Dynamic endoscopy of the upper airways.

Obstructive disorders of the upper airways have been recognised as important causes of abnormal respiratory noise and poor performance in the equine athlete for many years. Resting endoscopy may be useful to identify static forms of upper airway obstruction, such as subepiglottal cysts, epiglottal entrapment, arytenoid chondritis, and laryngeal hemiplegia. However, exercising endoscopy is considered to be the “gold standard” for making a definitive diagnosis of dynamic upper airway collapse in horses where resting findings are frequently unreliable or absent. Through the use of exercising endoscopy, it has been possible to identify the collapse of a range structures within the nasopharynx and / or larynx, with complex dynamic collapse (whereby multiple structures are affected) occurring frequently.¹ A number of systems are now available commercially for performing dynamic endoscopy in the field and this technique has a number of advantages over treadmill endoscopy because the horse can be examined in its normal environment and the effects of the tack and rider can be accounted for.²

When performing exercising endoscopy, the type of exercise test performed is crucial in enabling an accurate diagnosis of dynamic airway obstruction to be made. In cases where horses make obvious respiratory noise during training conditions, a diagnosis should be straightforward. However, for investigation of those cases that make abnormal respiratory noise or perform poorly only during competition or racing it is essential to replicate the conditions encountered during competition. If the clinical signs reported during competition are not replicated, false negative findings may occur.³ For the examination of racehorses, it is recommended that exercise testing be performed at an appropriate track where the distance and speeds encountered during racing can be replicated. In many cases dynamic upper airway obstruction only occurs or is at its most severe during peak exercise. This is because inspiratory airway pressures become more negative at higher speeds and fatigue of the upper airway musculature may also be involved. However, there are occasions when upper airway collapse appears more severe at slower speeds. In horses that are involved in non-racing disciplines, factors other than exercise intensity such as head and neck position, tack modifications and other rider interventions have been shown to induce or exacerbate dynamic airway collapse and need to be considered.^4-6 There is increasing evidence that these factors may also play a role in the development of dynamic airway collapse in both harness and Thoroughbred racehorses. Furthermore, many of these factors may occur concurrently, for example when pulling up at the end of the gallops the horse will slow down and at the same time might show concurrent head-neck flexion and mouth opening, making it difficult to separate the possible inciting causes.

It is important to note that whilst dynamic endoscopy enables visualisation of any dynamic airway collapse, it does not enable quantification of the functional effects of an obstruction. In order to definitively assess any respiratory limitation, it is necessary to measure upper airway mechanics. However, this is challenging to perform in the field and is not commonly performed in clinical practice. Not all upper airway obstructive conditions have the same impact on respiratory function, but they commonly create an increase in respiratory resistance along the upper respiratory tract, which may result in either reduced airflow or an increase in the trans-upper airway pressures required to maintain airflow. This increase in airway resistance will lead to an increase in respiratory workload, and where airflow is reduced, the resulting hypoventilation may lead to decreased oxygen consumption, increased blood lactate concentration and exacerbation of arterial hypoxaemia and hypercapnia.⁷ It should also be recognised that concurrent lower airway disease is also present in many horses and may exacerbate the situation further.

The impact of respiratory disease will not only depend on the nature and severity of the disease but also on the equestrian discipline performed. In horses competing at maximal and supramaximal intensities (ie. at or above speeds that elicit maximal oxygen consumption (VO2Max)), optimal efficiency of all body systems including the respiratory system is essential. Therefore, upper airway obstructions in racehorses will more markedly impair their performances than those of horses exercising at less strenuous levels, such as dressage horses or show jumpers. The oxygen consumption associated with these equestrian disciplines is less than for racing and a smaller fraction of the total respiratory capacity is required. Nevertheless, several studies have confirmed that upper airway obstruction is a cause of poor performance in sport horses. Whilst effort perception cannot be measured in horses, it is plausible that the increased work of breathing may affect the horse’s attitude and willingness to work.

In conclusion, athletic horses may be afflicted with a wide range of upper airway obstructions. The clinical significance of this in any individual may be influenced by a number of factors, including the type and severity of the obstruction as well as the level and type of activity being undertaken and rider / driver interventions. It is important to make a definitive diagnosis and take into consideration these other factors when making decisions relating to treatment