Dorsal Displacement of the Soft Palate
Dr. Patricia Provost
When a horse makes an unusual noise associated with respiration during exercise, you should always take it seriously. Some noises indicate a self-limiting problem. However, noises can also indicate that a more significant airway obstruction, like constriction or narrowing, is developing, possibly even requiring surgery. Exercise intolerance
Noises, described as blowing, whistling, roaring, gurgling, and fluttering are created by turbulent airflow, which results from a change in the shape of the internal airways, sometimes very slight. Abnormalities in just about any structure of the nose, throat, or sinuses can cause noise and exercise intolerance. Only a thorough examination including endoscopy can sort them out.
Dorsal Displacement of the Soft Palate
Dorsal displacement of the soft palate (DDSP) is the inappropriate positioning of the soft palate above the epiglottis during breathing.
The soft palate is an extension of the hard palate and both serve as a barrier between the nasal passages and the mouth. It is not rigid and moves upward ("dorsal") when horses swallow to form a channel between the mouth and esophagus. This prevents food and water from entering the nasal passages. When horses are not swallowing the soft palate should remain beneath the epiglottis. This streamlines air movement from the nose to the windpipe (trachea) and back.
There are two basic forms of DDSP. In the first type, dorsal displacement of the soft palate is present continuously. The horse appears unable to ever position the soft palate beneath the epiglottis, as in a normal horse. These horses may have other symptoms including, but not limited to, inability to swallow, coughing when eating and abnormal respiratory noise during slow exercise.
In the second form, the soft palate displaces dorsally only during exercise, or intermittently This displacement usually occurs abruptly and without warning. This is the most common form of DDSP and it occurs primarily in racehorses (thoroughbreds and standardbreds) at race speeds.
The inappropriate dorsal position of the soft palate during exercise significantly reduces the diameter of the airway and can triple the effort that it takes the horse to breathe.
The position of displaced soft palate in the airway results in a characteristic gurgling or flutter respiratory noise that is loudest on expiration. The horses are said to "choke-up" or "swallow the tongue," and since the horse cannot breathe through his mouth, until he swallows a few times and replaces the soft palate, he cannot breathe. There are, however, some horses that displace their soft palate that do not produce an abnormal noise.
To date there is no known single cause of DDSP. Inflammation of the airway, small epiglottic size, and inflammation of or trauma to the nerves and muscles responsible for soft palate position or movement have all been proposed as possible causes.
What to Watch For
Sudden respiratory noise and simultaneous loss of exercise tolerance
All racehorses that have a history of poor performance in the absence of lameness should be examined for DDSP. It should also be considered in all horses, regardless of their occupation, if the horse has a history of exercise intolerance and produces an abnormal respiratory noise during exercise.
Treadmill endoscopic exam. Direct visualization of the upper airway using a fiber optic endoscope that is passed up the horse's nose is necessary to evaluate the position of the soft palate. Exercising the horse on a treadmill during the endoscopic examination is the most definitive method to make the diagnosis and is the best way to identify those horses that displace the soft palate only during exercise.
Closed nostril endoscopic exam. In the absence of a treadmill endoscopic examination, the horse can be evaluated using the endoscope while holding his nostrils closed. As the horse increases his effort to breathe in air, affected horses will sometimes displace the soft palate and begin to breathe through their mouths.
A radiograph (X-ray) of the area may also be taken to confirm the size of the epiglottis. A small epiglottis may predispose to DDSP.
For those horses that have continuous DDSP, endoscopic examination of the airway will be performed (including the guttural pouches) as well as a complete neurologic evaluation. Radiographs may be taken to check for the presence of a foreign body, bone fracture or infection, and abnormal epiglottis position and function.
Recently, sound recordings of the upper airways made during routine training or treadmill exercise, have been used for diagnosis of DDSP versus other upper airway obstructions. DDSP can be distinguished on the basis of increased levels of specific sound frequencies, and greater noise production during exhalation. This method can be helpful since DDSP can be difficult to reproduce in some horses with intermittent signs, and audible sounds cannot always be detected with the human ear.
Use of a tongue tie and head halter. These conservative methods function to keep the tongue pulled forward and the mouth closed. The tongue is pulled forward and tied in place to the lower jaw of the horse using a soft cotton, nylon or leather band. Because of its attachments, pulling the tongue forward also bring the epiglottis forward. This creates a more secure positioning of the epiglottis above the soft palate. Using a head halter, which is pulled tightly, will keep the horse from opening its mouth and pulling its tongue further back into the mouth, a movement that sometimes causes DDSP.
Staphylectomy. This is a surgical procedure in which a crescent shaped piece of tissue is removed from the free border of the soft palate. Removal of the edge of the soft palate shortens the soft palate and therefore reduces the amount of soft palate that obstructs airflow. As it heals, the cut edge may also become stiffer and more resistant to abnormal displacement.
Myectomy. In this surgical procedure a section of the paired throat muscles are removed via an incision in the ventral mid-neck region. These act in part to retract the larynx and may predispose to displacement of the soft palate. Removal prevents the retraction.
Llewellyn procedure. In essence this is the same procedure as the myectomy only it is performed at the level of the larynx. The tendon of insertion for both throat muscles are severed along with removal of a small section of other muscles. The procedure is often performed in conjunction with the staphylectomy procedure.
Epiglottic augmentation. This procedure can be performed in horses with confirmed epiglottic hypoplasia. A synthetic paste is injected beneath the epiglottis to stiffen it. The increase in stiffness helps to prevent displacement of the soft palate.
Home care is tailored for each specific surgical procedure. Horse's undergoing a staphylectomy are generally treated initially with antibiotics and pain medications along with rest. Stall rest with hand walking is prescribed for the first 3 to 4 weeks, then a gradual return to regular exercise.
Horses undergoing a myectomy procedure are also treated with antibiotics, pain medications and a period of rest. Regular exercise is generally resumed in two weeks.
Success rates for the treatments range from 50 to 60 percent.
As there is some concern that infection of the upper airway may lead to abnormal function of the nerves and muscles responsible for airway function, proper treatment and adequate rest should be provided for horses with upper airway infections. This may reduce the incidence of soft palate displacement. There is no known prevention for dorsal displacement of the soft palate.