Fatigue in the thyro-hyoid muscles has been identified by researchers as a key factor in displacement of the soft palate during exercise in racehorses — a debilitating condition that can result in poor track performance.
Horses can breathe only through their nose. The soft palate, essentially a flap of tissue, blocks off the pharynx from the mouth, except when the horse is swallowing.
Any disruption of this normal anatomical relationship causes increased airflow turbulence and respiratory problems.
A problem whereby the free caudal border of the soft palate moves dorsally 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.
The definite cause remains unclear, but researchers have found that dysfunction of the thyro-hyoid muscles plays a role. These paired flat muscles lie on each side of the larynx.
Researchers from Cornell University in New York and the Royal Veterinary College in London set out to learn more about the nature of this dysfunction by investigating the 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, recruited based on upper airway function evaluated through wireless endoscopy during exercise.
Five horses with normal airway function were used as controls, while four 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 was recorded for analysis during incremental exercise tests on a high-speed treadmill.
The affected horses had palatal instability with displacement on repeated exams before the surgical implantation of the electrodes.
Although palatal instability persisted after surgery, only two of these horses displaced their palate after the electrodes were implanted, Marta Cercone and her colleagues reported in the open-access journal PLOS ONE.
The results identified fatigue as the main factor leading to exercise-induced palatal instability and DDSP in the affected racehorses.
Further studies are required to evaluate the characteristics of the thyro-hyoid muscles that could predispose them to fatigue, they said.
Discussing their findings, the authors said fatigue is defined as “a loss in the capacity for developing force and/or velocity of a muscle, resulting from muscle activity under load and which is reversible by rest”.
The increased fatigability of the thyro-hyoid muscle could arise from either an intrinsic problem of the muscle (physiological or neuromuscular), sub-optimal fiber type distribution, small size muscles fibers, or sub-optimal training methods for the upper airway musculature. Microscopic assessment of these muscles in DDSP-affected horses would be the next step.
Alternatively, it may develop because of other upper airway problems (palatal billowing, other upper airways obstruction, laryngo-hyoid anatomic relationship) that cause an overload of the muscle during exercise, leading to its fatigue.
The research team said it would be sensible to evaluate the effect of different types of training protocols on thryo-hyoid muscle characteristics to identify a new therapeutic approach for DDSP prevention and cure.
“The positive effect of endurance training exercise on muscle fatigue resistance has been well described, with an increase in number of capillaries, activity of oxidative enzymes, mitochondrial biogenesis and changes in muscle fiber type composition, and increased inspiratory muscle strength and fatigue resistance.”
They continued: “Confirmation of different thyro-hyoid muscle characteristics (fiber type composition, metabolic features) in DDSP-affected horses would open the possibility to use muscle reconditioning through specific training as a non-surgical option for the management of DDSP in young racehorses.”
The study team comprised Cercone, Jonathan Cheetham, Lisa Mitchell and Norm Ducharme, all from Cornell University; and Emil Olsen and Justin Perkins, with the Royal Veterinary College.
The horses used in the study belonged to the research herds of the two institutions. At the end of the study, the horses were enrolled in other research programs.
Cercone M, Olsen E, Perkins JD, Cheetham J, Mitchell LM, Ducharme NG (2019) 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. PLoS ONE 14(10): e0224524. https://doi.org/10.1371/journal.pone.0224524