Dynamic endoscopy system shows what happens in horses with varying poll flexion

A fresh study has improved our understanding of what degree of poll flexion may be appropriate from a welfare standpoint in dressage and showjumping horses, with a difference of just 15 degrees having a major effect.

Horses are elite athletes with a high oxygen demand under exercise. But they are obligate nasal breathers and are unable to switch to mouth breathing in order to decrease resistance. This causes severe ventilatory limitations, thus constraining their athletic performance, Paula Tilley and her fellow researchers wrote in the journal Animals.

Still, the horse’s respiratory system has some ways of decreasing resistance by dilating the external nostrils, abducting the larynx fully, and achieving some degree of bronchodilation.

“However, the friction and turbulence caused by the large increase in air speed during exercise compromises these adaptations,” the researchers, with the University of Lisbon, in Portugal, said.

“When we ride horses, we add to frictional resistance, often unknowingly. During exercise, horses generate very high air flows with resultant negative pressures inside their upper airways, risking their collapse.

“Therefore, they want to extend their heads and necks as this stiffens the trachea, preventing this collapse.

“In disciplines such as dressage and showjumping, we ask horses to bend the extra-thoracic airway at the level of the larynx and upper trachea, narrowing the lumen; in association with the increase in air flow speed during exercise.”

This leads to problems, with the potential of dynamic collapse and eventually lower air flows.

The impact of various degrees of poll hyperflexion on the welfare of ridden horses has previously been evaluated in studies, they noted. However, the International Society for Equitation Science has proposed that lesser degrees of poll flexion should also be investigated.

Tilley, together with Joana Simões and José Paulo Sales Luis, set out to evaluate the effects of two poll flexion positions, with a difference of only 15°, on the respiratory system and behaviour of horses during ridden exercise.

Twenty high-level dressage and 20 showjumping horses were ridden twice for 40 minutes, three weeks apart, with the first ridden exercise at an 85-degree ground angle and the second at a 100-degree ground angle (the angle between the ground and the line from the forehead to the muzzle).

The degrees of flexion are shown in the illustration above, with the 85-degree angle being a more forward, or open, position than the higher angle.

Conflict behaviour was monitored, as were signs of upper airway collapse, as observed on an over-ground endoscopy using a dynamic system.

Arterial blood oxygen and lactate, pleural pressure, pharyngeal diameter, and heart and respiratory rates were also evaluated.

For horses from both disciplines, at 100°, both conflict behaviours and upper airway abnormalities were seen more frequently, the intrathoracic pressure was higher, and the pharyngeal diameter was lower.

The conflict behaviours seen in greater abundance at the 100-degree angle included tail swishing, head shaking, mouth opening, and excessive salivation/drooling.

At 85°, relaxation behaviours – ear play and having the ears turned forward – were more frequent.

Compared to the first ridden exercise, at the lesser angle, the heart rate and respiration rate were lower at the beginning of the second ridden exercise but higher at the end. The lower rates at the start of the second test suggest that the horses had become familiar with the space, the people, and the exercise test, the study team said.

“The differences found here support the idea that an increase of just 15° in ridden poll flexion can have negative effects on the respiratory system and behaviour of a horse and therefore on its welfare,” the study team said.

“We believe that the relevance of keeping the amount of time spent riding with a poll flexion ground angle greater than 85° to an essential minimum should be further explored for the benefit of horse performance and the horse’s quality of life in sport, thereby reassuring public concern.”

Discussing their findings, the authors noted that, in the evaluation of the over-ground endoscopy videos, multiple upper airway tract dynamic dysfunctions were significantly more common when the 100°-ground-angle position was used in both equestrian disciplines.

“These were aryepiglottic fold axial deviation, palatal instability/dysfunction, and the collapse of various structures, such as the nasopharynx, vocal fold, intermittent bilateral arytenoid cartilage, and cricotracheal ligament.

“In showjumping horses, ventroaxial luxation of the arytenoid corniculate process was also significantly more frequent with this degree of flexion.

“Still, the highest scores for both the dressage and the show-jumping horses with the 100° ground angle were attributed to nasopharyngeal collapse, palatal instability/dysfunction, and intermittent bilateral arytenoid cartilage collapse.”

The effect of these upper airway abnormalities on the performance and welfare of dressage and showjumping horses must be further evaluated, they said.

Blood lactate levels were significantly higher after the dressage horses were ridden with the 100°-ground-angle poll flexion position (second test), but this was not true for the showjumping horses.

“Still, when considering the blood samples from all horses together, the lactate levels were again significantly higher after they were ridden with the 100°-ground-angle poll flexion position.”

Pleural pressure was significantly higher with the 100°-ground-angle position in both equestrian disciplines. The significant reduction in the pharyngeal diameter found in the present study with the use of greater poll flexion in both equestrian disciplines likely contributed to this result.

In conclusion, the researchers said multiple upper airway dynamic dysfunctions were significantly more frequent among the dressage and showjumping horses with the 100°-ground-angle poll flexion positions, in comparison to the 85° poll flexion position.

The highest scores were attributed to nasopharyngeal collapse, palatal instability/dysfunction, and intermittent bilateral arytenoid cartilage collapse. In addition, most conflict behaviours were significantly more frequent at the greater poll flexion angle.

They said the most significant differences between the equestrian sports evaluated in the present study, dressage and show jumping, centered on the conflict and relaxation behaviours, as some variations were seen between the two groups. There were no significant differences in this respect for the upper airway dynamic dysfunctions.

“The fact that it was possible to discriminate between the two riding poll flexion positions based on a few conflict behaviours and a few relaxation behaviours could contribute to an easier detection of discomfort in the ridden horse.”

Tilley, P.; Simões, J.; Sales Luis, J.P. Effects of a 15° Variation in Poll Flexion during Riding on the Respiratory Systems and Behaviour of High-Level Dressage and Show-Jumping Horses. Animals 2023, 13, 1714. https://doi.org/10.3390/ani13101714

The study, published under a Creative Commons License, can be read here.