Investigations into the role of the thyrohyoid muscles in the pathogenesis of dorsal displacement of the soft palate in horses

Summary Reasons for performing study: Contributes to the understanding of the pathogenesis of dorsal displacement of the soft palate during exercise so that management of this condition could be enhanced. Hypothesis: That the thyrohyoid muscles play an important role in the stability of the laryngo‐palatal relationship and that dysfunction of these muscles leads to dorsal displacement of the soft palate (DDSP) during exercise. Methods: Ten horses were exercised on a high‐speed treadmill under 4 different treatment conditions: control conditions (n = 10), after resection of thyrohyoid muscles (TH, n = 10), after sham‐treatment (n = 5), or after restoration of function of the thyrohyoid muscles with surgical sutures (prosthesis‐treatment, n = 6). During trials, the following determinations were made: videoendoscopy of the upper airway, gait frequency and pharyngeal and tracheal static pressures. Results: None of the 10 horses developed DDSP during 2 separate treadmill‐exercise trials under the control conditions. Seven of the 10 horses developed DDSP after resection of the TH muscles, 4 of 5 of these horses still experienced DDSP aftersham‐treatment, but 5 of 6 horses no longer experienced DDSP at exercise after the prosthesis‐treatment. There were significant anomalies in airway pressures, respiratory frequency, and occurrence of DDSP in both the TH resection and sham‐treatment conditions compared to control conditions. In contrast, no statistical differences were noted in any of the parameters measured between the prosthesis‐treatment and control conditions. Conclusions: That the function of the TH muscles is important to the stability of the laryngo‐palatal relationship and plays a role in the pathophysiology of exercise‐induced DDSP. Potential relevance: Management of horses with DDSP could be enhanced by restoring the function of the TH muscles.