Dorsal Displacement of the
Soft Palate in Horses
Stacy Brown
Veterinary Student
Dr. Jeremy D. Hubert
Assistant Professor of Equine Surgery
Equine Health Studies Program
School of Veterinary Medicine
Louisiana State University
Baton Rouge, LA 70803
Although
horses can breathe through their mouth, they differ from people in that they
are much more efficient at moving air through their nasal passages and
nostrils. This results in them being unable to breathe sufficiently through
their mouth to sustain exercise. The epiglottis, a relatively rigid structure
in the back of the throat, is positioned above the back edge of the soft
palate, which is an extension of the hard palate (roof or mouth) and serves to
separate the nasal and oral cavities (Figure 1). This anatomical arrangement
helps assure that the air is directed into the trachea (windpipe).
However, during eating and swallowing, the soft palate moves upward as the
epiglottis flips backward to cover the entry to the trachea. This shift in
the position of the epiglottis occurs so that food and saliva are directed into
the esophagus and not into the trachea. Dorsal displacement of the soft
palate (DDSP) is a condition whereby the epiglottis becomes positioned above
the soft palate (Figure 2). Swallowing should replace the epiglottis to
its normal position; however, if this does not occur then a tentative diagnosis
of DDSP is provided.
DDSP may be
intermittent (the most common) or persistent. With intermittent
displacement, the horse is able to replace the soft palate when
swallowing. When a horse is persistently displaced, the displacement is
not corrected when the horse swallows. Because the displacement is not
corrected with swallowing in horses with permanent DDSP, these horses are not
capable of covering the opening of their trachea during eating, which may lead
to coughing and ultimately aspiration pneumonia.
DDSP most
commonly occurs in racehorses, but can occur in other types of performance
horses, particularly those required to over flex at the poll (i.e. Hackney
ponies and Saddlebreds). Owners and trainers often complain that these horses
are “choking down” or are “gurgling”. These horses are often observed to be
open-mouthed breathing during episodes of this loud, expiratory (while breathing
out) gurgling noise. Once the palate displaces they are unable to breathe
sufficiently, which leads to rapid slowing or stopping, at which time, they
usually swallow and replace the palate into normal position, causing the
gurgling noise to dissipate and the open-mouth breathing to stop. Substantial
exercise intolerance occurs during DDSP due to disruption in airflow. The
exercise intolerance and gurgling noise are due to the soft palate creating an
expiratory airway obstruction because of its abnormal position. While gurgling
is relatively common, DDSP cannot be ruled out in a horse that is exercise
intolerant, but does not make a noise. Approximately 30% of horses
affected with DDSP reportedly do not make a noise.
Although much
research has been conducted, the exact cause of DDSP remains unknown.
Numerous factors are believed to contribute to this performance-limiting
disease. These factors include inflammation of the airway, including the
throat and guttural pouches, abnormal epiglottic size and shape, abnormal soft
palate rigidity (flaccidity), abnormal retraction of the larynx, excessive
flexion at the poll, neuromuscular disease, excitement and malpositioning of
the horse’s tongue over the bit.
Making a
diagnosis of DDSP requires a thorough history be obtained and a complete
physical exam be performed. Important historical questions include how
long has it been occurring, has it gotten worse, does the horse make a noise,
does the condition improve or worsen, has it responded to treatment, and other
pertinent questions. During the physical examination, a re-breathing bag may be
placed over the horse’s nostrils to make the horse breathe deeply so that lower
airway noises can be better assessed. Horses with DDSP should be
evaluated closely for evidence of inflammatory airway disease and/or
exercise-induced pulmonary hemorrhage (bleeders) because horses with lung
disease seem to have to work harder to breathe during exercise, which leads to
increased negative airway pressure during inspiration (breathing in),
predisposing the soft palate to collapse and displacement. Horses that cough
during or after placement of the re-breathing bag or have abnormal lung sounds
should be further assessed for evidence of lower airway (lung) disease.
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Endoscopy is the
most useful diagnostic test and is usually performed in the standing, unsedated
horse to visualize the inside of the throat. It is important not to sedate the
horse during endoscopy because many of the drugs that are used actually cause
muscle relaxation and can alter the function of the upper airway, which
prohibit an accurate assessment. Some horses require the use of a nose twitch
or other method of physical restraint to safely perform upper airway endoscopy
while standing. Endoscopy involves placement of the flexible, fiberoptic
endoscope through the nostril, nasal passages and into the throat, which
enables thorough examination of the structures of the upper respiratory tract
(Figure 3). The endoscope can also be passed into the guttural pouches to
assess for evidence of inflammation or infection. Because many nerves important
for the function of the muscles of the soft palate, epiglottis and the wall of
the throat course along the guttural pouch, it is believed that inflammation in
or around the guttural pouch can cause palate dysfunction, which is a possible
contributing factor to the development of DDSP. The nostrils should be occluded
with the hands while the scope is positioned in the throat to make the horse
breathe harder, which simulates the higher airway pressures that occur during
exercise. Horses predisposed to DDSP often will displace their palate during
this maneuver, however, some horses that do not have DDSP will displace during
nasal occlusion, and others that are known to have intermittent DDSP will not
displace during this procedure. Although the test is not 100% accurate, it
still remains a useful test.
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Figure 3: Photograph of a
horse undergoing standing endoscopy to assess the upper respiratory tract. |
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Figure 4: Horse being
exercised on a high-speed treadmill for dynamic endoscopy. |
In horses in
which DDSP is not confirmed with standing endoscopy with or without nasal
occlusion, it may be more beneficial if endoscopy is performed during or
immediately after exercise. A high-speed treadmill is used for dynamic
endoscopy as an ancillary diagnostic test to standing endoscopy (Figure 4).
Exercise on a treadmill helps to simulate racing conditions and speeds similar
to racing or performance. Dynamic endoscopy enables the veterinarian to
assess the structure and function of the airway during exercise, which often
helps with obtaining of confirming the diagnosis. Radiographs (x-rays) are also
a useful aid in determining epiglottic size and the presence of calcified or
soft tissue masses.
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.
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.
A third surgical
treatment option for DDSP is epiglottic augmentation. This procedure must be
done under general anesthesia and is done when it is believed that the
epiglottis is abnormally small or flaccid. During this procedure, the underside
of the epiglottis is injected with medical grade Teflon paste. After
injection of the paste, the epiglottis becomes quite inflamed and swollen, which
may persist for several weeks. It is expected to find a permanently
displaced soft palate for a few days to weeks after the surgery. An
approximate success rate of 60% has been reported with this treatment.
The fact that
there are many options available for the treatment of DDSP would suggest 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. If surgery becomes the only option, then a
decision must be made regarding which procedure(s) to perform. The
decision is based upon many factors including the age, past performance,
suspected cause (i.e. a small, abnormal epiglottis), post-operative care
required, post-operative healing time, and costs involved. Owners will
sometimes choose to have all of the procedures performed at the same time. This
is typically done in three- or four-year-old horses that has a limited time
before they must race successfully. In these cases, this is done because a
single procedure may not completely eliminate the problem, thus necessitating
the need for a different surgical procedure to be performed in the future. By performing all procedures at the same
time, it potentially maximizes the chances of successful resolution of the
problem while minimizing the time of return to performance. The overall
prognosis for improvement or resolution of DDSP in horses is approximately 60%,
regardless of which single treatment is performed. Although there is no hard
and fast evidence, there may be some additive effect of some of combining these
procedures.
For more equine
health-related articles or information, please visit the LSU Equine Health
Studies Program website at http://www.equine.vetmed.lsu.edu/.