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\begin{document}

\title{Unilateral and bilateral compression of the epiglottis during poll
flexion in harness racehorses}



\author[1]{Hanna Vermedal}%
\author[2]{John Mark O'Leary}%
\author[1]{Anna Emilie Klemsdal}%
\author[1]{Gina Margrete Roen}%
\author[1]{Zoe Fretheim-Kelly}%
\author[1]{Eric Strand}%
\affil[1]{Norwegian University of Life Sciences Faculty of Veterinary Medicine}%
\affil[2]{University College Dublin}%


\vspace{-1em}



  
  \date{June 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Background: Horses exercised ``onto the bit'' with periods of induced
poll flexion can demonstrate a unique array of upper respiratory tract
(URT) disorders. Objectives: To describe a previously unreported
exercise-induced disorder of the epiglottis associated with poll flexion
in harness racehorses. Study design: Retrospective observational study.
Methods: Medical records of all harness racehorses that presented for
exercising URT endoscopy between 2005 and 2022 were reviewed. These
horses were exercised on a high-speed treadmill using a previously
standardised protocol with alternating one-minute phases of free head
carriage (no rein tension on the bit) and poll flexion (driven onto the
bit with long reins) until they could no longer maintain the trotting
gait. Results: Seven Standardbreds and 11 Norwegian-Swedish Coldblooded
trotters were diagnosed with unilateral or bilateral compression of the
epiglottis during exercise in poll flexion. These horses demonstrated
progression of this disorder during the phases of induced poll flexion
and showed no signs of epiglottic compression during phases exercised
with free head carriage. Main limitations: Retrospective nature of study
and limited sample size due to low prevalence. Conclusions:
Exercise-induced compression of the base of the epiglottis is an URT
disorder only evident videoendoscopically when horses are driven onto
the bit leading to poll flexion. It is most often seen in association
with dynamic laryngeal collapse but can also be witnessed as a solitary
disorder. Videoendoscopically, it appears that the compression is
initiated by the rostral advancement of the larynx and hyoid apparatus
within a progressively narrowing intermandibular space during poll
flexion. Visually this leads to local inward compression of the lateral
nasopharyngeal walls and base of the epiglottis. Further studies are
ongoing to confirm this hypothesis and to objectively quantify the
degree of URT obstruction caused by this conformational narrowing of the
epiglottis during poll flexion.%
\end{abstract}%



\sloppy


\textbf{Unilateral and bilateral compression of the epiglottis during
poll flexion in harness racehorses}

RUNNING TITLE: Epiglottic compression in harness racehorses

Hanna Vermedal\textsuperscript{1} \textbar{} John Mark
O'Leary\textsuperscript{2} \textbar{} Anna Emilie
Klemsdal\textsuperscript{1} \textbar{} Gina Margrete
Roen\textsuperscript{1} \textbar{} Zoe
Fretheim-Kelly\textsuperscript{1}\textbar{} Eric
Strand\textsuperscript{1}

\textsuperscript{1}Faculty of Veterinary Medicine, Norwegian University
of Life Sciences, Oslo, Norway\textsuperscript{2}School of Veterinary
Medicine, University College Dublin, Dublin, Ireland

Correspondence: Hanna Vermedal. Email:\emph{hannaver@hotmail.com}.
Keywords: horse, upper airway, respiratory, epiglottis, harness
racehorse.

Clinical relevance:

\begin{itemize}
\item
  This manuscript describes a novel exercise-induced upper respiratory
  tract (URT) disorder occurring during periods of induced poll flexion
  in harness racehorses; unilateral and bilateral compression of the
  epiglottis during poll flexion. Horses affected by this disorder
  presented with signs of abnormal respiratory noise and/or poor
  performance.
\item
  This manuscript describes a novel exercise-induced URT disorder
  involving the epiglottis, not previously described in any horse breed
  or disciplines.
\item
  The pathophysiology likely involves inward compression of the
  epiglottis from the hyoid apparatus, in the area of thyrohyoid bone
  and thyroid cartilage articulation, as the larynx and hyoid apparatus
  advances rostrally during poll flexion.
\item
\end{itemize}

\textbf{SUMMARY}

\textbf{Background:} Horses exercised ``onto the bit'' with periods of
induced poll flexion can demonstrate a unique array of upper respiratory
tract (URT) disorders.

\textbf{Objectives:} To describe a previously unreported
exercise-induced disorder of the epiglottis associated with poll flexion
in harness racehorses.

\textbf{Study design:} Retrospective observational study.

\textbf{Methods:} Medical records of all harness racehorses that
presented for exercising URT endoscopy between 2005 and 2022 were
reviewed. These horses were exercised on a high-speed treadmill using a
previously standardised protocol with alternating one-minute phases of
free head carriage (no rein tension on the bit) and poll flexion (driven
onto the bit with long reins) until they could no longer maintain the
trotting gait.

\textbf{Results:} Seven Standardbreds and 11 Norwegian-Swedish
Coldblooded trotters were diagnosed with unilateral or bilateral
compression of the epiglottis during exercise in poll flexion. These
horses demonstrated progression of this disorder during the phases of
induced poll flexion and showed no signs of epiglottic compression
during phases exercised with free head carriage.

\textbf{Main limitations:} Retrospective nature of study and limited
sample size due to low prevalence.

\textbf{Conclusions:} Exercise-induced compression of the base of the
epiglottis is an URT disorder only evident videoendoscopically when
horses are driven onto the bit leading to poll flexion. It is most often
seen in association with dynamic laryngeal collapse but can also be
witnessed as a solitary disorder. Videoendoscopically, it appears that
the compression is initiated by the rostral advancement of the larynx
and hyoid apparatus within a progressively narrowing intermandibular
space during poll flexion. Visually this leads to local inward
compression of the lateral nasopharyngeal walls and base of the
epiglottis. Further studies are ongoing to confirm this hypothesis and
to objectively quantify the degree of URT obstruction caused by this
conformational narrowing of the epiglottis during poll flexion.

\textbf{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; Jo\selectlanguage{ngerman}ó 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.

\textbf{MATERIAL AND METHODS}

\textbf{Population}

Medical records and high-speed treadmill endoscopy (HSTE) recordings of
harness racehorses that presented with abnormal respiratory noise and/or
poor performance which underwent HSTE using a standardised protocol at
the Norwegian University of Life Sciences between 2005 and 2022 were
reviewed. Horses were included in the study if they had videoendoscopic
evidence of epiglottic compression during phases of poll flexion, prior
to appearance of any other URT abnormalities (if present). Epiglottic
compression was defined as inward compression of the base of the
epiglottis resulting in a local U-shaped conformation which often
partially or fully (visually) obscured the vocal fold(s). Data reviewed
for these cases included breed, sex, age, presenting complaint and HSTE
findings. Horses were excluded from the study if they were visually lame
at the time of examination, diagnosed with DLC associated with poll
flexion at the time of examination, if they had been diagnosed with DLC
previously or if they had undergone any form of URT surgery prior to the
HSTE examination.

\textbf{High-speed treadmill endoscopy protocol}

Horses were exercised using a previously standardised HSTE protocol
(Fjordbakk et al. 2008; Strand et al. 2012) and driven on the treadmill
by an experienced driver with full racing tack including bit, bridle,
harness, conventional overcheck and long reins. Horses were equipped
with a heart rate monitor and following a warm-up period of
approximately 2500 m, the horses were exercised in 1-minute phases,
alternating between free head carriage (phases 1, 3, 5) and poll flexion
(phases 2, 4 ± 6). During phases with poll flexion, horses were driven
``onto the bit'' by applying even tension on the long reins as if they
were being driven on a racetrack. During ``free head carriage'', tension
on the reins and bit were released. Standardbreds (STBs) were trotted at
9 m/s and Norwegian-Swedish Coldblooded Trotters (NSCTs) at 8.5 m/s, all
on a 1.5-degree treadmill incline. Exercising racehorses typically
achieve a heart rate over 200 bpm during phase 1 and reach exhaustion at
the trotting gait by phases 5 or 6. Videoendoscopic evaluation of the
larynx was carried out through the length of the exercise protocol, with
the endoscope being passed through the right ventral meatus to the level
of the caudal nasopharynx in all cases. Videoendoscopic recordings were
evaluated in real time and slow motion retrospectively for horses
included in the study. Exercise-induced compression of the base of the
epiglottis was identified as unilateral or bilateral if present. Other
concurrent URT abnormalities that developed subsequent to compression of
the base of the epiglottis were recorded if present.

\textbf{Cadaver sample}

A cadaver sample of a larynx with the hyoid apparatus attached was
obtained from a harness racehorse euthanised for reasons unrelated to
the URT. This larynx was carefully examined to physically explain the
structures which were potentially involved with causing the local
compression of the base of the epiglottis.

\textbf{RESULTS}

\textbf{Population}

Population summary is presented in Table 1. On average 60 harness
racehorses present each year at the Norwegian University of Life
Sciences for HSTE using our standardised protocol. This represents
approximately 1080 horses during the study period between 2005 and 2022.
Of these, eighteen harness racehorses were diagnosed with compression of
the base of the epiglottis during periods of poll flexion and met the
inclusion criteria, giving an estimated prevalence rate of 1.7\%. Of
these eighteen horses, 11 were NSCTs and 7 were STBs. There were 50\%
(9/18) mares, 39\% (7/18) stallions and 11\% (2/18) geldings. The horses
as a group presented at a mean ± SD age of 4.7 ± 2.0 years. The STBs
presented at 4.0 ± 0.8 years (range 3-5 years) and the NSCTs at 5.1 ±
2.4 years (range 3-10 years). The presenting complaint was poor
performance alone in 44.4\% (8/18), abnormal respiratory noise alone in
33.3\% (6/18) and both abnormal respiratory noise and poor performance
in 11.1\% (2/18). One horse (5.6\%) presented due to physical collapse
during a race and one horse (5.6\%) presented for a routine fitness
test. Three horses had abnormal respiratory noise mostly evident when
driven on the bit (horse nr. 6, 9 and 15).

\textbf{High-speed treadmill endoscopy findings}

All horses developed exercised-induced compression of the base of the
epiglottis from the first phase of induced poll flexion (phase 2), often
with exacerbation during subsequent phases of poll flexion. Compression
was bilateral in 13/18 horses (72.2\%) (Figure 1 and 2) and unilateral
in 5/18 horses (27.8\%), with equal distribution between left and right
sides (Figure 3 and 4). Bilateral compression was seen in 10 of 11 NSCTs
(90.9\%) and in 3 of 7 STBs (42.9\%), whilst unilateral compression was
seen in 4 of 7 STBs and in 1 of 11 NSCTs. All horses demonstrated a
unilateral or bilateral inward bulging of the nasopharyngeal wall
adjacent to the compressed side of the epiglottis (green arrows in
figures) during periods of exercise in poll flexion.

One horse (horse nr. 1) also developed moderate ventromedial luxation of
the apices of the corniculate processes during the first phase of poll
flexion after development of epiglottic compression, which persisted
during phases without poll flexion. Two horses developed intermittent
dorsal displacement of the soft palate (iDDSP) (horse nr. 8 and 11) at
the end of the examination protocol (phase 6 and 5, respectively) when
the horses were completely fatigued and could not maintain the trotting
gait and position on the treadmill. This was several minutes after
developing compression of the base of the epiglottis. Two additional
horses developed palatal instability (horse nr. 7 and 9) and MDAF (horse
nr. 12 and 13) respectively, also several minutes after the onset of
epiglottic compression (phases 4-6).

\textbf{Cadaver sample}

An equine larynx/hyoid specimen from a harness racehorse was acquired to
explain the cause of this unusual disorder (Figure 5). When manual
pressure was applied on the thyrohyoid bone (only) approximately 1 cm
distal to the articulation between the thyrohyoid bone and thyroid
cartilage (thyrohyoid articulation) a similar compression of the base of
the epiglottis (Figure 5 bottom right) as witnessed videoendoscopically
was created. This compression visually obscures the vocal folds as the
base of the epiglottis narrows and forms a U-shape.

\textbf{DISCUSSION}

This manuscript describes a novel exercise-related (dynamic) disorder of
the epiglottis induced during periods of poll flexion in harness
racehorses. Compression of the base of the epiglottis appeared visually
during the first phase of induced poll flexion (phase 2) in all 18
horses. In all cases, whether unilateral or bilateral, it appeared
videoendoscopically as if there was extraluminal compression of the base
of the epiglottis from the lateral nasopharyngeal wall (green arrows in
figures) locally on the affected side(s), potentially caused by a change
in position of the larynx/hyoid apparatus within the intermandibular
space during poll flexion. In most cases the compression of the
epiglottis was of a magnitude that it visually obscured the abducted
vocal fold(s) on the affected side(s). In certain cases, the lateral
margins of the epiglottis displayed progressive inward deviation with
prolonged duration of exercise, potentially due to greater inspiratory
luminal pressures in the region of the larynx.

Poll flexion during exercise, defined as a decreased angle between the
mandible and ventral neck, affects upper airway function in horses
competing in multiple athletic disciplines (Petsche et al. 1995; Strand
et al. 2009; Davidson et al. 2011). Harness racehorses perform in a
position of ``high'' poll flexion, due to the overcheck which prevents
lowering of the head and neck and aids steering the horse tactically in
races. These horses are gathered onto the bit with long reins and are
restrained from premature exhaustion or galloping. Poll flexion during
maximal exercise in clinically normal Standardbreds causes an increase
in inspiratory impedance and greater negative inspiratory pressures (7-8
cm H\textsubscript{2}O difference), relative to when these horses are
exercised in free head carriage (head and neck extended), creating a
mild inspiratory obstruction (Petsche et al. 1995). Similar findings (6
cm H\textsubscript{2}O difference) have been documented in clinically
normal elite Norwegian-Swedish coldblooded trotters during maximal
exercise in poll flexion relative to maximal exercise with free head
carriage (Strand et al. 2009). This physiologic event is explained by
the Bernoulli principle and Venturi effect, occurring as a consequence
of narrowing of the URT during poll flexion (Cehak et al. 2010).
Narrowing of a region of the URT will lead to increased velocity of air
through the constricted region and therefore lower intraluminal
pressures according to Poiseuille's Law (fluid dynamics notation). If a
form of URT obstruction is present, local narrowing (collapse) of the
region can become self-perpetuating with progressive lowering of
inspiratory pressures over time and worsening of the obstruction. This
has been demonstrated to occur in harness racehorses diagnosed with DLC
(Strand et al. 2009).

Poll flexion can also induce or exacerbate both simple and complex forms
of exercise-induced URT disorders reported in harness racehorses (Strand
et al. 2004; Fjordbakk et al. 2008; Strand et al. 2012). Exacerbation of
already existing URT disorders during poll flexion may be due to the
increase in airway turbulence, impedance and intraluminal pressures
occurring during poll flexion, while the pathophysiology behind
induction of new URT disorders solely evident during poll flexion is
likely more complex.

In addition to altering airway dynamics, poll flexion also causes
anatomical conformational changes of the laryngohyoid apparatus which
can be readily measured from lateral radiographs of the region
(McCluskie et al. 2008; Fjordbakk et al. 2013). The larynx moves
rostrally in relation to the hyoid apparatus during poll flexion;
McCluskie et al. (McCluskie et al. 2008) demonstrated a rostral
advancement of the ossified portion of the thyroid cartilage in relation
to the thyrohyoid bone. Achieving this local conformation change is the
basis of the ``laryngeal tie-forward'' procedure for treating iDDSP
(Woodie et al. 2005).

Fjordbakk et al. (2013) demonstrated that horses affected by DLC showed
a statistically significantly greater rostral advancement of the larynx
in relation to the thyrohyoid bone during poll flexion compared to
unaffected control horses. This rostral advancement within the
intermandibular space resulted in external compression of the larynx as
shown by the authors in form of a reduced laryngeal lumen width in
affected horses measured ultrasonographically (Fjordbakk et al. 2013).
Though DLC to this date has been diagnosed in a number of different
breeds and disciplines, the aforementioned study and more recent
research into genetics illustrate that URT disorders occurring solely
during poll flexion often result from a combination of phenotypic
predisposition along with the consequences of a rostral laryngohyoid
displacement (Fjordbakk et al. 2013; Velie et al. 2020).

Epiglottic compression appears soon after inducing poll flexion by
gathering the reins and driving the horse onto the bit, similar to how
DLC visually appears. The authors have seen compression of the
epiglottis manifesting most commonly as part of a DLC diagnosis, and
epiglottic compression may be a variation of a complex array of
exercise-induced laryngeal disorders seen only during phases of poll
flexion. The occurrence of the disorder primarily, without concurrent
DLC, has however not been described before.

Dynamic laryngeal collapse is a form of bilateral arytenoid cartilage
and vocal fold collapse occurring during exercise in poll flexion in
predisposed horses, not associated with recurrent laryngeal neuropathy
(Fjordbakk et al. 2015; Ducharme 2016). It is most commonly diagnosed in
NSCTs but is also reported to be common in the Colombian Criollo Paso
breed (Joó et al. 2021) and other gaited horses that exercise in
``high'' poll flexion (Hanche-Olsen et al. 2010; Allen et al. 2011;
McCarrel \& Woodie 2015; Hackett \& Leise 2018). In DLC, a phenotypic
predisposition combined with rostral laryngohyoid advancement in the
intermandibular space occurring during poll flexion leads to inward
displacement of the arytenoid cartilages with secondary collapse of the
vocal folds (Strand et al. 2004; Fjordbakk et al. 2008). The phenotypic,
or conformational traits include: a rostrally positioned larynx relative
to the vertical ramus of the mandible, a high head and neck position
during exercise, a narrow intermandibular space and an energetic
attitude when driven ``onto the bit'' (Strand et al. 2004; Velie et al.
2020). A rostrally positioned larynx, relative to the vertical ramus of
the mandible, is a trait of many draft and pony breeds of horses (Hawe
et al. 2001). In the cases presented here, the rostral laryngohyoid
advancement caused by periods of poll flexion leads to compression of
the base of the epiglottis while the arytenoid cartilage's ability to
abduct is unaffected. As opposed to horses diagnosed with DLC, horses
showing epiglottic compression likely have more compression of a
rostroventrally positioned articulation between the thyrohyoid bone and
thyroid cartilage which is not interfering with the ability of the
cricoarytenoideus dorsalis muscles to abduct the arytenoid cartilages,
as seemingly occurs with DLC (Figure 5) (Strand et al. 2004; Fjordbakk
et al. 2008).

The majority of horses in this study presented for investigation of poor
performance and/or abnormal respiratory noise (Table 1), however one
horse (nr. 3) presented after physical collapse during a pari-mutuel
race. This horse had visually the most severe degree of URT obstruction
of all 18 horses in this review during HSTE with both compression of the
base of the epiglottis and dorsomedial deviation of the epiglottic
margins. This highlights the importance of examining harness racehorses
during exercise using bit, bridle, long reins and overcheck to induce
different head and neck angles similar to what these horses experience
under competition if one is to fully understand the clinical condition.
This can be accomplished using high-speed treadmill videoendoscopy, as
described here, or utilising a vigorous exercise protocol on the
racetrack with overground endoscopy.

Unilateral compression of the base of the epiglottis was seen in 57.1\%
of STBs and in 9.1\% of NSCTs, whilst bilateral compression was seen in
42.9\% of STBs and in 90.9\% of NSCTs. This uneven breed distribution
may be related to the low sample size, or to a potential anatomical
conformational difference of the hyoid apparatus and/or thyrohyoid
articulation between these two genetically distinct breeds. Another
plausible cause of unilateral compression of the base of the epiglottis
is uneven head carriage while being driven, but this variable was not
investigated in this study. However, the experienced drivers of the
horses in this study always attempted to drive them evenly onto the bit
during the examination. Additionally, any horses showing signs of
lameness during the examination, a common cause of uneven head carriage
in harness racehorses, were excluded from the study. The position of the
endoscope should also not affect the unilateral appearance of the
disorder as the endoscope was passed via the right ventral meatus in all
cases, and there was an even distribution between left and right
unilateral epiglottic compression. Finally, the compression of the
epiglottis occurred only during phases of poll flexion and was never
evident in phase 1 (free head carriage) prior to first induction of poll
flexion.

\textbf{CONCLUSIONS}

Horses affected by epiglottic compression in the current manuscript
presented primarily with signs of abnormal respiratory noise and/or poor
performance, as well as one horse presenting after physically collapsing
in a race. The pathophysiology of this disorder likely involves inward
compression of the epiglottis from the hyoid apparatus, in the area of
thyrohyoid bone and thyroid cartilage articulation, as the larynx and
hyoid apparatus advances rostrally through a progressively narrowing
intermandibular space during poll flexion. This is supported by the
inward movement of the lateral nasopharyngeal walls seen visually during
poll flexion in all cases, combined with current knowledge of how the
laryngohyoid apparatus changes in position during poll flexion. The
present treatment recommendation is to limit the degree of poll flexion
by use of tack modifications in affected horses. Further studies are
necessary to confirm the proposed pathogenesis of this URT disorder and
objectively define how this disorder impacts upper airway dynamics in
exercising horses.

\textbf{AUTHOR CONTRIBUTIONS}

H. Vermedal, AE. Klemsdal, GM. Roen, Z. Fretheim-Kelly and E. Strand
contributed with collection of data and data analysis. H. Vermedal, JM.
O'Leary and E. Strand contributed with manuscript preparation. All
authors revised and approved the submitted manuscript.

\textbf{CONFLICT OF INTEREST}

The authors declare no conflict of interest related to this report.

\textbf{ETHICS STATEMENT}

All animal materials used in the current study were received from client
owned animals and used with owner consent.

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\textbf{FIGURE LEGENDS}

\emph{Figure 1:} Endoscopy image of horse nr. 3, a 3-year-old
Norwegian-Swedish Coldblooded Trotter (stallion) during phase 1 (A/left)
with free head carriage and during phase 4 (B/right) with induced poll
flexion demonstrating compression of the base of the epiglottis (yellow
arrows) along with inward deviation of the left nasopharyngeal wall
(green arrow).

\emph{Figure 2:} Horse nr. 18, a 5-year-old Standardbred (mare)
demonstrating bilateral compression of the base of the epiglottis
(yellow arrows) during phase 2; the first phase of induced poll flexion
(A). The epiglottic compression is accompanied by simultaneous inward
deviation of the nasopharyngeal walls (green arrow). Once relieving rein
tension and reducing the angle of poll flexion, epiglottic conformation
returns to normal as seen in phase 3 (B). Note the transtracheal
pressure sensor passed through a polyethylene catheter for
simultaneously measuring tracheal pressures.

\emph{Figure 3:} Horse nr. 15, a 3-year-old Standardbred (stallion) with
a normally appearing epiglottis during phase 1 (A) and unilateral
compression of the left side of the base of the epiglottis (yellow
arrow) with inward deviation of the left nasopharyngeal wall (green
arrow) (B) during poll flexion in phase 2. Note the transtracheal
pressure sensor passed through a polyethylene catheter for
simultaneously measuring tracheal pressures.

\emph{Figure 4:} Horse nr 4, a 4-year-old Standardbred (mare) in phase 1
(A) with a normal conformation of the epiglottis, and during poll
flexion in phase 2 (B) with left sided compression of the base of the
epiglottis (yellow arrow) and inward deviation of the left
nasopharyngeal wall (green arrow).

\emph{Figure 5:} An equine larynx/hyoid specimen illustrating the
thyrohyoid articulations (red arrows top picture and bottom left) and
how manual pressure applied ca. 1 cm distal to the thyrohyoid
articulation causes compression of the base of the epiglottis (bottom
right). Note how the vocal folds are visually obscured during epiglottic
compression (bottom right).

\textbf{TABLES}

\textbf{Table 1: Summary of signalment, presenting complaint and
unilateral or bilateral nature of epiglottic compression for the 18
harness racehorses included in this study:}\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Nr. & Breed & Age & Sex & Presenting complaint &
Unilateral/bilateral\tabularnewline
\midrule
\endhead
1 & STB & 3 & Stallion & Abnormal respiratory noise &
Bilateral\tabularnewline
2 & NSCT & 3 & Stallion & Poor performance & Bilateral\tabularnewline
3 & NSCT & 3 & Stallion & Collapse in race & Bilateral\tabularnewline
4 & STB & 4 & Mare & Routine fitness test & Unilateral
(left)\tabularnewline
5 & NSCT & 3 & Mare & Abnormal respiratory noise &
Bilateral\tabularnewline
6 & NSCT & 6 & Stallion & Abnormal respiratory noise &
Bilateral\tabularnewline
7 & NSCT & 8 & Mare & Poor performance & Unilateral
(right)\tabularnewline
8 & NSCT & 7 & Mare & Poor performance & Bilateral\tabularnewline
9 & NSCT & 3 & Mare & Abnormal respiratory noise &
Bilateral\tabularnewline
10 & STB & 5 & Mare & Abnormal respiratory noise &
Bilateral\tabularnewline
11 & NSCT & 3 & Stallion & Abnormal respiratory noise &
Bilateral\tabularnewline
12 & NSCT & 5 & Gelding & Poor performance & Bilateral\tabularnewline
13 & NSCT & 10 & Stallion & Poor performance & Bilateral\tabularnewline
14 & STB & 4 & Mare & Abnormal respiratory noise and poor performance &
Unilateral (right)\tabularnewline
15 & STB & 3 & Stallion & Abnormal respiratory noise and poor
performance & Unilateral (left)\tabularnewline
16 & NSCT & 5 & Gelding & Poor performance & Bilateral\tabularnewline
17 & STB & 4 & Mare & Poor performance & Unilateral
(left)\tabularnewline
18 & STB & 5 & Mare & Poor performance & Bilateral\tabularnewline
\bottomrule
\end{longtable}

NSCT = Norwegian-Swedish Coldblooded Trotter, STB = Standardbred.\selectlanguage{english}
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