Advances in the approach to DDSP

Dorsal displacement of the soft palate has been recognized as a common cause of upper airway obstruction in racehorses with a reported prevalence of 10 – 20% in 2 – 3-year-old racehorses1,2. In affected horses the caudal border of the soft palate moves dorsal to the epiglottis during exercise. This results in increased airflow turbulence and respiratory impedance3. Horses will often present with clinical signs of exercise intolerance and a gurgling or vibrating noise on exhalation. These signs can be exacerbated by head and neck flexion4 . Resting endoscopy often fails to diagnose horses with DDSP, occasionally ulcers will be seen on the caudal soft palate and there will be bruising in the nasopharynx secondary to DDSP. Exercising endoscopy is the gold standard for diagnosis of DDSP5-7. In racehorses’ treadmill endoscopy has been shown to be superior to overground endoscopy for identification of horses with DDSP8 whereas in sport horses overground endoscopy is often required for a diagnosis of DDSP9 . This is because in racehorses DDSP is related to exercise intensity and hypothesized thyrohyoid muscle fatigue10 whereas in sport horses it can be related to rider effects, head, and neck position9 or the presence of equine asthma11. This is evidenced by the increased prevalence of DDSP in dressage horses when compared with showjumpers9 . Numerous etiologies have been proposed as causative factors in horses with naturally occurring DDSP including dysfunction of the pharyngeal branch of the vagus nerve12, dysfunction of the hypoglossal nerve13, dysfunction of the thyrohyoid muscles14, inflammation of the upper and lower airways and the use of certain items of tack and training methods. Identifying the cause of DDSP in individual cases is essential as treatment should be tailored to the cause. A wide variety of surgical and conservative treatments are used by clinicians to manage horses with this condition with similar published success rates of around 60 – 70%. These treatments address the proposed etiologies by reducing inflammation, altering the position of the hyoid apparatus or by increasing the stiffness of the soft palate. Conservative methods include the use of systemic and topical anti-inflammatories, tack changes such as the use of a tongue tie, spoon bit, Cornell® collar and a cross or grackle noseband. Surgical treatment options include laryngeal tie-forward with a recent report describing a technique for performing this in the standing horse15, myectomy, staphylectomy, and palatoplasty to reduce compliance of the soft palate including tension, thermal and laser palatoplasty techniques. Recently injection of the soft palate with Genipin® (a self-polymerizing collagen cross linker) has been utilized to reduce soft palate compliance. This treatment has the benefit of minimal convalescence and good initial reported success rate16. There is also emerging evidence for the efficacy of training of inspiratory muscles17 which may alter our future approach to the management of horses with DDSP.