Diagnosing & Managing Upper Respiratory Tract Disorders
Abnormalities of the upper respiratory tract are second only to musculoskeletal disorders as leading causes of poor performance in athletic horses. While a small number of horses with mild upper airway issues can be managed medically, many horse owners elect surgery as a primary treatment option.
"Any abnormality of the respiratory system can negatively impact performance," comments Norm G. Ducharme, DMV, MSc, Dipl. ACVS, medical director of the Equine and Farm Animal Hospitals at Cornell University's College of Veterinary Medicine in New York. "An exercising horse needs to move large volumes of air between the environment and the lungs. Disorders of either the upper or lower respiratory tracts that impede or disturb the flow of air can decrease the amount of oxygen being delivered to the horse's lungs and, therefore, the skeletal muscles."
This article describes the most common upper airway disorders contributing to performance problems in horses, the available surgical treatment options for these disorders, and some potential complications associated with surgery of the upper respiratory tract.
The upper respiratory tract extends from the nose to the back of the throat where the larynx is located. Major anatomic structures in this region of the respiratory system include the:
- Nasal passages;
- Paranasal sinuses and guttural pouches;
- Nasal septum;
- Hard and soft palates;
- Larynx; and
- Upper trachea.
While abnormalities can occur at virtually every point along this muscular and cartilaginous tube, the majority of performance-limiting abnormalities occur near the back of the throat and involve the palate, the muscular wall of the pharynx, the epiglottis, and larynx. Specifically, the "big three" conditions are dorsal displacement of the soft palate (DDSP), epiglottic entrapment, and roaring (i.e., laryngeal hemiplegia).
DDSP In horses the soft palate is extremely long, extending from the end of the hard palate to the base of the epiglottis. DDSP occurs when the soft palate displaces dorsally (abnormally moves in an upward direction) so the end of the soft palate rests above the epiglottis, instead of below.
"DDSP is problematic because it decreases the volume of air passing through the horse's airways during exercise by obstructing the flow of air during exhalation," says Ducharme.
Roaring Left laryngeal hemiplegia, also called "roaring" due to the classic sound produced by affected horses, occurs in approximately 3-5% of Thoroughbreds. Other breeds of horses can develop this abnormality as well, including Standardbreds and large-breed horses (e.g., those over 17 hands).
Roaring is usually caused by weakness or paralysis of the left arytenoid cartilage and vocal fold, resulting in a failure to achieve full abduction of these structures during respiration. As a result, these tissues droop or hang in the lumen (opening) of the larynx, which reduces the volume of air that can pass through the larynx to the lungs. The underlying cause of roaring is a degeneration of or damage to the nerve (the left recurrent laryngeal nerve) that innervates the dorsal cricoarytenoid muscle on the left side of the larynx. It is this muscle that normally holds the left arytenoid cartilage open during exercise.
Epiglottic entrapment The epiglottis--the movable, leaf-shaped cartilaginous structure located at the base of the tongue and above the soft palate--serves as a "diverter valve" during swallowing. That is, during swallowing, the epiglottis covers the opening of the larynx to ensure that food and water do not enter the trachea, but instead pass into the esophagus.
When a fold of tissue, called the aryepiglottic fold, abnormally enfolds the epiglottis, the epiglottis is said to have become "entrapped." Clinical signs of epiglottic entrapment include noise on inspiration and/or expiration while exercising, poor performance, and, less commonly, headshaking. Again, the presence of abnormal tissues and suboptimal function of the anatomic structures within the airway are thought to impede airflow through the respiratory system, resulting in poor performance.
Because of the current limitations in diagnosing upper respiratory tract disorders, determining the underlying cause of poor performance (be it real or perceived) remains a challenge for veterinarians.
While veterinarians consider direct visualization of the upper respiratory tract with endoscopy the "gold standard" test, scoping horses at rest is not a reliable tool to diagnose dynamic obstructions of the upper respiratory tract. In fact, researchers have estimated that 85% of horses with a treadmill diagnosis of upper respiratory tract obstruction did not have an abnormality at rest. The solution, according to various veterinarians, is endoscopy during exercise. This can be achieved using either a high-speed treadmill or a portable endoscope that can be used in horses exercising under natural conditions.
This relatively new technology was described by Youssef Tamzali, DVM, PhD, Dipl. ECEIM, of the Ecole Veterinaire de Toulouse (National Veterinary School) in France, at the 2008 American Association of Equine Practitioners (AAEP) conference. According to Tamzali, "The portable endoscope, called overground endoscopy, allows one to examine horses from one day to another at the client's location of choice. In addition, overground endoscopy is less expensive than treadmill endoscopy, which is not available everywhere."
This technology is currently available commercially in the United States through a number of veterinarians practicing at such facilities as Palm Meadows Thoroughbred Training Center and Gulfstream Park in Florida, Cornell University's Equine Performance Testing Service, as well as Woodland Run Equine Veterinary Facility in Grove City, Ohio, whose veterinarians incorporated this technology into the practice over a year ago.
One additional diagnostic technique worth mentioning is the use of ultrasound to diagnose upper airway problems.
Ducharme explains, "An ultrasound examination can easily be performed to visualize parts of the walls of a horse's throat, including the hyoid apparatus, laryngeal cartilages, and associated muscles and other soft tissues." (See diagram on page 26.)
The technique is described in detail in the article, "Ultrasonography of the Equine Larynx," which was published in the September-October 2006 edition of the journal Veterinary Radiology & Ultrasound.
"Using the acoustic windows described in the article and a description of the normal appearance of the anatomic structures of interest, this technique can easily be introduced into virtually any equine practitioner's armamentarium of diagnostic tools to diagnose upper airway problems," suggests Ducharme.
DDSP Considering the number of different procedures currently available for the management of DDSP, it is reasonable to suggest that a "treatment of choice" for DDSP remains to be discovered. Nonetheless, one of the more effective choices at present is the "tie-forward" surgery. The surgery is approximately 20% more effective than strap muscle resection alone (i.e., Llewellyn procedure), in that nearly 80% of horses have improved performance after surgery. A tie-forward involves placing two sutures on each side of the larynx and wrapping them around the basihyoid bone. As a result, the larynx is pulled both upward and forward to minimize displacement of the soft palate.
"It is this upward movement that has recently been shown to correlate best with postoperative results," says Ducharme.
Roaring Like DDSP, there are various surgeries described for the management of roaring, but the current treatment of choice is a "tie-back" surgery (also referred to as a prosthetic laryngoplasty). This procedure involves using suture material to replace the function of the dorsal cricoarytenoid muscle to effectively tie back the left arytenoid cartilage into a permanently abducted position.
Many surgeons also perform a ventriculocordectomy (i.e., removing both the ventricles and vocal cords) using a laser a day or two after the tie-back to improve the success rate of the procedure. Nonetheless, the success rate of this procedure is only estimated to be about 50-70%.
In cases of a failed tie-back surgery, a procedure is available that, according to Eric Parente, DVM, Dipl. ACVS, an associate professor at the University of Pennsylvania's New Bolton Center, can resolve the persistent upper airway obstruction and return horses to their athletic endeavors within about six months post-surgery.
"The procedure, called a unilateral partial arytenoidectomy with primary mucosal closure, involves removing the abnormally thick or immobile cartilage without removing the entire mucosal lining," explains Parente.
In a study of 73 Thoroughbred racehorses that underwent this operation, 82% returned to racing, and 63% of these horses raced five or more times. While the procedure's success has not been specifically studied in other breeds, Parente suggests that similar success rates are expected for other athletic horses, and this operation offers "the best chance for horses with failed tie-backs to return to competition."
Epiglottic entrapment The treatment of choice for epiglottic entrapment is called "axial division of the aryepiglottic fold." In simpler terms, the veterinarian cuts tissue trapping the epiglottis so that it is no longer capable of trapping the epiglottis. There are several different approaches that can be used. Veterinarians most commonly use either a laser or a curved knife under endoscopic guidance to sever the aryepiglottic fold.
Ducharme says, "Success rates are very good for horses treated via these described approaches, with 80-95% of horses considered 'cured' postoperatively."
Despite the good prognosis, a small number of horses develop problematic complications such as recurrence and permanent DDSP. Further, the aryepiglottic fold can regrow and subsequently re-trap the epiglottis. Using a laser to surgically cut the fold of tissue is reportedly associated with a lower rate of recurrence as compared to using a knife.
Improving Surgical Outcomes
In light of the less-than-ideal success rate for horses treated with tie-backs, Jon Cheetham, VetMB, PhD, Dipl. ACVS, a colleague of Ducharme's at Cornell, has been identifying ways to test novel techniques for managing left recurrent laryngeal hemiplegia. Cheetham created a model of the equine larynx to test various means of manipulating the equine larynx to achieve maximal airflow through the paired arytenoid cartilages. Cheetham's research led to the discovery that injecting the cricoarytenoid joint of the larynx with bone cement (polymethylmethacrylate) can help hold the airway in an open position.
"This 'fusion' of the cricoarytenoid joint could prove to be a useful adjunct technique to minimize loss of arytenoid abduction postoperatively in horses undergoing laryngoplasty," Cheetham says. "Hopefully this will ultimately improve the success rate and post-surgical return to function."
Ducharme summarizes, "Any abnormality of the upper respiratory tract that results in obstruction of airflow can potentially cause poor performance and noise while exercising. Remember that some horses with airway obstructions will not make an audible noise, so quiet does not necessarily indicate a perfect airway."
While the surgical techniques currently available for horses with DDSP, roaring, or epiglottic entrapment remain less than perfect, surgery is still the best option for returning horses to their athletic careers.
"Owners and trainers simply need to be aware that postoperative complications can and do occur and that in some cases, the procedure may fail," advises Ducharme. "Continued research is needed to offer our clients the best chances of successful resolution of the underlying cause of their horses' poor performance. The new wireless endoscope is a significant diagnostic advancement" that has the potential to aid this research.
About the Author
Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.
POLL: University Equine Hospitals