Within the upper airway of the horse there are a number of cartilages and soft tissues making up the opening of the windpipe or trachea which, combined, are called the larynx. It’s a highly dynamic and intricate structure, which opens and closes with respective breathing and swallowing of the horse.
An exercising horse has huge oxygen requirements to energise the muscle contractions needed to move such weight at high speed. With this in mind, any obstructions or narrowing of the opening of the airway can limit the oxygen intake and therefore limit performance. A look at the anatomy of the larynx can give some idea of the range of problems that can arise.
The arytenoid cartilages (1) create the upper archway of the larynx. These should open widely and symmetrically at exercise to allow maximum air flow. The sides of the opening are made up of aryepiglottic folds (2) with the vocal cords (3) just inside. The base or floor is the epiglottis (4); a triangular shaped cartilage sitting over the soft palate. This flaps up over the opening of the windpipe, along with collapsing of the arytenoid cartilages to close the trachea when swallowing occurs, preventing inhalation of food into the lungs.
Any of these structures in isolation or combination can behave abnormally at exercise and reduce the diameter of the airway. Thereby creating an obstruction to the airflow which can be audible as the horse breathes at exercise.
If we look at the arytenoid cartilages first, these have the biggest of influence over the size of the upper airway. The cartilages are controlled by muscles above them contracting and pulling them in an upwards and outwards direction. These muscles are innervated by the recurrent laryngeal nerve which, almost exclusively on the left hand side, is prone to damage for a number of known and unknown reasons. Damage to this nerve then causes atrophy or weakness in the muscle leading to the reduction in the diameter of the airway. Airflow turbulence around the collapsed cartilage is often audible and sounds like a whistle or a roar on inspiration. This is known as laryngeal hemiplegia or recurrent laryngeal neuropathy. Depending on the severity of the condition, the larynx can be assigned a grade, from a perfectly symmetrical grade 1 which is able to open fully and hold the airway open, to a grade 5 where there is no movement and complete collapse of the left arytenoid.
The folds and cords at the sides of the larynx can also display problems. As air is sucked into the lungs, the inspiratory pressure can cause the aryepiglottic folds and/or vocal cords to diverge inwards, again reducing the diameter of the airway. While the diameter reduction is usually less than with the arytenoid cartilage collapse, it can still be performance limiting and often cause a similar inspiratory whistle to the laryngeal hemiplegia described above.
The epiglottis sits above the soft palate during exercise. Negative inspiratory pressure which occurs to suck air into the lungs, can cause the soft palate to billow up. This can progress and the soft palate can flip over the top of the epiglottis covering the airway and an audible gurgle is often heard. This is often associated with a dramatic reduction in speed as the horse swallows to try to ‘flip his palate back’ to continue to breath normally. This is known as dorsal displacement of the soft palate or DDSP.
While the anatomy and structure of the larynx can be assessed easily on resting endoscope, many of these problems are only seen at exercise. A horse that displaces its palate at rest can be entirely normal at exercise, likewise, a horse who has a normal looking arytenoid cartilage at rest, can have complete collapse of the left cartilage at exercise. While resting endoscopy can give useful clues, a definite diagnosis will often require dynamic endoscopy with the horse wearing an endoscope during exercise, recording a true picture of what the larynx is actually doing at high intensities of work.
It is not unusual for horses who are required to exercise at maximal intensity, mainly racehorses and eventers, to undergo investigations for upper airway abnormalities. Surgeries, prognosis and recovery times are dependent on the problem found. They vary from standing surgeries such as laser cautery of the soft palate, vocal cords or aryepiglottic folds, to surgeries completed under general anesthesia such as tie back or tie forward procedures.
Advances in diagnostics, treatments and surgeries have improved our understanding and outcomes when it comes to problems of the upper airways. However, they can remain challenging cases.