Palatal sclerotherapy: A potentially useful treatment of intermittent dorsal displacement of the soft palate in juvenile standardbred racehorses

Abstract

This study was aimed at evaluating the tolerability and the efficacy of palatal sclerotherapy in juvenile standardbred racehorses with easily audible “snoring-like” respiratory noises suspected to be the result of intermittent dorsal displacement of the soft palate. The palate of 8 horses was injected with sodium tetradecyl sulfate under videoendoscopic guidance. Palatal sclerotherapy resulted in resolution of the respiratory noise in 7 horses, improvement of performance in 6 horses, and mild side effects in only 3 horses.

This preliminary study suggests that palatal sclerotherapy is a safe, repeatable, inexpensive, and promising technique that should be considered as an alternative to existing treatments of intermittent dorsal displacement of the soft palate.

Introduction

In horses, intermittent dorsal displacement of the soft palate (IDDSP) is the most common cause of exercise intolerance (1–4). It obstructs the upper airways, mostly during the expiratory phase (5,6), leading to clinical signs of poor performance and respiratory noise during exercise (7,8). Some authors have suggested that flaccidity of the soft palate due to neuromuscular disorders could play a role in its displacement during exercise (9–12). Based on these observations, one goal of treatment is to rigidify the tissues of the soft palate (13). Several therapies have been reported, including staphylectomy (14) and thermal palatoplasty with either laser (15–17) or cautery (18).

In humans, soft palate flaccidity is responsible for snoring and contributes to obstructive sleep apnea. A recent treatment for these conditions consists of a soft palate stiffening procedure called “injection snoreplasty” or “palatal sclerotherapy.” This technique involves injecting a sclerosing agent (sodium tetradecyl sulfate) into the submucosal layer of the soft palate to induce fibrosis. It has proven efficient in stiffening the soft palate in a canine model (19) and has shown promising results in human patients affected with snoring and sleep apnea (20,21). More recently, a similar approach, injecting poly-L-lactic acid into the soft palate, has been described in horses (22).

The main objective of the present study was 1) to evaluate the feasibility and efficacy of injecting a sclerotic agent into the soft palate of young standardbred horses showing typical clinical signs of IDDSP and 2) to register possible side effects of palatal sclerotherapy. For this purpose, a technique for injection of the soft palate under endoscopic guidance was developed.

Materials and methods

All experimental procedures were performed in accordance with the guidelines of the Canadian Council for Animal Care and were approved by the animal care committee of the Faculty of Veterinary Medicine of the Université de Montréal.

Horses included in this study were 8 young standardbred horses that had been showing a “snoring-like” respiratory noise when subjected to slow or moderate exercise ever since the beginning of training and because of which they were not able to race. The group consisted of 4, 2-year-olds (2 males, 2 females) and 4, 3-year-olds (1 male, 3 females) that were presented to the Veterinary Teaching Hospital of the Université de Montréal between January and July 2004. A putative diagnosis of IDDSP had been made by the referring veterinarian, based on a history of respiratory noise related to exercise and on postexercise endoscopy. For each horse, a complete physical and lameness examination was performed, in addition to upper airway videoendoscopy with a 30 s nasal obstruction test.

A semirigid plastic tube (#076X038-pc; Putnam Plastics Corporation, Dayville, Connecticut, USA), to the distal end of which the metal part of a 20-g needle had been snugly fitted and glued in place (Cyanoacrylate adhesive; Permabond, Somerset, New Jersey, USA), was passed through the instrument channel of a 10-mm videoendoscope (GIF type 100-1 meter; Olympus, Tokyo, Japan). The channel lining was protected from puncture by a Teflon sleeve (Teflon sheath-HX-20L-1, model maj-251; Olympus, Tokyo, Japan) inserted in the instrument channel so that 1 cm protruded from the end of the endoscope. The channel was also protected by covering the end of the needle with a 1-cm long accessory piece of the tube during passage through the channel. When the tube was completely inserted in the channel, the accessory piece covering the needle was removed and the tube was retracted so that the needle would be located within the protection sleeve at about 1 cm from the end of the videoendoscope.

Injection of the soft palate with the sclerosing agent was performed with the horses restrained in stocks by a nose-twitch and sedated with xylazine (Rompun; Bayer, Etobicoke, Ontario), 0.5 mg/kg bodyweight (BW), IV, and butorphanol (Torbugesic; Fort Dodge Animal Health, Fort Dodge, Iowa, USA), 0.025 mg/kg BW, IV.

The videoendoscope was inserted into the pharynx through a nostril. The nasal and pharyngeal mucous membrane was then anesthetized with 40 mL of lidocaïne delivered through the accessory channel of the videoendoscope. Before injection, in order to facilitate visualization of the distal border of the soft palate, dorsal displacement of the free distal margin of the soft palate was obtained with a blunt-end hook inserted through the contralateral nostril. The injections were aimed at the submucosal level of the distal two-thirds of the soft palate and each horse received 8 to 10 injections of 1 mL of 3% sodium tetradecyl sulfate (Tromboject; Omega Laboratory, Montreal, Québec) depending on its body weight (1 injection/50 kg BW). The pattern of injection followed, as closely as possible, that represented in Figure 1. Phenylbutazone (generic), 4.4 mg/kg BW, IV, was administered at the end of the procedure; subsequently, the horses received phenylbutazone, 2.2 mg/kg BW, PO, q12h for 4 d. The horses were discharged immediately after treatment with instructions to the owner that they were to be stall rested or hand walked for 15 d, followed by moderate exercise at slow speed for the next 15 d.

Figure 1.

Sites of submucosal injection of the soft palate with 3% sodium tetradecyl sulfate performed under endoscopic guidance. Round locations were used when the free margin was normal, and square locations were used when of an ulcer was present at the free margin. Each horse was injected with 1 mL at each site. The total volume injected corresponded to 1 mL/50 kg BW and when fewer than 10 injections were needed, most rostral injection sites were not used.

After 1 mo, the horses were submitted to a control videoendoscopy; if the soft palate still seemed more flaccid than normal on visual evaluation, the procedure was repeated.

A subjective postreatment performance evaluation was obtained from the trainer or the referring veterinarian by using a phone survey. Side effects, persistence of respiratory noise, and horse performance were recorded. Furthermore, evaluation of performance was assessed objectively by reviewing the racing records obtained from the Official Quebec Racing Authorities.

Results

Eight horses met the inclusion criteria. In addition to the persistent problem of respiratory noise during exercise in all horses, owners had reported exercise intolerance in 7 horses and 1 horse could not be trained at high speed. Of the 7 horses, 1 showed intermittent coughing and 2 had serious difficulty in recovering from exercise. In the 6 mo prior to presentation, 1 horse had been treated for guttural pouch empyema and 2 others for lower respiratory tract infection, but all were clinically normal in regard to these lesions at the time of examination. None of the horses was lame. Results from physical examination of 5 of the 8 horses were unremarkable; 2 horses had tracheal wheezes and 1 had a scar from a previous tracheal trauma.

According to the Raker’s classification (23), based on endoscopic findings, horses had grade 3 (n = 2) or grade 4 (n = 6) hypertrophic lymphoid hyperplasia, aryepiglottic folds vibration (n = 2), partial pharyngeal roof collapse (n = 1), grade 2 laryngeal hemiplegia (n = 1), minor tracheal secretions (n = 4), and a fibrous callus involving a fracture of 3 tracheal rings that induced a minimal reduction of the tracheal diameter of less than 10% (n = 1). Seven of the horses showed IDDSP, secondary to the nasal occlusion performed on examination, and 1 of them had ulceration of the free margin of the soft palate. Except for the IDDSP, none of the abnormalities were considered sufficient to explain the clinical signs and the “snoring-like” respiratory noise heard during light exercise.

The injection procedure was relatively easy to perform in all horses and did not induce major side effects. One horse had temporary mucosal bleeding, and 2 horses displayed slight coughing for 1 wk; 1 of the latter showed moderate fever of unknown origin that resolved without any treatment.

All horses were presented for endoscopic reevaluation at the Veterinary Teaching Hospital between 28 and 99 d after the initial treatment. Reevaluation did not show any signs of inflammation or swelling of the soft palate, as was the case in experimental horses that had been injected with the same product and on whom videoendoscopic pictures had been taken 72 h, 1 wk, and 2 wk post-treatment (unpublished observations) (Figure 2). On reevaluation, 1 horse had a small lump (< 2 mm) on the soft palate at an injection site, which was neither inflamed nor painful and was not considered important. Among the 7 horses that had IDDSP on forced inspiration, only 2 displaced their soft palate during the post-treatment videoendoscopy. Seven horses underwent a 2nd treatment, because the soft palate still seemed more flaccid than normal on visual examination, which was not the case on 1 horse. During the 2nd procedure, in most cases, needle insertion into the soft palate mucosa seemed more difficult than during the 1st procedure.

Figure 2.

Endoscopic view of the pharyngeal region of a horse 72 h (A), 1 wk (B), and 2 wk (C) after a palatal sclerotherapy procedure.

After completion of soft palate injections, subjective evaluation of performance by the trainer or the referring veterinarian revealed that there had been an improvement in relation to abnormal upper respiratory sounds in 7 of 8 horses. The improvement was considered as > 75% in 4 cases, between 50% and 75% in 3 cases, and < 25% in 1 horse that showed persistent respiratory noise during mild exercise and could not tolerate high speed training.

Objective evaluation of racing performance was available for all horses from 7 to 13 mo post-treatment. One horse could not be qualified, 1 horse had qualified but had not raced, and 6 horses had raced more than once. Two of them were winners.

Discussion

Previous studies have reported variable success rates for different treatments for the correction of IDDSP in horses that had already performed as racehorses: 58% to 70% for sternothyrohyoideus myectomy (24,25), 60% for staphylectomy (14), 53% for epiglottic augmentation (26), 63% to 92% for laser thermoplasty (16,27), 60% to 78% for different composite surgeries (28,29), and, more recently, 80% for laryngeal tie-forward (30).

The pathophysiology of IDDSP seems to be multifactorial and is not completely understood (5,31). Whereas increasing the size of epiglottis and inhibiting retraction of the larynx have become controversial (33,34), increasing soft palatal rigidity seems promising, because studies have shown tissular lesions in the soft palate of horses that show displacements and it is proposed that a lack of tissular rigidity could be the cause of these displacements (9,31). Moreover, the diagnosis of IDDSP is extremely difficult and often based on history and exclusion of other causes of upper airway obstruction, which makes it difficult to select cases for studying the efficacy of a treatment. These factors, and the fact that the problem is intermittent, probably account for some of the variation in success rates reported in relation to the correction of IDDSP. In these, otherwise normal, juvenile horses in this study, clinical and upper airway videoendoscopic examinations, “snoring-like” sounds typical of IDDSP present ever since the beginning of training, and displacements of the soft palate on forced inspiration strongly supported the clinical diagnosis of a soft palate lesion.

It would have been interesting to examine all horses while they exercised on the treadmill, but this was not acceptable to owners because of the cost and the guarded prognosis for these horses as future race animals.

In this study, palatal sclerotherapy with 3% sodium tetradecyl sulfate represented a noninvasive, repeatable, secure, and effective procedure for the treatment of IDDSP in juvenile racehorses; it required no sophisticated equipment and was easy to perform. The efficacy of this procedure was probably due to the sclerosing effect of the drug, which, logically, should, as in humans (35), increase the rigidity of the soft palate and thus help to prevent its displacement during exercise.

In the juvenile standardbred racehorses studied here, the subjective evaluation survey showed an improvement of more than 50% in 7 of 8 (87.5%) horses, and objective data revealed a success rate of 6 of 8 (75%) for racing in a population of animals that had not shown such a possibility before. It could be argued that the rest period after treatment could be responsible for some of the results, but this is not likely for animals that had continuously shown respiratory noises ever since the beginning of training, and for which training had often been stopped for a few weeks and then restarted. Results could have been influenced negatively by the inclusion criteria: 1) the population of horses treated had been showing clinical signs since the beginning of training, even at moderate speed, and was unable to race, and 2) the sample was limited to the most severely affected cases. In the past, this category of cases has shown very few, if any, positive results from conventional therapeutic regimens.

In this study, a treadmill evaluation could possibly have identified horses with other causes of dynamic obstruction or displacement, but, logically, these horses would not have responded positively to treatment. Overall, the results obtained in this study are either superior to, or as good as, those obtained with most of previously reported surgical techniques attempted on mature racehorses.

Compared with other techniques, palatal sclerotherapy does not require general anesthesia, does not require sophisticated equipment, is relatively easy to perform, is not costly, and did not cause significant side effects. The treatment can be repeated easily, if clinical results are not satisfactory; in this study, only 2 of 7 horses still displaced their soft palate on reexamination when the nasal obstruction test was used. Since, subjectively, the structure of the palate in these 2 horses still seemed more flaccid than normal, it was decided to proceed with another infiltration. These results need to be confirmed on a greater number of horses before palatal sclerotherapy treatment can be fully recommended. Further experiments would also help to determine the success rate in less severe cases and help to optimize both dosage and post-operative recommendations.