A flaccid appearance of the epiglottis is linked with the occurrence of dorsal displacement of the soft palate in racehorses, researchers in Italy have found.
The finding was made by a study team that set out to evaluate the upper and lower airways of Thoroughbred and Standardbred racehorses with dynamic upper airway obstruction.
Chiara Maria Lo Feudo and her fellow researchers said horses are obligatory nasal breathers. They cannot avoid the high pressures at the level of the nasopharynx during exercise by switching to oral breathing like other species.
In addition, the nasopharyngeal region is not supported by bone or cartilaginous structures, relying only on muscle activity to maintain its stability.
Therefore, during exercise, when airflow turbulence and negative pressures occur at the floor of the nasopharynx and within the larynx, these structures may collapse, and horses may develop different forms of dynamic upper airway obstruction.
As a consequence, respiratory function and gas exchange in the lungs may be impaired, resulting in poor athletic performance, especially in racehorses working at maximum effort.
Dynamic upper airway obstructions are a common cause of poor performance in racehorses. They can occur in different forms which vary in severity.
Previous studies have reported contrasting results concerning the contribution of abnormal pharyngo-laryngeal appearance and airway inflammation to the development of upper airway obstruction problems.
The University of Milan researchers, reporting in the journal Animals, sought to evaluate possible associations between the development of the disorder and resting airway endoscopic findings. They looked at epiglottis size, inflammation of the airways, and evidence of exercise-induced pulmonary hemorrhage (bleeding into the lungs).
Their study centered on 360 Thoroughbred and Standardbred racehorses with poor performance or abnormal respiratory noises. All had been referred to the Equine Sports Medicine Unit of the Veterinary Teaching Hospital of the University of Milan between 2000 and 2021.
All underwent a highly standardized protocol for airways evaluation, including treadmill work.
“A flaccid appearance of the epiglottis was associated with the occurrence of the dorsal displacement of the soft palate,” the researchers said. No relationship was detected between the condition and epiglottis length, nor was it linked with inflammation of the upper or lower airways.
Horses affected by dynamic upper airway obstructions did not appear to be more prone to lung bleeding, they said.
The results suggest that the epiglottis may contribute to upper airway stability, they said, while inflammation does not predispose horses to the onset of upper airway obstructions.
Discussing their research, the study team said although dynamic upper airway obstructions have long been known to cause poor performance in racehorses, their precise mechanisms are not yet fully understood.
In their study, the most frequently observed forms of the disorder were dorsal displacement of the soft palate (29.72%), medial deviation of aryepiglottic folds (17.5%), and nasopharyngeal collapse (16.39%). Horses with multiple causes for their airway obstruction were identified relatively frequently in the study, they observed.
Airway obstruction was more common in Thoroughbreds than in Standardbreds in the study group, confirming a possible breed predisposition.
“The detection, at resting endoscopy, of the abnormal appearance and function of the pharyngo-laryngeal region was associated with the development of dynamic upper airway obstruction,” they concluded. “In particular, a flaccid appearance of the epiglottis, with a loss of convexity and rigidity, was associated with the occurrence of dorsal displacement of the soft palate, both at rest and during high-speed treadmill endoscopy.”
In contrast, the epiglottis length measured in x-rays was not associated with dynamic upper airway obstruction, suggesting that the epiglottis may be involved in maintaining the stability of the upper respiratory tract, not based on its dimensions but on its conformation.
They said their findings do not support the theory of a possible contribution of upper and lower airway inflammation to the onset of the airway obstruction problems, nor did they see a cause-effect relationship between the condition and lung bleeding.
“However, given the contrasting results reported by different studies, these hypotheses cannot be ruled out.”
The study team comprised Lo Feudo, Giovanni Stancari, Luca Stucchi, Bianca Conturba, Enrica Zucca and Francesco Ferrucci, all with the University of Milan; and Federica Collavo, a veterinarian in Germany.
Lo Feudo, C.M.; Stancari, G.; Collavo, F.; Stucchi, L.; Conturba, B.; Zucca, E.; Ferrucci, F. Upper and Lower Airways Evaluation and Its Relationship with Dynamic Upper Airway Obstruction in Racehorses. Animals 2022, 12, 1563. https://doi.org/10.3390/ani12121563