Upper Airway Problems in Horses

Noise and exercise intolerance...those are the usual signs of an upper airway disorder. Many things can go wrong with a horse's breathing mechanisms that don't involve the lungs. The diagnoses aren't always accurate, and treatments aren't totally effective, but researchers are finding new ways to handle these complicated problems.

Diseases of the upper airway can occur in the hard or soft palate, larynx, pharynx, nasal passages, and sinuses. Researchers at various institutions are working to remedy the problems that can affect your horse's upper respiratory tract.

It Sounds Like...

Researchers at Michigan State University (MSU) are engaged in several different projects. One of the most intriguing is developing the means to more easily and accurately diagnose specific upper airway disorders through sound analysis.

Fred Derksen, DVM, PhD, MSU professor of large animal clinical sciences with an interest in respiratory diseases, explains, "There are a number of different upper airway conditions in horses. These upper airway abnormalities generally don't bother the horse when he is resting, only during exercise. This makes these conditions difficult to diagnose in the standing horse. During exercise, the horse must breathe hard, and forces a lot of air through the upper airway. These forces change the upper airway, making the abnormalities apparent. Standard procedure is to do an endoscopy of the upper airway, but often you can't determine what's wrong because whatever goes wrong only does so during exercise."

Alternatively, one can put the horse on a treadmill and scope him during exercise. This is a useful, but involved and expensive, procedure that entails shipping a horse to a facility with a high-speed treadmill, training him to exercise on the treadmill, and scoping him during exercise.

One feature of upper airway obstructions is that during exercise, horses make noises that might be specific to the type of airway obstruction or abnormality they have.

"In other words, a roarer (a horse with laryngeal hemiplegia) would make a different kind of noise than a horse that has a dorsal displacement of the soft palate," Derksen states. To determine that, Derksen placed a small microphone on a caveson (noseband) of an exercising horse, recorded the animal's airway noise, then analyzed the noise via a computer, making a "voiceprint" of the noise.

"Hopefully, by looking at the voiceprint, we can diagnose the specific condition," says Derksen. "The beauty of this method is that it's simple and inexpensive, and can be done anywhere the horse is exercised. The sound analysis can be performed on a personal computer, and sound files can be e-mailed to an expert for interpretation."

Currently, Derksen is studying the accuracy of sound evaluation for laryngeal hemiplegia (roaring), dorsal displacement of the soft palate, and some less common upper airway disorders. Although study results are still pending, should sound analysis prove to be an accurate diagnostic tool, it will provide a simpler, safer, and much less expensive way of making a diagnosis than an endoscopic exam on a treadmill. See samples of voiceprint analyses here.

Additionally, Derksen and his coworkers (Jennifer Brown, DVM, graduate student; John Stick, DVM, professor; and Ed Robinson, BVetMed, PhD, professor) have been evaluating the efficacy of several surgical procedures used to reduce the noise associated with laryngeal hemiplegia.

"A horse that is a roarer is often disqualified from shows because he is said to be of unsound wind," he says. "Lots of owners want the respiratory noise that their horses make to disappear."

Derksen found that ventriculocordectomy, which removes the vocal cord and the associated lateral ventricle (a tissue outpouching beside the vocal cord), is effective in reducing noise, while tying back the arytenoid cartilage is less effective at reducing noise. Other surgical procedures are still under investigation.

Naso-Pharyngeal Collapse

Also at MSU, Susan J. Holcombe, VMD, MS, PhD., Dipl. ACVS, Dipl. ACVECC, assistant professor in the Department of Large Animal Clinical Sciences, is studying dorsal displacement of the soft palate (DDSP) and naso-pharyngeal collapse. The latter is a disease in which the soft palate, the roof, and the walls of the naso-pharynx displace into the airway. (The naso-pharynx is the area between the end of the nasal passage and the larynx or voice box.) In severe cases, naso-pharyngeal collapse can be career-limiting or career-ending. There is no cure.

Explains Holcombe, "We're trying to define the pathology of this type of collapse. Our goal is to investigate the nerves and muscles and the dysfunction of those nerves and muscles that are responsible for this type of collapse. If we can figure out what causes the disease, the next phase would be coming up with treatment--medical therapies to help reverse/correct muscle or nerve damage, or surgical correction that creates a prosthesis for a muscle--although that is probably a bit further away."

To learn more about nerves and muscles that could be involved with the collapse, Holcombe and graduate student Caroline Tessier, DVM, selectively block nerves that create specific muscle dysfunction, then observe the horse on the treadmill while those nerves are blocked to see if they can get a model that mimics the disease. They also measure the electromyographic activity in those muscles while the horse is exercising to determine if those muscles are firing during respiration and if muscle activity is increasing. Thirdly, they perform selective sensory nerve anesthesia to see if they can affect the electromyographic activity of those muscles.

"This gives us information about how these muscles function in the naso-pharynx in normal horses," Holcomb says. "Understanding how the normal airway works may provide information about what might be happening in the horse that has the disease."

To date, Holcombe and her team have identified some of the muscles in the airway that create some of the airway collapses, defined certain muscles important in stabilizing the naso-pharynx, and succeeded in reproducing certain types of collapse by blocking specific nerves and muscles.

Soft Palate Displacement

The soft palate normally sits underneath the epiglottis (a cartilage that forms the larynx or voice box) and therefore is not in the way of airflow. With certain airway conditions, the soft palate becomes displaced on top of the epiglottis and disrupts airflow (much like a sail in the wind). Dorsal displacement of the soft palate (DDSP) is an upper airway disease in which the palate partially obstructs the airway, usually (but not always) toward the end of maximal exercise intensity. Although exact incidence is unknown, DDSP might affect up to 25% of racehorses.

Management of the disease includes the use of specific equipment (i.e., tongue tie, figure-eight nose band, and Serena Song bit, a bit with a U-shaped extension in the center of the snaffle mouthpiece to keep the tongue from getting over the bit), increasing fitness, and decreasing local inflammation. If all else fails, surgery is recommended. Many surgical treatments have been proposed, but the most frequently used consists of either removing a small section of the soft palate or cutting some muscles on the lower side of the neck to prevent the larynx from slipping back. Regardless of the surgical treatment or combination thereof, the success rate is about 60%.

Investigators (Norm G. Ducharme, DVM, MSc, Dipl. ACVS, medical director and professor of surgery; Brett Woodie, DVM, MS, Dipl. ACVS, assistant professor of clinical sciences; and Richard Hackett, DVM, MS, Dipl. ACVS, chair of the Department of Clinical Sciences) at the Equine Performance Testing Clinic at Cornell University hope to improve those odds. They are clinically evaluating a new surgical technique they developed to stabilize the larynx in a position that they feel is optimal for preventing the disease. This surgical procedure is called "tie-forward." Although the study is not complete, preliminary results suggest that the new surgical treatment is effective in clinical situations, says Ducharme.

In addition, the same investigators are exploring the efficacy of a "throatlatch stabilizer" for preventing DDSP. Worn externally during exercise and performance, this device is placed over the larynx, applying pressure and support to the area in order to prevent displacement of the soft palate.

"We finished the experimental trial of horses running on the treadmill," Ducharme says. "In 13 of 14 trials, DDSP was prevented with this device. Now we're working on the field study, where there are a lot more variables--horses' performance at race tracks, in show situations, etc." They hope to complete the field trial this summer.

Laryngeal Hemiplegia

At the Virginia-Maryland Regional College of Veterinary Medicine, researcher and clinical instructor Celia Goodall, MRCVS, is investigating what might be an effective procedure for treating laryngeal hemiplegia (roaring). The condition occurs due to paralysis of one side of the larynx (usually the left side); this allows laryngeal tissue called the arytenoid cartilage and vocal cords to protrude into the airway, obstructing airflow and affecting the horse's ability to breathe correctly. This results in upper respiratory tract noise and exercise intolerance.

"It is a common problem in large-breed horses," notes Goodall, "and long-necked horses appear to be predisposed."

The tie-back is the surgical procedure that is usually performed to treat the condition. It involves suturing the paralyzed cartilage with either nylon or polyester out of the airway so it can't interfere with airflow. Goodall is studying the use of a special nylon suture of fishing line, which acts as a prosthesis, replacing the action of the muscle, that is secured with a precision crimping device to tie the protruding tissue out of the airway so the horse can breathe easier. Previous tie-back procedures sometimes failed due to knot slippage or suture breakage, allowing the tissue to fall back into the airway. The hope is that this extremely strong nylon suture will decrease the occurrence of failure of the prosthesis.

"We have placed the nylon prosthesis with precision crimping device in five horses," says Goodall. "It was extremely easy to place and secure, and is working very well. We just have to see if it works long-term." Study completion is expected this fall.

In related studies, Goodall is looking at a technique called laser ventriculocordectomy. Previous work has not clearly indicated whether this procedure should be done along with a tie-back. Goodall's research will investigate whether this technique is beneficial in horses with tie-back procedures.

Preliminary study results are expected in July, and the study will be completed in the fall. The research is a collaborative study with Eric Birks, DVM, PhD, assistant professor of exercise physiology at the University of Pennsylvania's New Bolton Center.

Take-Home Message

If your horse starts making noise while exercising, or he's failing to live up to his previous competition standards, it's time for your veterinarian to do a thorough examination, including the upper respiratory tract. While not all treatments for upper respiratory problems are 100% effective, there are many ways to help your horse breathe easier.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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