Vet turned trainer turned college lecturer, Nick Roe looks at Roaring

After an excellent round at Spray Farm and then a sensational third in the Four-Star Adelaide International Horse Trials Kirby Park Irish Hallmark (Paddy) is certainly one of the most exciting Australian event horses at the moment. What makes Paddy even more interesting is that he was a ‘roarer’ and has had a relatively new surgical procedure done to effect a ‘cure’ for this condition.

Roaring is the common name given to a condition known as laryngeal hemiplegia and hemiparesis. The larynx is the opening of the windpipe.

Hemi- refers to one side, -plegia means paralysis and -paresis means weakness or partial paralysis. One side of the larynx, usually the left side, is partially or totally paralysed. This paralysis or paresis results in a noise as the horse breathes in that sounds like a cross between a wheeze and a roar.

This is what you see when you place an endoscope up a horses nostril and look at the larynx from front to back while the horse has a normal respiratory rate:

The black hole in the centre is the start of the windpipe (trachea). During exercise, airflow can increase by up to 20 times resting values, so that it is important that the laryngeal airways can also increase so as not to cause turbulence or airflow resistance.

This shows how the arytenoid cartilages open up the diameter of the airway during inspiration (breathing in) of a normal larynx.

This shows the appearance of a larynx where the left side is paralysed. This paralysis reduces the amount of air (oxygen) that can flow into the lungs and thereby limits the top speed that a horse can obtain.

Now plenty of eventers, showjumpers and dressage horses can be highly successful competitive horses without ever having to go at their maximum speed. So just because your horse makes a noise it does not mean that you have to have surgery done before you go to compete that horse.

Laryngeal hemiplegia is usually seen in the larger breeds (Thoroughbreds, Draught horses and Warmbloods) and in larger individuals within their breeds, and has been reported to occur to some extent, in around 3% of Thoroughbreds.

It occurs due to the paralysis/paresis of the muscle which pulls the arytenoid cartilage open (cricoarytenoideus dorsalis (CAD) muscle). The recurrent laryngeal nerve controls this muscle and it is some form of damage to this nerve, which results in roaring. The recurrent laryngeal nerve is the longest nerve in the horse (a few centimeters longer on the left side). It comes from the brain down the neck into the thoracic cavity, around some of the major blood vessels near the heart, then all the way back up the neck to the larynx. Now this is definitely not the shortest or simplest way to set up the wiring to the larynx and there seems to be no need for such a long nerve!

Damage to this nerve can occur if an irritant drug, which was meant to go into the jugular vein misses and is injected next to the vein. Guttural pouch infections can also damage the nerve as it passes through the pouch. There has been a link postulated with stringhalt as this also results from damage to a long nerve and total laryngeal paralysis has been seen in a number of stringhalt cases.

Unfortunately in the majority of cases a cause cannot be identified and these cases are labeled as idiopathic laryngeal hemiplegia.

There is a definite genetic component to this problem i.e if you mate a roarer to a roarer there is a much greater possibility of getting a roarer than if you mated a non-roarer to a non-roarer. Now the extent of this component is difficult to quantify, but it is a matter of percentages.

There are a higher percentage of roarers in some families. An interesting example of this is in the offspring of the legendary Sir Tristram there appears to be a much higher percentage of roarers.

Diagnosis of this condition is highly suspected just from the characteristic inspiratory noise heard as the pace is increased. Confirmation is obtained via the use of fibre-optic endoscopic examination of the upper airways to grade the degree of paresis/paralysis and to also exclude other conditions of the upper airways, which can also cause noisy breathing.

It is now possible to use a combination of video endoscope and high-speed treadmills to view the larynx of a horse as it is travelling at high speed. This is very interesting but these facilities are only available at a few locations.

An acceptable diagnosis can usually be achieved by blocking both nostrils for about forty seconds while the endoscope is in place, this causes the horse to increase the rate and depth of respiration and thereby make any partial paralysis more obvious. The same effect can be achieved by scoping the horse straight after a gallop, while they are still huffing and puffing.
Once we have the diagnosis you can grade the extent of paralysis/paresis from one to four, where four is total paralysis and one is very slight.

Horses with total paralysis (Grade 4) of one side of their larynx can quite happily survive a normal life as long as you don’t ask them to gallop too fast! They will start to make a noise at a fast trot or slow canter, but they will still cope at these lower speeds albeit a little noisy. The worse the paralysis obviously the slower the speed at which the problem will start to limit performance.

If you have a horse that is Gr1 or 2 and you want it to win a 1000m sprint down the straight at Caulfield in Cup Week then your chances would probably be better with another horse or you may consider surgical intervention. To win that 1000m race your little speedster would have to gallop the stretch in sub 60 seconds, whereas a three day eventer would never need to travel that stretch much under 90 seconds and would more than likely cope with being a Grade1 or 2 roarer.

Whenever we talk surgery in horses we talk about the potential risks. For whatever reason we anaesthetise a horse we take the risk that quite a bizarre range of problems can occur. From death by numerous means, both slow and expensive to quick and dramatic, to infections, founder etc etc. Both the surgeries that are available for laryngeal hemiplegia carry these inherent risks.

Laryngeal Laryngoplasty +/- Ventriculectomy

This surgery involves an incision being made either just behind the angle of the jaw or underneath the throat lash area. A strong prosthesis (i.e. 100lb nylon fishing line) is used to tie back the arytenoid cartilage in a partially open position. This position is then hopefully permanent. If it is too far open then when the horse swallows food will pass down the windpipe causing chronic coughing and discharge from the nostrils. If it is not held open far enough then the airway will still not be open enough to allow maximum speed to be obtained.

Other complications can include infection of the cartilage through which the prosthesis passes, the prosthesis can pull through the cartilage therefore requiring another surgery, and surgical wound infections and breakdown can occur.

Nerve Muscle Pedicle Graft

This procedure has been pioneered largely by Dr Ian Fulton of Ballarat Veterinary Practice. It is adapted from similar procedures in people and dogs to horses. The aim is to provide the paralysed muscle with a nerve that fires (depolarises) during inspiration. The omohyoideus muscle, part of which is situated near the larynx, contracts during inspiration. Some of the nerves, which innervate this muscle, are relocated, with a small piece of omohyoideus muscle still attached, and reattached to the paralysed CAD muscle. These nerve muscle pedicle grafts reinnervate the paralysed and atrophied CAD muscle and gradually return the muscle mass back to normal.

This process can take 6 to 18 months. At rest the larynx still appears to be paralysed but when inspiration is stimulated laryngeal function is normal.

Results so far:

• 53 horses have been treated with the nerve muscle pedicle graft, 48 Thoroughbreds, 4 Standardbreds and 1 Warmblood (Paddy). Thirty-six horses have been re-assessed, 13 are still convalescing or in training and 4 have died (one got salmonellosis after surgery, this is not a result of anything done to the throat of a horse but rather one of those bizarre things that happen to horses when you operate on them!)
• 29 have raced – 1 bowed a tendon
• 9 horses considered too slow despite having an adequate airway
• 1 horse considered a failure
• of the remaining 18, 14 winners and 4 placegetters

Average time from surgery to racing was 11 months but ranged from 6 – 21 months and in the racing world this is considered a drawback to this kind of surgery as it is a considerably longer recovery time than compared to the laryngoplasty procedure. The only complications of this procedure are those associated with any anaesthetic and surgical site complications. A disadvantage of this procedure is that it is possible for the horse to still make an inspiratory noise at the slower gaits, so that a dressage horse could still do a very noisy passage.

For dressage horses it is very unlikely that laryngeal hemiplegia effects performance, but rather is a little distracting for the judges. For racehorses grade 1 or 2 roarers are capable of winning long distance races if they are run in a way that they never exceed say 80% of their top speed.

Eventers with grade 4 hemiplegia usually require surgical intervention, as they tend to struggle once they get above say 500 metres per minute. Now the surgical procedure should then be chosen on the merits of each individual case, but remember that if you can spare 18 months (which most owners of 3 or 4 year old racehorses cannot) then if the nerve pedicle graft procedure does not work then laryngoplasty is still an option, whereas once laryngoplasty has been performed then the nerve pedicle graft is no longer an option. If time is of the essence then the laryngoplasty may be the method of choice due to the much shorter convalescence time. Remember that because your horse is a roarer this does not end his competitive career. It depends upon the degree of paresis, the intended level of performance and the way of going of that individual horse, each case should be judged on its own merits.