Dorsal Displacement of the Soft Palate in Horses
Dorsal displacement of the soft palate (DDSP) is a performance-limiting condition of the upper respiratory tract and is a relatively common cause of upper respiratory noise during exercise. During DDSP, the caudal free margin of the soft palate moves dorsal to the epiglottis, creating a functional obstruction within the airway. The cross-sectional area of the pharynx is reduced, and airflow resistance and turbulence are increased.
DDSP may result from several pathophysiologic mechanisms. Inflammation of the upper respiratory tract due to infection may cause neuropathy of the pharyngeal branch of the vagus nerve as it traverses the floor of the medial compartment of the guttural pouch, resulting in neuromuscular dysfunction of the pharyngeal muscles that control the soft palate. The retropharyngeal lymph nodes are in direct contact with the pharyngeal branch of the vagus nerve, and retropharyngeal lymphadenopathy may result in compression and irritation. Clinical signs can be induced by local anesthesia of this nerve. Congenital hypoplasia of the epiglottis may contribute to DDSP due to insufficient epiglottal tissue to maintain the position of the caudal border of the soft palate ventral to the epiglottis. Horses that have undergone laryngoplasty for left laryngeal hemiplegia are more likely to develop DDSP.
DDSP creates a characteristic gurgling respiratory noise, primarily during expiration, due to vibration of the soft palate. Horses may make no noise at the onset of exercise but displace their palate during high-speed exercise, causing them to “choke down.” Head position (flexed) may contribute to displacement.
The most effective treatment for DDSP in young horses (2-yr-olds) and horses with evidence of upper respiratory tract infection is rest and anti-inflammatory therapy. Caudal retraction of the tongue elevates the soft palate and pushes the larynx caudally, both of which may predispose to DDSP. Placing a tongue tie during exercise reduces caudal retraction of the tongue. Sternothyrohyoideus myectomy performed in horses prone to DDSP to alter the anatomy of the upper respiratory tract is successful in ~50% of horses. Soft palate resection (staphylectomy) is frequently performed in horses with DDSP and also has a success rate of ~50%; however, the mechanism of improvement after surgery is unclear. Success has been attributed to reduction in the mass of soft palate obstructing the airway, easier replacement of the shorter soft palate to the subepiglottic position, and firming of the caudal edge of the soft palate to keep it ventral to the epiglottis. Palatal sclerotherapy via endoscopic-guided sodium tetradecylally sulfate injection has demonstrated success in a small number of horses, resolving respiratory noise in 7 of 8 horses and improving performance in 6 of 8 horses. A laryngeal tie-forward procedure can be performed to alter the position of the larynx with respect to the caudal edge of the soft palate.