What’s that noise? An overview of exercise-induced upper airway disorders
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By Kate Allen and Geoffrey Lane
The majority of upper airway (‘wind’) disorders affect the regions of the pharynx and larynx. Most of these conditions are only present during exercise, when the upper airway is exposed to large changes in pressures associated with increased breathing rate and effort. This is the reason why performing endoscopy at rest may not give an accurate diagnosis. Endoscopy during strenuous exercise (overground endoscopy) has become key for veterinary surgeons to be able to give an accurate interpretation of upper airway function.
There are many different forms of upper airway disorders. They occur when part of the pharynx or larynx collapses into the airway, causing an obstruction to airflow. This obstruction causes turbulence to airflow, which in turn creates the abnormal noise. Observations of upper airway function during exercise enable veterinary surgeons to estimate the impact of the abnormalities with respect to race performance. Generally speaking, the more the structure collapses and the more the airway is narrowed, the greater the detrimental effect to performance. The mechanisms by which upper airway disorders affect performance are surprisingly complex, but in brief they influence the amount of air the horse can breathe in and also how hard the horse has to work to get that air into the lungs.
A full understanding of an individual horse’s upper airway function allows targeted treatments to be performed. Although the more common treatments have been included here for completeness, it is important for you to discuss individual horses with your own veterinary surgeon.
Understanding the anatomy is the first step to interpreting upper airway function during exercise. When looking at an endoscopic image, the left side of the horse is on the right side of the image as we look at it, and vice versa (figure 1).
With good upper airway function, we are looking for full abduction (which means opening) of the arytenoid cartilages while the vocal cords and aryepiglottic folds remain stable, and the epiglottis retains a curved shape; the soft palate and pharyngeal walls also remain stable. This gives a wide opening called the rima glottidis for air to enter the lungs (Figure 2 a, b, c).
Palatal instability and dorsal displacement of the soft palate
In the normal horse, the soft palate is positioned beneath the epiglottis. Palatal instability comprises billowing movement of the soft palate and often coincides with flattening of the shape of the epiglottis. The appearance of palatal instability can differ between horses (Figure 3 a, b, c). Palatal instability often causes an inspiratory noise.
Dorsal displacement of the soft palate (DDSP) occurs when the free border of the soft palate becomes displaced and comes to lie above the epiglottis (Figure 4 a, b, c). In this displaced position, there is a substantial obstruction of the rima glottidis. Sudden onset ‘gurgling’ expiratory noises are characteristic of DDSP. Palatal instability almost invariably precedes DDSP, and it is thought these conditions may arise through weakness of the muscles within the palate itself.
Thus, in younger racehorses, palatal instability and DDSP will often improve with fitness and maturity. In the UK, the two most commonly performed surgical treatments are soft palate cautery and laryngeal tie-forward. The purpose of the soft palate cautery is to induce scar tissue to tighten the soft palate. The tie-forward has a different rationale. In some horses, the larynx slips backward just prior to DDSP, therefore the tie-forward holds the larynx in a more forward position, thereby inhibiting displacement.
Arytenoid cartilage collapse
This condition is also called recurrent laryngeal neuropathy, laryngeal hemiplegia or laryngeal paralysis because it is caused by nerve damage to the muscles of the larynx. During exercise, we observe collapse of the arytenoid cartilage almost always on the left side. …
