SUMMARY
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.