NEWS
Veterinary Spotlight: Clear the air
Posted: Saturday, July 17, 2004
Careful diagnosis indicates type of treatment for wind problems
FORTY PERCENT of horses that perform poorly despite exhibiting no obvious physical problems have undiagnosed upper airway obstruction. The two most common types of wind problems are dorsal displacement of the soft palate, which causes flapping of loose tissue in the throat, and laryngeal hemiplegia, a paralysis of the arytenoid cartilage (flapper) usually on the left side of the horse's larynx.
Other less common problems are entrapment of the epiglottis, when the flap of cartilage that covers the opening of the larynx is sucked into the airway upon inhalation; epiglottic fold collapse, caused by weak epiglottic cartilage; and pharyngeal collapse, which occurs when suction from the horse inhaling causes the walls of the pharynx to collapse.
The soft palate is the tissue at the back of the roof of the mouth that separates the entrance of the esophagus from the entrance to the windpipe. Normally, it forms a tight seal that allows air into the windpipe and food into the esophagus, but not vice versa.
Dorsal displacement of the soft palate, or a flipped palate, occurs in certain horses that break this seal and begin breathing through the mouth and the nose at the same time, typically during peak exertion. The soft palate begins to flap, just as it does in humans who snore, and the flapping of this loose, soft tissue causes severe airway obstruction. The jockey will hear a gurgling or choking noise, and the horse will stop running until it swallows, which allows the soft palate to return to its proper position.
Three types of traditional surgery--myectomy, tenectomy, and staphylectomy--
routinely are performed on horses to stop them from flipping their palates.
Surgeries explained
Myectomy and tenectomy are similar surgeries, and most veterinarians opt to perform them in a clean stall rather than a surgical facility.
Susan Holcombe, V.M.D., Ph.D., a researcher at Michigan State University who specializes in displacement of the soft palate, explained the differences in these three surgeries.
"A myectomy is a cutting of the two [strap] muscles to change the position of the larynx in the airway," Holcombe said. "Exactly what it does for horses with soft-palate displacement isn't known. The theory is that it prevents a backward traction on the larynx that might effectively be pulling the epiglottis off the soft palate. By transecting those muscles, that tension is released. It's a surgery that really doesn't have a proven efficacy; it doesn't have a known effect, although some people do feel it helps horses that have this problem.
"A myectomy doesn't seem to have any adverse side effects at rest, but there have been some experiments that show that transecting these muscles in normal horses actually was detrimental to their upper airway tract.
"The tenectomy is functionally quite similar to a myectomy. The myectomy transects the strap muscle; the tenectomy is done where the tendon inserts on the thyroid cartilage. That tendon is resected and a small bit of muscle is removed.
"There is a procedure that is a trimming of the [rear] margin of the soft palate that is referred to as a staphylectomy. Once again, there really is no known efficacy for that surgical procedure."
Berwitz procedure
Gregory Beroza, D.V.M., who operates Long Island Equine Medical Center near Belmont Park, has collaborated with human throat specialist Mark Shikowitz, M.D., to adapt a human procedure to alleviate snoring for use in horses with a displacing soft palate. Beroza uses a special needle connected to a radio frequency generator to pierce the bulky tissue and then heat the underlying tissue to form a lesion that naturally is resorbed over six to eight weeks.
Dubbed the Berwitz procedure, it reduces tissue volume and stiffens the remaining tissue in the treated area while the outer tissue remains intact, just tighter. The procedure, which has the horse back in training in three days, is performed in a sterile surgical facility under a short-acting, intravenous anesthesia.
Flapper removal
Laryngeal hemiplegia, or roaring, occurs when one of two flappers, which open when the horse needs air and close when he swallows to protect the windpipe, becomes paralyzed. The affected flapper is sucked into the airway when the horse inhales, obstructing airflow.
Traditionally, tying the paralyzed flapper out of the way has been the preferred surgical correction, but Norman Ducharme, D.V.M., M.Sc., medical director of the Equine Hospital at Cornell University who specializes in wind problems, believes removal of the flapper produces better results.
According to Ducharme's research, only 60% of horses returned to racing after tie-back surgery, and the trainers complained that those horses rarely returned to their former level of performance.
"After reviewing our results of horses treated with removal of the flapper, we noted that the success rate and owner satisfaction was higher after removal of the flapper than after tie-back," Ducharme reported. In his study, 16 out of 17 horses returned to race after flapper removal versus only 12 of 17 that had tie-back surgery.
Difficult diagnosis
Horses who displace the soft palate in a race seldom do so when they are tested on a standard treadmill. Some researchers believe anxiety and stress of competition cause some horses to tense the throat muscles to trigger the displacement, and those conditions cannot be created in the laboratory.
At the Marion duPont Scott Equine Medical Center near Middleburg, Virginia, Jennifer Brown, D.V.M., uses an in-ground, high-speed treadmill capable of up to 55 mph, a video endoscope, and sound analysis to pinpoint the cause of wind problems. While formerly at Michigan State University, Brown collaborated with Frederik Derksen, D.V.M., Ph.D., who pioneered a method for diagnosing upper airway problems by recording and analyzing the breathing sounds of an exercising horse.
Each specific wind problem creates a unique sound pattern called a signature. For example, the noise created by a displacing soft palate is distinct because the problem is only heard when the horse exhales; all other wind problems cause noise when the horse inhales.
Brown uses a special microphone placed on a noseband three to six centimeters from the horse's nostrils. The microphone is connected to a cassette recorder containing a compression circuit, which suppresses environmental noises associated with exercise. While most of the sound analysis work is done using the high-speed treadmill, a portable system allows Brown to record horses exercising on the track.
Spectrogram analysis of the voiceprint can reveal subtle breathing noises before they can be heard by the human ear, thus enabling early intervention to resolve the problem.
Horsemen who do not have access to such sophisticated technology must rely on veterinarians in the field to diagnose the cause of a breathing problem. Holcombe stressed that before opting for surgical correction for a suspected displacing soft palate, horsemen should be absolutely sure of the diagnosis.
"The problem with displacement of the soft palate is that we really don't know yet what causes it and, therefore, correcting it surgically is very difficult," she commented.
Holcombe suggested that horsemen first attempt to solve the problem by changing bits and bridles or by using a tongue tie, a figure-eight noseband, or both. She also advised that upper airway inflammation could cause the horse to flip its palate. If a veterinarian finds this to be the cause, administration of antibiotics and corticosteroids should correct the problem, she said.
Beroza advised horsemen who suspect a horse has a breathing obstruction to seek help from a veterinarian who is an expert in the field because the exact problem could be difficult to pinpoint.
"With scoping, sometimes you will see it and sometimes you won't," he said. "Sometimes during endoscopic examination, a horse will displace the palate and it will stay displaced for several seconds and you will see it. Some horses only do it in the heat of a race or in a final workout. Sometimes diagnosis just depends on the process of elimination of variables."
Ongoing research
In her laboratory at Michigan State, Holcombe has caused horses to displace the soft palate by blocking the nerve to the palatinus muscle that keeps the soft palate taut. The same nerve runs through the guttural pouch, often the site of upper airway infections. This new information may implicate a previous guttural pouch infection as a predisposing factor in a horse that displaces its soft palate.
Ducharme has identified the thyrohyoid muscle as being responsible for causing displacement of the soft palate, and he and his colleagues have developed the Cornell DDSP collar, intended to solve the problem without surgery.
"Hopefully, the external collar will reposition and hold the larynx and hyoid bone, which connects the muscles of the tongue to the muscles of the throat, to prevent the palate from moving over the airway," Ducharme said. "This repositioning will hopefully prevent exercise-induced [dorsal displacement of the soft palate] and reduce the need for surgery.
"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 also could play an important role in identifying horses that are the best candidates for surgery."
Denise Steffanus is a contributing editor of Thoroughbred Times who writes frequently on veterinary and farm management topics.
