The Equine Larynx – on a Knife Edge!

Men have been interfering with the equine larynx for centuries, but so far with only limited success. When a horse is heard to be making a noise for the first time, it is of serious concern. Sometimes the concern is only short lived as the horse may be unfit, have a mild respiratory infection or perhaps a sore throat. However, on other occasions the equine athlete in question is on the verge of being diagnosed with a problem that will limit its performance for the rest of its life.

The equine athlete is anatomically designed on a knife edge in so many ways. Firstly, rather than having five digits like a human, the horse is precariously balanced on the equivalent of our middle finger. Add to this the obscure meandering anatomy of the horse’s gut leading to regular occurrences of painful and life-threatening colic episodes, and it is easy to get a sense of just how the thoroughbred has been built for athletic ability rather than soundness – the horse’s respiratory system is no exception. The horse has a massive, powerful cardio-respiratory system but unfortunately air is inhaled and exhaled through a small unreliable larynx and a rather narrow complex nasal system, especially considering that the horse is an obligate nasal breather and thus does not receive any air through its mouth. It is for this reason that any abnormality in the upper respiratory tract of the horse causes a reduction in the amount of oxygen it receives. Clearly, the result of this is an adverse effect on performance.
When faced with a horse that makes a respiratory noise we have a few diagnostic tools at our disposal. Firstly, and perhaps most importantly, we must analyze the noise that the horse is making at exercise. Is the noise inspiratory (when the horse is breathing in) or expiratory (when the horse is breathing out), or are there both excess inspiratory and expiratory breathing sounds? Also, the noise must be accurately described as certain noises are characteristic of certain abnormalities. For example, an inspiratory ‘whistle’ or ‘roar’ made all the way up the canter often indicates laryngeal hemiplegia (paralysis of the left side of the larynx), whereas an expiratory ‘gurgling’ or ‘choking’ sound whilst the horse is at peak exercise or pulling up at the top of the canter usually indicates dorsal displacement of the soft palate.
Young, unfit horses coming into training for the first time often sound ‘thick’ in their wind and can also make an expiratory gurgle when pulling up at the top of the gallop, especially if they have a sore throat (pharyngitis). This condition is essentially inflammation of the pharynx characterized by enlarged white spots (lymphoid follicular hyperplasia). It is a condition that is easily diagnosed by endoscopic examination and will affect almost all horses at some stage and is present in nearly one hundred percent of horses in training under two years of age. The exact cause is unknown but it is probably initiated by challenge to the young horse’s immune system. It is not a serious condition and it usually self-resolves with time. However, when it is causing problems, various treatments may be attempted including anti-inflammatories, antibiotics and immuno-stimulants.
Endoscopy is a crucial diagnostic aid; however, it can have its limitations when carried out in a horse at rest. If the horse has a respiratory infection, pharyngitis or an obviously paralyzed larynx then endoscopy is an excellent diagnostic aid, but in other cases scoping a horse at rest can provide little in the way of information as to why the horse is making such a noise. For this reason, equine veterinary medicine has looked to more advanced technology for assistance. The idea of ‘scoping’ horses on a treadmill whilst galloping came first. Whilst this certainly has obvious merits it does come with some downsides such as the question of whether a treadmill truly represents an equivocal test to a gallop or race and the surface on which the horse has to gallop. In fact, many of the treadmills around the country are currently not in use as too many injuries have occurred. There is now a new idea of fixing a scope in the horse’s nostril, which stays in place whilst the horse canters or gallops. It transmits a signal that can be viewed on a monitor and so we could see exactly what the horse’s larynx was doing as it makes the noise. As yet only a prototype of this ‘over-ground’ endoscope exists but could this be the future of accurate diagnosis of equine wind problems?
By far the most common condition that causes an abnormal inspiratory sound, and possibly the most common cause of any abnormal respiratory sound in the thoroughbred racehorse, is idiopathic left laryngeal hemiplegia (paralysis of the left side of the larynx). This condition is caused by degeneration of the nerve that supplies the left side of the larynx so that that it ‘hangs’ into midline causing an inspiratory ‘whistling’ or ‘roaring’ sound during cantering or galloping and thus obstructing airflow to the lungs. The cause of this nervous degeneration is not known but this again leads me onto yet another poor anatomical design point of the horse. The right laryngeal nerve has a simple route, branching off from the vagus nerve (which comes from the brain) travelling directly to the larynx. However, God decided that the left laryngeal nerve shouldn’t have it so easy and instead it must travel all the way to the heart, where it wraps around a large pulsing artery, before coming all the way back to the larynx. The left laryngeal nerve is also the longest nerve in the body and so it stands to reason that it is commonly damaged and perhaps unsurprisingly, there is also data to suggest that the bigger the horse, the greater its chance of developing laryngeal hemiplegia.
This disorder is not desirable for a number of reasons, not least the fact that it is a progressive disease and hence a small problem in a two-year-old can rapidly become a huge problem in a three-year-old. Nevertheless, surgical treatment is commonly attempted and there are three main operations. A ‘Hobday’ operation refers to the removal of a large portion of the left side of the larynx and thus theoretically reduces the amount of respiratory obstruction. However, many veterinary surgeons argue that although this may alleviate the noise (as the left vocal cord has been removed) it struggles to reduce the obstruction significantly and hence they prefer the ‘tie-back’ operation. Here, the larynx is permanently tied open and so the obstruction should be alleviated. However, things are never so simple in wind surgery and occasionally the larynx can end up in a mess if things do not go well, for example, the stitch breaks down. Hence, the last resort is to insert a permanent metal tube into the horse’s throat through which it can breathe, by bypassing the larynx altogether. This can also be very messy and it is not easy to keep the tube clean, however, Party Politics did win a Grand National with a tube in his windpipe!
Perhaps the most common cause of an expiratory ‘gurgling’ sound is dorsal displacement of the soft palate. During normal breathing, the soft palate sits in front of the larynx just below the epiglottis allowing maximal airflow through the larynx. During eating on the other hand, the soft palate rises above the larynx, directing food into the food pipe rather than the windpipe. What happens in this condition is that the soft palate rises up during exercise thus blocking airflow and often causing an expiratory gurgling or choking sound. Although the clinical signs of this problem are quite characteristic, confirmation of the diagnosis can be difficult as the larynx often looks normal at rest and thus the use of a treadmill or over-ground endoscope may be necessary for an absolute diagnosis.
There are many possible treatments for soft palate displacement, probably because none of them are one hundred percent effective. Starting with the simple solutions, if there is respiratory infection, it should be treated. Next, if the horse is unfit, it should be trained further before considering anything more radical. Then various items of tack can be tried – these include a cross-noseband, a tongue-tie, a spoon-bit, a ring-bit or an Australian noseband. If none of these treatments works then surgery is often attempted. There are a number of possible operations but two are more commonly carried out than the rest – soft palate cautery and the ‘tie-forward’ operation. This is because most soft palate operations are approximately 60% effective; therefore the easiest operation with the shortest layoff is usually tried first. The soft palate can be cauterized with a hot iron to make the palate firmer so that it does not displace during breathing. This may sound a little unsophisticated and slightly barbaric but it is very easy to do, it hardly interrupts the horse’s training and it can make a large difference in some horses, although it often has to be repeated. The second most commonly carried out operation, the ‘tie-forward’, tackles the problem from a different angle. Here, the larynx is manually tied forward with steel stitches, which reduces the amount of soft palate that is available to rise up and block the airway. Some horses have performed much better after such an operation and examples include Royal Auclair, who had his best season following the surgery culminating in finishing fourth in the Cheltenham Gold Cup and second in the Aintree Grand National.
There is a piece of tack that acts in a similar way to the tie-forward operation called the ‘Cornell Collar’ or throat support device. Researchers at Cornell University in the state of New York believe that a deficit in one particular muscle contributes to soft palate displacement and the device intends to mimic the effect of this muscle.
However, although it is in use in some American states, Canada, Australia and Hong Kong, it is banned by most racing authorities including most of Europe. There may be many reasons for this but perhaps the main one is the possibility of cheating as unlike an operation the tack is not permanent and so it could be fitted correctly one day and deliberately incorrectly another day.
Another common upper respiratory condition is epiglottic entrapment or aryepiglottic fold entrapment as it is sometimes known. The epiglottis is the tongue-like structure that should sit in front of the larynx. However, the epiglottis can become enveloped by a mucosal fold and so it becomes trapped in front of the larynx causing a partial obstruction. This usually results in a gurgling or choking sound that may be inspiratory or expiratory. The cause is not completely understood but diagnosis can be made relatively easily at rest if the horse has an ulcerated epiglottis representing the regularity with which the horse entraps its epiglottis, or alternatively a treadmill or over-ground scope could be used to visualize the horse entrapping at exercise. Treatment again involves checking for infection and using different tack, however, surgery can often be successful, at least in the short term, by cutting the mucosal fold and thereby preventing the epiglottis from becoming entrapped.
No discussion of equine wind problems would be complete without at least touching on respiratory infections. Respiratory infections can predispose horses to many of the conditions mentioned above but they can also target the larynx itself. Such laryngeal infections must be treated quickly and aggressively as any scarring or permanent damage to these important structures can leave the horse with a significant problem for the rest of
its life. The cause of laryngeal infections is not fully understood. Some have suggested that kick-back may cause damage to the horse’s larynx, which then becomes infected. However, if this were true then we should expect an increased incidence of laryngeal infections associated with dirt racing due to the large amount of kick-back, an idea that has no statistical evidence to support it.
In summary, the horse’s larynx is a complex topic and I have only succeeded in scraping the surface of a very large subject. There are essentially two major obstacles that so often cause us to fail in its treatment. Firstly, we are not always certain about a horse’s specific problem as we cannot scope it in the final furlong of a race. Secondly, even when we know what the problem is, the area is so delicate and there is so little margin for error that surgery fails to improve equine wind issues with alarming regularity.

James Tate BVMS MRCVS 
(14 October 2008 - Issue 10)