Dr. Normand G. Ducharme and Dr. J. Brett Woodie
Although we know some young racehorses experience dorsal displacement of the soft palate (DDSP) due to inflammation of the throat area or lack of fitness, most of the time we don't know why horses displace and choke down. This common problem in racehorses first was described in 1949. Originally, veterinarians thought the palate itself was abnormal—that the palate was weak or elongated. This perspective led to surgical treatments such as removing a small portion of the palate. The success rate of this treatment is only 60%, with chronic coughing as an uncommon major complication (some horses aspirate water and feed into the windpipe after this surgery). The next treatment approach was based on the assumption that palate displacement was due to a problem with the larynx (voice box): it is the voice box's fault! In response to this belief, the popular treatment in the late 1970s and 1980s consisted of cutting the strap muscles on the neck's underside to prevent the voice box from pulling back and away from the palate. This procedure also had a 60% o success rate with fewer serious complications. Indeed, the most common complication was incisional infection, which was relatively easy to treat. In the last decade, it became fashionable to do both procedures at the same time, based on the assumption that one or the other would have a beneficial effect. Dr. Llewellyn cleverly modified the combined procedure to decrease complications, so it has become the current "in vogue" treatment. However, the Llewellyn procedure is also associated with a 60% success rate.
Recently, measuring airway pressures in exercising horses after strap-muscle resection (previously thought to be a harmless procedure) revealed that it caused a respiratory obstruction in normal horses; so it might not be so harmless after all. It is clear we need a better understanding of this disease. During the last decade, we have performed many studies concerning this problem. In a recent study, we noted that one throat muscle's function (the thyrohyoid muscle) decreased immediately before naturally occurring palate displacement in one horse. In a following study, we were able to reproduce palate displacement in 4 out of 5 horses by transecting 4 muscles in the throatlatch area, including the thyrohyoid-muscle. In a study just completed, we were able to reproduce palate displacement in 7 out of 10 horses by transecting the thyrohyoid muscle in the throatlatch area. In the same study, we were able to correct palate displacement through a surgical procedure that restored the cut thyrohyoid muscle's function. It now appears that palate displacement is due to inadequate control by muscles in the throatlatch area, so an abnormal position and tension between the palate and larynx results, but neither the palate nor larynx (voice box) is at fault. In some of the horses, we experimented by manually holding their voice box while they were running on the treadmill, and we were able to prevent palate displacement. Using our hand to prevent palate displacement was a stunning finding, and we believe a specially designed "Cornell DDSP Collar" should be able to perform the same function. We have already designed one collar to use during exercise, but it compresses the trachea (wind pipe) to an unacceptable degree. This proposal would enable us to perfect the collar's design so it prevents DDSP without compressing the trachea (without upper airway mechanical obstruction).
This study would use normal horses to test various collar modifications to ensure that: 1) it helps prevent DDSP, 2) it does not cause any airway obstruction, 3) the fit is appropriate, and 4) it is easy to place. We will cut the thyrohyoid muscle, which we identified as most important for preventing DDSP, to create DDSP in the study's horses. The horses then will be used to test how effectively the various collar-design modifications prevent DDSP. The best matches of effective prevention of DDSP, absence of airway obstruction, ease of application, quality of fit (comfort) and any other considerations that may arise will be used to select the best collar for final testing in a clinical population of horses with the naturally occurring disease.
If successful, this collar would have widespread application for the racing industry, because nearly 90% of racehorses wear tongue-ties at least partly, to stabilize their upper airway. This collar could become widely used during training and racing to prevent palate displacement. Conceivably, the collar could be used to "teach" horses where to place their voice box during exercise training. The collar could play an important role in identifying horses that are the best candidates for surgery, whether or not its use during racing is accepted by the racing and wagering boards. Therefore, we feel a relatively simple, low-cost tack-change design that might have a major impact on the industry is very close to completion.