INTRODUCTION
Exercise-induced upper respiratory tract (URT) disorders are a common cause of poor performance and abnormal respiratory noise at exercise in sport horses and racehorses (Morris & Seeherman 1991; Kannegieter & Dore 1995; Lane et al. 2006; Davidson et al. 2011; Strand et al. 2012). The literature suggests a breed, gait and sporting discipline difference in the prevalence (Kannegieter & Dore 1995; Tan et al. 2005; Strand et al. 2012) and predisposition (Strand et al. 2012) to different forms of URT disorders. Certain disorders appear to have a phenotypic (conformational) cause, especially in horses where poll flexion is induced, such as in harness racehorses (Fjordbakk et al. 2008; Strand et al. 2012; Velie et al. 2020). Harness racehorses, along with draft horses, dressage horses and gaited horses can also demonstrate a unique array of exercise-induced URT disorders only during poll flexion compared to horses not examined or exercised in this manner (Hanche-Olsen et al. 2010; Allen et al. 2011; McCarrel & Woodie 2015; Hackett & Leise 2018; Joó et al. 2021).
Exercise-related laryngeal disorders associated with poll flexion are often induced by changes in the relative positioning of the larynx and hyoid apparatus within the intermandibular space (McCluskie et al. 2008; Fjordbakk et al. 2013) and exacerbated by the increased negative inspiratory pressures caused by narrowing of the pharyngeal/laryngeal lumen width during poll flexion (Petsche et al. 1995; Strand et al. 2009), as predicted by the Bernoulli Principle and Venturi Effect. Upper respiratory tract disorders evident during poll flexion can be primary in nature (Strand et al. 2004) or secondary to the increased regional airway pressure gradients occurring with other URT disorders (Strand & Skjerve 2012) and/or surgical interventions that alter laryngeal and/or hyoid conformation (Vermedal & Strand 2020).
Currently reported exercise-induced disorders of the epiglottis include intermittent epiglottic entrapment (Morris & Seeherman 1991; Kannegieter & Dore 1995; Tan et al. 2005), epiglottic retroversion (Tan et al. 2005) and flaccid epiglottis (Strand et al. 2012; Strand & Skjerve 2012). These are less commonly reported than other exercise-induced disorders of the larynx and/or pharynx. There is sparse literature regarding exercise-induced epiglottic disorders in the horse in general and compression of the base of the epiglottis during poll flexion has not, to the authors knowledge, been reported as an exercise-induced URT disorder. Similar manifestations have been mentioned in the literature, namely dynamically flaccid epiglottis or dorsomedial deviation of the margins of the epiglottis, however in these cases the epiglottic disorder occurred concurrent with or subsequent to other URT disorders such as dynamic laryngeal collapse (DLC) associated with poll flexion and/or medial deviation of the aryepiglottic folds (MDAF) (Fjordbakk et al. 2008; Strand & Skjerve 2012; McCarrel & Woodie 2015; Joó et al. 2021).
The aim of this study was to report a clinically specific URT disorder involving the epiglottis, seen videoendoscopically only during periods of exercise with induced poll flexion, in harness racehorses.