoxon’s signed-rank test against the null hypothesis of no increase in the rima glottidis volume was 0.0039, indicating a statistically significant growth. As none of the values were less than 103%, none of the larynxes analysed were grouped into the unchanged category, and 2/9 cases were categorised under the moderately lateralised group, with volume increases of 169% and 195%, respectively. Four cases were categorised into the sufficiently lateralised group, with values in the range of 229–303%. Three cases reached the value for the over-abducted category, with a volume increase from 324% to 335%. The agreement between the objective measurements and the subjective evaluation was quantified at a kappa estimate of 0.82 (95% CI 0.509–1.000), demonstrating a strong agreement with the point estimate (Table 2).
Subjective and objective measurements in each cadaver
The median value of volume increase was 250%, which fell into the sufficiently lateralised category. Furthermore, there was no evidence of mucosal perforation caused by the suture material. However, this does not exclude the possibility of this complication in the source population of the sample: observing zero events out of nine cadavers translates to a one-sided 95% CI for the probability of suture penetration in the range of 0–28%. As none of the nine cadavers in this study had suture perforations, the hypothesis of no mucosal perforation was accepted (Figure 2).
Video endoscopic image of the larynx showing no penetration of the inner mucosal lining by suture material
Discussion
One of the most comprehensive clinical studies on arytenoid lateralisation in cats was published in 2009 by Hardie et al. 10 They examined 10 cats with laryngeal paralysis that underwent arytenoid lateralisation and concluded that 9/10 animals had bilateral paralysis and one had unilateral paralysis. According to the current literature, unilateral cricoarytenoid lateralisation is the best treatment option for cats with unilateral or bilateral laryngeal paralysis.6,10,11,14 Very few detailed surgical descriptions have been published on unilateral cricoarytenoid lateralisation in cats;10,13 however, the present study includes a detailed surgical description. In 2023, Bonanno and White 13 investigated arytenoid lateralisation in an excised feline larynx. They measured the increase in abduction of the arytenoid cartilage and provided a detailed surgical description, differentiating between the two surgical techniques. In one group, the cricoarytenoid joint was completely disarticulated, whereas in the other group, no disarticulation occurred. In the present study, a complete disarticulation of the joint was performed. The outcome and success of surgical intervention can be estimated by evaluating the degree of arytenoid lateralisation and the increase in the rima glottidis. Furthermore, performing subjective postoperative laryngoscopy at the surgeons’ discretion to assess the outcome of unilateral cricoarytenoid lateralisation correlates with the objective measurements of volume increase of the rima glottidis, which was supported by the kappa estimate suggesting a strong agreement (κ = 0.82). The exception was one case in which visual inspection concluded that the lateralisation was sufficient; however, objective measurements showed that the abduction was only moderate. A possible over abduction of the arytenoid cartilage can be recognised during postoperative laryngoscopy; however, in all cadavers, the epiglottis completely covered the entrance of the larynx. Nevertheless, the contralateral side of the arytenoid cartilage appeared deformed, with a volume increase of more than 312%.
No percentile ranges or categories have been previously established to evaluate the postoperative outcomes of unilateral cricoarytenoid lateralisation in cats. In the present study, the median volume increase was 250%; therefore, the hypothesis that the rima glottidis volume increased with unilateral cricoarytenoid lateralisation by a minimum of 200% was accepted. To evaluate the reliability of subjective postoperative laryngoscopy after unilateral cricoarytenoid lateralisation, the results were compared with objective measurements. In the clinical setting, this information can be useful, as the grading scheme can be applied to classify the outcome of surgery, as only one of the subjective classifications did not match the objective measurement in this study.
In this study, percentile increases in the rima glottidis volume were not only subdivided into four different categories, but percentile ranges were also established for each category. The percentile ranges for unchanged, moderately lateralised, sufficiently lateralised and over abducted were less than 103%, 104–207%, 208–311% and more than 312%, respectively. The values and ranges were established based on previous studies, where the maximum value of volume increase in the rima glottidis after cricoarytenoid lateralisation and complete disarticulation of the cricoarytenoid joint was 311.5%. 13 No deformation was observed at this maximum value. In this study, we showed that a volume increase of more than 324% caused the deformation of the larynx. This is because the contralateral arytenoid cartilage is displaced to the median by traction on the lateral side. The displacement of the arytenoid cartilage of the resting larynx towards the median decreases the overall opening of the rima. The concern is that, in clinical settings, this may cause increased airway resistance and upper airway obstruction or aspiration (due to the deformity). However, this observation requires further investigation to make appropriate conclusions because, despite the over abduction of the arytenoid cartilage, the epiglottis completely covers the volume of the larynx once manipulated upwards. Even if complete coverage of the epiglottis of the laryngeal volume is achieved by manual manipulation of the epiglottis in a cadaveric study, this does not exclude the possibility of postoperative aspiration in clinical settings.
Suture perforation did not occur in any of the cases described in this study; therefore, this hypothesis was accepted. No previous studies have investigated suture perforations related to this procedure in living cats or cat cadavers.
To the best of our knowledge, no previous study has investigated the volume increase in the rima glottidis after unilateral cricoarytenoid lateralisation in whole feline cadavers and relating the obtained results to postoperative laryngoscopic evaluation. Furthermore, the categories and ranges for evaluating the degree of arytenoid abduction after unilateral cricoarytenoid lateralisation have not been proposed for cats. Perforation of the mucosa by the suture material during this procedure has not been investigated in previous reports.
The limitations of this study include the use of cadavers, which may not fully reflect live patients in a clinical setting. All surgical procedures and postoperative laryngoscopies were performed by the same surgeon, leaving a possibility for conscious and unconscious bias. It is still unclear what percentage increase classifies as over abduction and at what degree it causes postoperative complications such as aspiration or airway obstruction of the contralateral displaced arytenoid cartilage. We did not obtain results between 303% and 324%; therefore, the exact percentage increase at which the larynx started to deform could not be stated.
Conclusions
The results of this study indicate that unilateral cricoarytenoid lateralisation in cats is a feasible option for increasing the rima glottidis volume. An objective grading scheme was established for this procedure, which is in strong agreement with the subjective results. The increase in the volume is comparable to the results published in dogs 14 and is related to the measurements made in excised feline larynxes. 13 Subjective postoperative laryngoscopic evaluation results correlated well with objective measurements. Therefore, it is a good method to assess the degree of arytenoid abduction and can be used in a clinical setting. A percentage volume increase of 169% is considered moderate; however, significant lateralisation can still be achieved.
Footnotes
Author note
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
Conflict of interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Ethical approval
The work described in this manuscript involved the use of non-experimental (owned or unowned) animals. Established internationally recognised high standards (‘best practice’) of veterinary clinical care for the individual patient were always followed and/or this work involved the use of cadavers. Ethical approval from a committee was therefore not specifically required for publication in JFMS. Although not required, where ethical approval was still obtained, it is stated in the manuscript.
Informed consent
Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or non-experimental animals, including cadavers, tissues and samples) for all procedure(s) undertaken (prospective or retrospective studies). No animals or people are identifiable within this publication, and therefore additional informed consent for publication was not required.
