Kate Allen, Samantha H. Franklin
Dorsal Displacement of the Soft Palate
In the investigation of upper respiratory tract (URT) function in the athletic horse, obtaining a definitive diagnosis is critical. The losses sustained as a result of incomplete or incorrect diagnoses may be considerable. Not only should the cost of inappropriate treatments be taken into consideration, but also the costs of convalescence, futile training, additional surgeries, loss of earnings, and horse devaluation must be weighed. Furthermore, the welfare implications of large numbers of horses undergoing inappropriate surgeries are important. Dorsal displacement of the soft palate is one of a number of forms of dynamic obstruction of the URT. This chapter summarizes clinical research evidence on the diagnosis and treatment of dynamic intermittent dorsal displacement of the soft palate (DDSP). A comprehensive list of references of efficacy studies can be found in the Allen et al, 2012, referenced in the Suggested Readings at the end of this chapter.
Diagnosis
Presenting Complaint
A clinical suspicion of DDSP may be gathered from the presenting complaint. Horses often have a history of poor performance or exercise intolerance, and in racehorses this may manifest as fading or stopping abruptly toward the end of a race. Studies suggest that 58% to 85% of horses with a diagnosis of DDSP make abnormal respiratory noise. Although a history of abnormal respiratory noise in itself is not specific for a diagnosis of DDSP, a history of gurgling URT noise has a specificity of 0.77 and sensitivity of 0.5. The characteristic gurgling expiratory sound is generated by vibration of the free border of the soft palate, and this is typically quite different from the inspiratory noises associated with other forms of dynamic URT obstruction. However, depending on the frequency of the vibrations, not all horses produce audible sounds. It has recently been suggested that sport horses with DDSP seldom make respiratory noise, and this is thought to be because of the lower airflow velocity in these horses, compared with that in racehorses.
Laryngeal Ultrasound
Laryngeal ultrasound is beneficial in the diagnosis of laryngeal dysfunction in horses, but its role in diagnosis of DDSP is debatable. Although one study has reported a significant relationship between the ultrasonographic depth measurement of the basihyoid bone at rest and the occurrence of DDSP during exercise, this finding was not reproduced in a subsequent study. At present, there is insufficient evidence for this technique to be used as a sole diagnostic method for DDSP.
Endoscopy
Endoscopy of the upper airways with the horse at rest is performed routinely. It has been suggested that intermittent DDSP, soft palate ulceration, and a small or flaccid epiglottis might be indicative of DDSP arising during exercise. In particular, it has been suggested that the ease with which DDSP is induced, the duration of displacement, and how readily the horse is able to correct it by swallowing are the most important signs indicative of DDSP. However, none of these factors have been critically appraised. The percentage of DDSP-confirmed cases that displace the soft palate at some point during the resting endoscopic examination varies from 8% to 51%. The specificity of DDSP observed during endoscopy of the resting horse is high (0.89 to 0.96). However, the sensitivity is generally low (0.02 to 0.64), and even when combined with a history of gurgling, this is considered a poor predictor of palatal malfunction, with one study reporting a 35% misdiagnosis rate.
Observing the URT during exercise is considered the gold-standard method for establishing a definitive diagnosis. This can be performed with a high-speed treadmill or during field exercise, using an overground (telemetric) endoscope. However, there remains the potential for misdiagnosis with both techniques. It is well known that treadmill exercise does not replicate exercise in the field. There are significant differences in heart rate, blood lactate, stride frequency, and stride length between field and treadmill exercise. Current research suggests that the prevalence of DDSP in racehorses is lower when the condition is diagnosed by use of overground endoscopy than when treadmill endoscopy is used to make the diagnosis, whereas for sport horses, the prevalence is greater during overground exercise. It is likely that the problems in obtaining a diagnosis of DDSP during overground endoscopy in racehorses are a reflection of less strenuous exercise tests, particularly in the United Kingdom, where the technique is often performed on trainers’ private gallops, which are shorter than the distances over which horses race. In contrast, the higher prevalence in sport horses is likely a result of the influence of rider intervention on head and neck flexion, factors that cannot be reproduced on the treadmill. In summary, diagnosis of DDSP is best made under conditions that most closely approximate those under which the horse is working.
Treatment
After an accurate diagnosis of DDSP has been made, the veterinarian must advise the owner on the most appropriate treatment option for that particular horse. To be able to advise on treatment options, the veterinarian must understand the rationale and scientific evidence that led to their development as well as being aware of the results of clinical efficacy studies. Numerous treatment options have been described for DDSP, which usually constitutes a warning that no one treatment is considered optimum in all instances. The number of treatments available is a reflection of our relatively poor understanding of the etiopathogenesis. Whenever possible, clinical decision making should be based on the best available evidence. A systematic review of the efficacy of interventions for DDSP was recently undertaken. The results of this review revealed that, in evidence-based medicine terms, the quality of the literature was considered to be low. It is therefore difficult to draw firm conclusions from the current evidence regarding the true efficacy of these procedures or to determine which procedures might be the most successful and least harmful for treatment of DDSP.
Because systematic reviews only include efficacy studies in clinical cases, several well-conducted research studies performed in clinically normal horses that might provide further evidence to support or refute a treatment were not included. Here, we summarize the findings of the systematic review and highlight some of the findings from other research studies in an attempt to draw all the evidence together.
Soft Palate Procedures
Although several surgical treatments have been described that aim to increase the tension or stiffness of the soft palate, there is presently insufficient evidence to confirm or refute if increasing the stiffness improves palatal function. These methods do not address muscular strength of palatal musculature but rather aim to reduce the compliance of the soft palate through the induction of fibrosis. The stiffening that results is thought to increase the intrinsic strength of the caudal soft palate so that it is able to resist the large pressure changes that occur within the nasopharynx during strenuous exercise.
Thermal Cautery
Thermal cautery is performed on the oral surface of the soft palate with the horse under general anesthesia. No research studies have assessed the histologic response to thermal cautery or its effect on soft palate stiffness. The five studies investigating the clinical efficacy of thermal cautery that were included in the systematic review varied in the inclusion population, whether a definitive diagnosis was obtained, and how outcome was assessed. Although in the initial pre-post study, a high number (72%) of trainers considered this treatment to be successful, only 48% of the horses that were reported to make gurgling noise before surgery ceased gurgling after surgery. In another study that reassessed six horses with DDSP by endoscopic examination on a high-speed treadmill, three still had DDSP, and three improved to palatal instability after the intervention. However, because a large proportion of horses in this study did not have a repeat endoscopic examination, the results may be biased, potentially toward the least successful cases. Three other studies used race form as the outcome measure. There was considerable variation in the number of races that were assessed and variation in whether earnings, ratings, or a performance index was used. The reported improvement (28% to 59%) in race performance varied, depending on which race parameter was assessed.
Other studies have compared the results of thermal cautery with the laryngeal tie-forward procedure or with conservative interventions. In no study was there a convincing statistical difference in efficacy between the procedures.
Most studies performed to date suggest that thermal cautery of the soft palate is likely to result in only modest success. Minimal adverse effects have been reported, primarily mild discomfort and occasional inappetence for the first 24 to 48 hours after surgery.
Laser Palatoplasty
Laser cautery of both the oral and the nasal surface of the palate have been described, using a variety of instruments (e.g., a diode laser, CO2 laser, and Nd:YAG laser). A histologic study assessing the effect of diode laser treatment of the nasal surface of the soft palate confirmed that it did induce a fibrous tissue reaction. Loss of soft palate skeletal muscle was also observed secondary to laser-induced thermal injury. Despite the development of fibrosis, the stiffness actually decreased, and the soft palates from treated horses were in fact more compliant than those of control horses. It has subsequently been proposed that loss of muscle mass may well be detrimental to soft palate stability. It is unclear whether the fibrosis failed to increase the palate stiffness or whether the loss of muscle mass was responsible for the decrease in stiffness despite the induction of fibrosis. These results are concerning: if the aim of treatment is to increase the stiffness of the soft palate, this does not appear to be achieved with this technique. Furthermore, loss of muscle tissue may be detrimental in the long term.
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