Neely Heidorn | 5/17/2011 11:55:10 PM
Colin Mitchell DVM
Neely Heidorn PhD
Horses are obligate nasal breathers, meaning that they cannot breathe through their mouths like small animals or people. The soft palate and epiglottis are important structures during inhalation and they form a tight seal when the horse is breathing to seal off the nasal cavity from the oral cavity. In a normal horse, the epiglottis sits on top of the soft palate and ensures that air moves directly from the horse’s nose into the trachea and on to the lungs. If this seal does not function properly, when the horse breathes out, the back edge of the soft palate will rise up and flap around, causing a respiratory noise and interfering with breathing. At the same time, the epiglottis is displaced under the soft palate, which is why this condition is call dorsal displacement of the soft palate. This can affect the horse’s ability to perform and cause a noise that is often heard as a gurgle, or choking noise as the horse is worked.
This is a condition that is intermittent, and usually only occurs when the horse has its head either in a flexed, collected position, or when it is fatigued (during hard exercise). When the palate displaces, the horse often will have problems breathing, resulting in the horse either slowing down, stopping or being unable to perform appropriately. For this reason, diagnosing this can be challenging, but resting endoscopy can be used to identify severely affected horses, whereas treadmill endoscopy or dynamic endoscopy during normal exercise may be necessary to identify some horses with this. It is important to rule out any other causes of inflammation in the upper airway, as some guttural pouch conditions can cause dorsal displacement of the soft palate as a secondary condition, which may resolve if the primary condition is treated. Not all cases will make a respiratory noise, but the decreased ability to perform is consistent between cases.
Treatment of DDSP involves both conservative and surgical approaches. Several conservative approaches exist. It is important to alleviate any inflammation by providing the horse with a period of rest and to resolve any respiratory tract infection and/or inflammation that may be present. This may include administration of antibiotics, anti-inflammatory drugs, topical throat sprays or washes and regular vaccination for influenza and rhinopneumonitis viruses. Young horses, particularly two-year-olds, may need to be laid off from work and even turned out for a few weeks to months to allow the inflammation to subside and allow them time to mature. If the horse is not physically fit, it is important to condition the horse so that his level of fitness is improved. Two other common conservative options are the application of a tongue-tie and/or a figure-eight noseband. Both of these methods are believed to be effective because they help to counteract the caudal retractile forces that are believed to contribute to DDSP. It is important to ensure proper application of these devices to increase their effectiveness. Other options include switching bits to one that aids in holding the tongue down and in place, and altering the horse’s headset. Each of these options may improve 50-60% of the horses with DDSP. More recently a collar has been designed that will keep pressure on the larynx to prevent it from moving caudally. This has been helpful to reduce the occurrence of dorsal displacement, although may not be legal in all states in race horses.
Numerous surgical treatment options are available. A staphylectomy (trimming the palate) is a procedure in which the back edge of the soft palate is surgically altered. It is not known for sure why this technique works, but in general it is believed that scar tissue is formed which stiffens the back border of the palate and gives it more rigidity, thus making it more difficult for displacement to occur. Several different methods for performing a staphylectomy exist, including using the use of a laser. Regardless of the technique, approximately 60% of horses seem to respond. Another surgical treatment option is a myectomy (removing a section of muscle) of the sternothyrohyoideus muscles (strap muscles in the neck), which involves the removal of a portion of muscle that results in the inability to retract the larynx. Depending upon which muscles are removed, this may be done standing with sedation and local anesthesia or may require general anesthesia to be used. After surgery, anti-inflammatory medications and antibiotics are often administered for several days. The horse will often need to be kept in a stall, with the neck bandaged, and hand walked daily for a period of two weeks. After the two-week recovery period and suture removal, training can often be resumed. A success rate of approximately 60% is associated with a myectomy. A more recently described treatment is to perform a tenectomy, which involves removal of a portion of the tendon of insertion of the muscle. This technique serves the same general function as the myectomy, but has fewer complications and requires less time for convalescence.
The most beneficial surgical treatment to date is a tie forward. In this technique, the horse is anesthetized and permanent suture is placed around the larynx and around the bones of the hyoid apparatus to bring the base of the tongue and larynx closer together. This moves the epiglottis closer to the back edge of the soft palate resulting in a tighter seal. This surgery has the best prognosis for a successful return to competition and alleviation of this problem.
The fact that there are many options available for the treatment of DDSP indicate that we do not completely understand the cause of this disease. Depending on the age of the horse and many other factors, it is probably best to proceed with a conservative approach first, and if proven unsuccessful, then a surgical approach should be considered. The tie forward has proven to be the most beneficial to date, although it does have a 3 week rest period postoperatively before the horse can return to competition.