34th Annual Scientific Meeting proceedings

A multitude of structures making up the equine upper airway may collapse or displace during exercise, leading to airway obstruction causing increased work of breathing. Complex disorders are defined as the co-occurrence of more than one exercise-induced abnormality and is reported in the trotting breeds at 30-60% prevalence in the Standardbred^1-6 and nearly 80% in the Norwegian-Swedish Coldblooded trotter (NSCT)⁵ when looking at horses in which an exercise-induced upper airway disorder was diagnosed.

The majority of cases with complex disorders present with abnormal respiratory noise and poor performance,^1-6 which might span from premature exhaustion during the race to collapsing on the racetrack. The prevalence of behavioral changes such as unwillingness to enter the racetrack or aversive behavior to training or racing is unknown.

Diagnosis is dependent upon endoscopy at an exercise intensity simulating the exertion and neuromuscular fatigue of racing effort, but also recreating the different head positions these horses experience during racing. In harness racing, the majority of trotters race with an overcheck, meaning that tension on the reins will result in high poll flexion. At an anatomical level, poll flexion induces rostral advancement of the larynx relative to the hyoid apparatus,^7,8 a latero-lateral narrowing of the airway lumen,^4,9 as well as a reduction in dorsoventral pharyngeal diameter.¹⁰ Even in normal trotters without endoscopic signs of airway collapse, poll flexion causes increased inspiratory impedance¹¹ and a drop in inspiratory tracheal pressures.¹² Together, these anatomical and aerodynamic changes may induce or exacerbate a number of exercise-induced upper airway obstructions, such as palatal disorders and nasopharyngeal collapse (NCP), dynamic laryngeal collapse associated with poll flexion (DLC), medial deviation of the aryepiglottic folds (MDAF), and epiglottis disorders.

Several of the most common upper airway abnormalities are believed to result from neuromuscular fatigue secondary to increasing pressure gradients occurring during maximal exercise.^13-15 The fatiguability of the upper airway musculature may be intrinsic, for instance due to suboptimal fiber type composition with low proportion of fatigue-resistant type I fibers,¹⁵ or lack of upper airway musculature conditioning due to sub-optimal training methods. Alternatively, the musculature may also just simply be overloaded by increased workloads. With complex disorders, the contribution to aerodynamic changes by one obstruction may cause or exacerbate another when reaching critical pressure fluctuations, leading to a self-perpetuating state of ever-increasing work of breathing. Not surprisingly, complex disorders also yield the largest impact on exercising metabolic parameters, such as a slower speed at a heart rate of 200 bpm, and at a lactate threshold of 4 mmol/liter, as well as increased peak lactate levels.¹⁶ 

In Standardbreds, obstructions at the nose, the pharynx, and the larynx may all contribute to complex disorders whereas in the NSCTs, co-occurrence of several laryngeal disorders are more prevalent. The most commonly reported clustering of airway disease in Standardbreds is the co-occurrence of MDAF with either palatal dysfunction such as palatal instability or iDDSP, and/ or NCP.^1-3,5,6 The vast majority of horses with MDAF have secondary disorders, and MDAF is the most common secondary disorder seen with iDDSP. NCP is part of complex disease in 20 – 30% of horses and is frequently accompanied by MDAF, palatal dysfunction, and dorsomedial deviation of the epiglottis margins (DMDEM). Another clustering of disease is the co-occurrence of alar fold collapse (AFC) and palatal dysfunction, exacerbating each other as they are both expiratory obstructions.¹⁷ Less commonly reported combinations are epiglottic entrapment and palatal dysfunction, recurrent laryngeal neuropathy and either palatal dysfunction or MDAF, and ventromedial arytenoid displacement with MDAF. In NSCTs, MDAF is commonly paired with DLC and/ or DMDEM, whereas a combination of DLC and palatal dysfunction is less common.⁴

Surgical treatment of exercise-induced upper airway disorders relies mainly upon resecting collapsing tissue or stabilizing laryngeal structures. Improved performance in approximately 75% of horses has been reported after surgical treatment of MDAF and AFC^13,17 and in 67% of horses with iDDSP.¹⁸ However, no specific surgical treatment exists to date for NPC, DLC, or DMDEM. In complex disorders, it may also be difficult to distinguish between the primary and secondary problem, as demonstrated in horses with concurrent AFC and iDDSP, where resecting the alar folds oftentimes alleviate the palatal problem also.¹⁷

Until recently, conservative treatment options have been limited to tack and management changes. Inspiratory muscle training (IMT) has however recently been investigated as a conservative treatment avenue for exercise-induced upper airway abnormalities in Thoroughbred racehorses.^19,20 In human athletes affected by exercise-induced laryngeal obstructions (EILO), IMT is currently used as first-line treatment whereas surgical options are only pursued in a few select cases.²¹ However, in humans, IMT can exacerbate some forms of EILO as a stronger diaphragmatic pull leads to more negative inspiratory pressures, and IMT focusing on lowering airway resistance is recommended.²¹ Preliminary data of IMT in horses have demonstrated improvement in exercise-induced conditions such as vocal fold collapse and palatal dysfunction in a small number of animals,²⁰ as well as a measurable increase in diaphragmatic size.²² Although promising, the technique is still in its infancy regarding treating or preventing complex exercise-induced upper airway disorders in the equine athlete, and further research is required to evaluate its usefulness.

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