Pathophysiology of dorsal displacement of the soft palate in horses.

Dorsal displacement of the soft palate (DDSP) is an intermittent obstructive upper airway condition that occurs in athletic horses during high-intensity exercise. The pathogenesis of this condition is unknown, but may involve epiglottic hypoplasia, malformation, or neuromuscular dysfunction. In this paper, we report on investigations into the pathophysiology of DDSP. In 3 separate experiments, Standardbred horses were exercised on a high-speed treadmill at speeds corresponding to 50, 75 and 100% of maximum heart rate. The upper airway was evaluated by videoendoscopy and measurement of tracheal pressures. In experiment 1, we examined the effect of bilateral hypoglossal and glossopharyngeal nerve block on epiglottic function in exercising horses. This nerve block caused epiglottic retroversion and inspiratory upper airway obstruction. However, DDSP did not occur. In experiment 2, we evaluated the hypothesis that tensor veli palatini muscle dysfunction is involved in the pathogenesis of DDSP. Bilateral tensor veli palatini muscle tenectomy did not cause DDSP, but caused instability of the rostral half of the soft palate and a modest inspiratory upper airway obstruction. In experiment 3, we hypothesised that palatinus and palatopharyngeal dysfunction causes DDSP. The pharyngeal branch of the vagus nerve was blocked bilaterally in the guttural pouch. This block caused DDSP in all horses within 2-15 min after nerve block. Furthermore, DDSP created an expiratory nasopharyngeal obstruction. Because the pharyngeal branch of the vagus nerve is in close proximity to the retropharyngeal lymph node chain, we suggest that retropharyngeal lymphadenopathy may cause neural dysfunction and thereby be involved in the pathogenesis of clinical DDSP.