Dorsal Displacement of the Soft Palate: Winning With Air
As the filly surged forward to hold off the challenger from behind, the jockey noticed she started making a funny noise, like she was choking, then she just "ran out of steam." She dropped from first place to fourth place in two strides.
Horses with dorsal displacement of the soft palate (DDSP) characteristically make a "choking" noise as they slow down from the resulting partial airway obstruction, hence the term "choking down" to describe them.
The above description is typical of a horse which has displaced her soft palate during a race. Right when the horse needs a last burst of energy, it appears as if someone turned off a switch. Horses with dorsal displacement of the soft palate (DDSP) characteristically make a "choking" noise as they slow down from the resulting partial airway obstruction, hence the term "choking down" to describe them.
Dorsal displacement of the soft palate is a disorder of the upper airway in horses. It is one of the most common upper airway obstructions, and it causes poor performance and abnormal noise while exercising. It's usually a disease of racehorses, for which it is often a performance-limiting problem. Over the years, a number of treatments have been developed for this condition; however, none are universally successful, mostly because we do not as yet fully understand the exact cause(s) of this disorder. But before we can further discuss this abnormality, you first need to understand what is normal for the equine upper airway.
Normal Upper Airway Anatomy
To understand DDSP, you must be familiar with the anatomy of the naso-pharynx and larynx of the horse. The nasopharynx is one part of the pharynx, which is divided into the nasopharynx and oropharynx. The nasopharynx is that section of the horse's head that lies caudal to (behind) the nasal passages and is rostral to (in front of) the larynx (voice box). Remember, the nasal passages sit directly above the oral cavity and hard palate. The nasopharynx is composed mostly of muscle tissue, while the soft palate is a direct extension of the hard palate and forms the floor of the nasopharynx. This is the partition between the mouth and the nasal cavity. The back edge of the soft palate (caudal free edge) is only attached on the sides of the nasopharynx, but extends back to connect with the larynx.
The larynx is essentially a conduit between the pharynx and the trachea (windpipe) of the horse. It is composed of cartilage and muscle tissue. The most easily recognizable structures of the larynx are the epiglottis and the arytenoid cartilages. The epiglottis is a triangular-shaped structure with smooth, scalloped edges and normally lies just above the soft palate. The arytenoid cartilages, often called the "flappers," are paired structures that lie above the epiglottis and form the opening to the trachea. There are many other structures that work together for the upper airway and larynx to function normally, but it is beyond the scope of this article to discuss all of them here.
What's the Function?
Besides joining the nasal passages with the trachea, the soft palate and larynx serve several functions in the horse; two of the most important involve swallowing and breathing. The arytenoid cartilages and epiglottis are very important during swallowing, as they coordinate to protect the trachea from a food or water bolus being inhaled instead of swallowed. During swallowing, the arytenoid cartilages close and the epiglottis flips upward to protect the opening of the trachea.
The soft palate and larynx also have a coordinated function during strenuous exercise. During exercise, the nasopharynx dilates and the arytenoid cartilages open maximally to accommodate as much airflow as possible.
What Went Wrong?
When DDSP occurs, the epiglottis--which normally lies on top of the soft palate--becomes displaced below the soft palate. If this happens when a horse is at racing speed, breathing very fast, then this creates a partial airway obstruction. On expiration (as the horse breathes out), some air goes underneath the now-exposed soft palate and it billows upward (like a sail) just in front of the opening to the trachea and blocks the airway.
Racehorses need maximal oxygen to keep up racing speed, so this decrease in airflow gives them no choice but to put on the brakes. After they slow down, they usually can swallow, allowing the soft palate and epiglottis to go back into their normal anatomical positions and correct the problem themselves. But by that time, the race is over. This scenario is called intermittent DDSP.
Some horses are affected by persistent DDSP--their soft palates are displaced all the time. Swallowing does not correct the problem. Although these horses can breathe normally at rest, they cannot exercise at speed. Many of these horses also have problems swallowing food and/or water correctly (dysphagia).
Since the soft palate and epiglottis also help direct food boluses, when they are not in their normal positions, they cannot protect the opening of the trachea. Therefore, theses horses sometimes inhale food and water while eating, leading to coughing, food and water being expelled through the nostrils, and aspiration pneumonia. This is obviously a much more serious condition, as it can be life-threatening.
What Causes DDSP?
There are several suspected causes for DDSP, but there's no one unifying theory. Intermittent DDSP, which occurs during racing, is thought to be caused by a combination of factors, such as laryngeal inflammation (especially in young horses), excessive backward movement of the larynx, and/or opening of the mouth during racing. Other causes include malformations or problems with the epiglottis.
Persistent DDSP is thought to be caused by severe inflammation around the horse's larynx--more specifically, inflammation that involves the nerves that control movement of the soft palate and larynx, such as fungal or bacterial infections of the guttural pouch.
Intermittent DDSP can be difficult to diagnose because it only occurs when the horse is exercising at strenuous levels. The first step is to have a physical examination performed by a veterinarian to rule out other causes of exercise intolerance. Then, a standing endoscopic examination of the upper airway can be performed to rule out any structural abnormalities of the upper airway. During the examination, many horses will displace their soft palates, then quickly swallow and replace them. This is normal, and it does not prove that DDSP is the reason for poor performance or noise made during exercise.
Persistent DDSP is easily diagnosed with standing endoscopy of the upper airway. In these cases, the epiglottis, which should be readily visible during endoscopy, is hidden underneath the soft palate (see images at left). If the upper airway is normal, then an endoscopic examination can be performed on the horse while he is exercising on a treadmill. Before the advent of high-speed treadmill endoscopy, DDSP was overestimated as a cause of poor performance in racehorses due to a lack of endoscopic findings.
Treatments--Medical vs. Surgical
Initial treatment for intermittent DDSP often involves equipment changes and a conservative approach. Tack such as a figure-8 noseband, tongue-tie, or bits that help keep the tongue in place are often used to prevent a horse from displacing his soft palate during exercise. Also, since DDSP has been associated with inflammatory conditions of the upper airway, treatment with anti-inflammatory agents is often successful in cases of intermittent DDSP.
Treatment for persistent DDSP will depend on the cause. For example, a horse with persistent DDSP from a bacterial infection of the guttural pouch would primarily be treated for the infection and to control inflammation. Once the infection and inflammation are under control, the normal function of the soft palate and larynx will usually return.
For horses in which conservative/medical therapy and/or tack changes have not helped, then there are several surgical options.
Myectomy--A myectomy is a surgical transection (cutting) of a muscle. For DDSP, a myectomy of the sternohyoideus, sternothyroideus, and possibly the omohyoideus muscles (often referred to as the "strap" muscles of the neck) are transected. Depending on which muscles are going to be cut, the procedure can be done in a standing, sedated horse with local anesthesia, or it can be done under general anesthesia. The theory behind this procedure is that these muscles might add to the problem of excessive backward retraction of the larynx, thus allowing DDSP to occur. Like many of the surgical procedures to correct DDSP, it has a success rate of about 60%.
Staphylectomy--A staphylectomy is a surgical procedure to remove a small amount of the back free edge of the soft palate. This procedure is performed in an attempt to stiffen the soft palate and make soft palate displacements less likely. It is performed under general anesthesia by removing a very small amount of the soft palate. To gain access to the palate, the larynx must be opened (laryngotomy). Once the soft palate tissue is cut, a scar forms, tightening the palate. The skin incision is left to heal by second intention (the skin is not closed with suture material) and the horses can return to work in 10 days to two weeks.
This procedure is very often coupled with a myectomy or tenectomy (cutting of the sternothyroideus tendon), but one complication of the procedure is persistent DDSP if too much of the soft palate is removed. The success rate for this procedure(s) is approximately 60%.
An alternative to the surgical staphylectomy was described by Patricia Hogan, VMD, Dipl. ACVS, of the New Jersey Equine Clinic, at the American Association of Equine Practitioners Convention in December of 2002. This procedure can be performed in the standing horse using a laser to cauterize the free edge of the soft palate. Using the laser technique does not require general anesthesia; however, the procedure is often combined with a sternothyroid tenectomy (removal of a portion of a tendon that is involved with control of the epiglottis and larynx), which does require general anesthesia. The benefit of the laser cautery is the larynx does not have to be opened (laryngotomy), so the aftercare is minimal and horses can return to exercise very quickly. Using Hogan's described technique, there was a 92% improvement in the performance of the 52 horses on which the procedure was performed.
Epiglottic augmentation--This procedure is performed under general anesthesia via a laryngotomy (an incision made into the larynx). The procedure is used for those cases of DDSP where the epiglottis appears underdeveloped or weak during high-intensity exercise. This might predispose horses to intermittent DDSP, so augmenting the epiglottis is used to correct the problem. There are a few products that can be used, but most commonly a Teflon paste is injected underneath the mucosa (lining) on the underside of the epiglottis in small rows. This helps thicken and stiffen the epiglottis and improves performance in horses affected this way in approximately 66% of the cases.
Because of the complexity of DDSP, researchers are trying new methods to treat this disorder. Veterinary researchers at Cornell University are using a new collar device christened the "Cornell DDSP Collar." The research group, headed by Norm Ducharme, DVM, MSc, Dipl. ACVS, of Cornell University, found that by stabilizing the larynx and hyoid bone, they could prevent intermittent DDSP.
This collar can be worn during exercise and has potential as a training tool for horses to teach them how to hold their larynx in a position that will not allow DDSP to occur. Researchers are excited about the collar and hope that it can decrease, if not eliminate, the need for surgery on these horses. In a laboratory setting, the device performed well. Field studies (with many more variables) are now underway.
Decreased performance can have many causes, but when combined with a "choking" noise at peak exercise, it could be DDSP. This is not a career-ending problem, but will require management and/or treatment in order to allow the horse to perform to his top ability. If your horse is making noises at peak exercise or is not performing up to par, have your veterinarian do an upper respiratory exam. Treatment of the problem(s) discovered might make your horse a winner!
Holcombe, S.; Ducharme, N. The Pharynx. Equine Surgery, Second Edition Philadelphia: W. B. Saunders Co., 337-348, 1999.
Stick J.; Tulleners, E.; Robertson, J.; et al. The Larynx. Equine Surgery, Second Edition Philadelphia: W. B. Saunders Co., 249-366, 1999.
Robertson, J.; Copelan, W. Surgical Treatment of the Dorsal Displacement of the Soft Palate. The Veterinary Clinics of North America, Equine Practice. Philadelphia: W. B. Saunders Co., 201-205, 1990.
Hogan, P.; Palmer, S.; Congelosi, M. Transedoscopic Laser Cauterization of the Soft Palate as an Adjunctive Treatment for Dorsal Displacement in the Racehorse. Proceedings of the 48th Annual Convention of the American Association of Equine Practitioners. Lexington: American Association of Equine Practitioners, 228-230, 2002.
About the Author
Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.
POLL: University Equine Hospitals