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

Abstract

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.

In vitro mechanical evaluation of equine laryngeal tie-forward constructs prepared with different suture materials and placement patterns

“…(Am J Vet Res 2015;76:373-383) tation of thyrohyoideus muscles. 3,4 Although LTF construct failure rate is apparently low (6%), 6 the modes of LTF construct failure in horses with DDSP are still unknown. It is our experience that braided polyester sutures can pull through the thyroid cartilage or break in the immediate postoperative period.…”

Update on Diseases and Treatment of the Pharynx

“…5,[31][32][33] There is increasing support implicating laryngohyoid positioning as a factor associated with DDSP. 5,6,9,20,21,24,34 As such currently the most widely accepted surgical intervention, following failed conservative therapy, is laryngeal tie-forward with or without myectomy of the sternohyoideus and sternothyrohyoideus. Success rates approaching upward of 80% have been reported for this treatment strategy and there seems to be minimal detrimental effects to the horse.…”

“…18,19 Neuromuscular dysfunction Neuromuscular dysfunction as a cause for DDSP is supported by recreation of the disorder with specific nerve blocks (pharyngeal branch of the vagus nerve or the Chesen & Whitfield-Cargile hypoglossal nerve) or muscle belly transection (thyrohyoideus). 8,9,20 A potential cause of neuromuscular dysfunction resulting in DDSP is inflammation caused by URT infection. This is supported by reports of increased prevalence of DDSP following URT infections.…”

“…[8][9][10][11][12][13] The unclear etiopathogenesis of this common condition has resulted in a large number of treatment options directed at each of the proposed causes. Recent improvements and changes in technology have resulted in new information related to the etiopathogenesis of DDSP and more diagnostic options.…”

Evaluation of Overground Endoscopy Findings in Sport and Pleasure Horses

“…Any factors that increase negative pressure in the URT, for example poll flexion, increased pressure in the LRT or multiple obstructions [7], or constitutional changes [8], can markedly influence the mechanics of the URT. When evaluating the URT, we also have to consider possible causes of nasopharyngeal instability, such as an immature nasopharynx or inflammation of the URT and/or LRT.…”