Investigation into pathophysiology of naturally occurring palatal instability and intermittent dorsal displacement of the soft palate (DDSP) in racehorses: Thyro-hyoid muscles fatigue during exercise.

Marta Cercone,Jonathan Cheetham,Justin D Perkins,Lisa M Mitchell,Emil Olsen,Norm G Ducharme

doi:10.1371/journal.pone.0224524

Abstract

Exercise induced intermittent dorsal displacement of the soft palate (DDSP) is a common cause of airway obstruction and poor performance in racehorses. The definite etiology is still unclear, but through an experimental model, a role in the development of this condition was identified in the dysfunction of the thyro-hyoid muscles. The present study aimed to elucidate the nature of this dysfunction by investigating the spontaneous response to exercise of the thyro-hyoid muscles in racehorses with naturally occurring DDSP. Intramuscular electrodes were implanted in the thyro-hyoid muscles of nine racehorses, and connected to a telemetric unit for electromyographic monitoring implanted subcutaneously. The horses were recruited based on upper airway function evaluated through wireless endoscopy during exercise. Five horses, with normal function, were used as control; four horses were diagnosed as DDSP-affected horses based on repeated episodes of intermittent dorsal displacement of the soft palate. The electromyographic activity of the thyro-hyoid muscles recorded during incremental exercise tests on a high-speed treadmill was analyzed to measure the mean electrical activity and the median frequency of the power spectrum, thereafter subjected to wavelet decomposition. The affected horses had palatal instability with displacement on repeated exams prior to surgical implantation. Although palatal instability persisted after surgery, only two of these horses displaced the palate after instrumentation. The electromyographic traces from this group of four horses showed, at highest exercise intensity, a decrease in mean electrical activity and median power frequency, with progressive decrease in the contribution of the high frequency wavelets, consistent with development of thyro-hyoid muscle fatigue. The results of this study identified fatigue as the main factor leading to exercise induced palatal instability and DDSP in a group of racehorses. Further studies are required to evaluate the fiber type composition and metabolic characteristics of the thyro-hyoid muscles that could predispose to fatigue.

Highlights

Horses are remarkable athletes whose performances are highly affected by their respiratory system function [1]
Of the four horses diagnosed as Dorsal Displacement of the Soft Palate (DDSP)-affected in exercise endoscopy prior to electrodes implantation, two demonstrated palatal instability followed by palatal displacement within the HRmax90 and HRmax100 intervals
As previous studies suggested that palatal instability (PI) and DDSP are manifestations of the same condition, and PI represents the preliminary stage of a disorder that may progress to DDSP, as part of a syndrome named palatal dysfunction [5,31,32], we decided to maintain the four horses within the original group identified at the time of enrollment (DDSP)

Summary

Introduction

Horses are remarkable athletes whose performances are highly affected by their respiratory system function [1]. The horse is an obligate nasal breather, where the caudal free border of the soft palate seals the nasopharynx ventrally to the epiglottis [2]; any perturbation of this normal anatomical relationship causes increased airflow turbulence and respiratory impedance [3]. A specific condition whereby the free caudal border of the soft palate moves dorsal to the epiglottis during exercise, named intermittent Dorsal Displacement of the Soft Palate (DDSP), has been recognized as a common cause of airway obstruction in racehorses with a reported prevalence of 10–20% in horses with poor performance [4,5,6,7]. The definitive cause of naturally occurring DDSP is still uncertain. The current standard surgical treatment for DDSP (laryngeal tie-forward) was developed to move the larynx to a more rostral and dorsal position and correct a dysfunction of the TH muscle [11]

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