AAEP Convention 2005: Upper Airway Obstructive Disease

"Before high-speed treadmill (HSTM) endoscopy, we did not recognize the complexity of maintaining a open airway under enormous pressure swings during inspiration and expiration (breathing in and out)," said Eric Parente, DVM, associate professor of surgery at the University of Pennsylvania's New Bolton Center, during the In-Depth Respiratory session at the 2005 American Association of Equine Practitioners convention, held Dec. 3-7 in Seattle, Wash. "The pharynx and larynx have limited structural strength and are highly dependent on muscular strength to maintain an adequate lumen (open space or cavity in the airway). The limits of muscular strength and the potential for muscular fatigue can often result in airway collapse. HSTM endoscopy is the best method to determine the specific source of collapse."

Parente discussed protocols for conducting an HSTM examination, which he said is best used in three cases, all of which might feature abnormal respiratory sounds:

• For horses that are suspected to have upper airway problems but don't have visible abnormalities on endoscopic examination at rest;
• When the clinical history and resting endoscopic examination don't agree for a diagnosis, and
• To evaluate patients who still have some problems after respiratory surgery.
Parente also reported that the following problems can be diagnosed with HSTM endoscopy. "Some of these diagnoses can often be made without HSTM endoscopy, whereas others are strictly HSTM diagnoses," he noted.

Laryngeal hemiplegia (LH)--In this disorder, the muscles of one side of the larynx are partially or fully paralyzed because of nerve dysfunction (called recurrent laryngeal neuropathy), so they can't fully pull the arytenoid cartilages (to which vocal cords attach) out of the airway. "While originally it was thought the majority of horses with grade III hemiplegia were able to support an adequate airway, more recent evidence with clinical patients yields evidence that most horses with a grade III hemiplegia experience dynamic arytenoid collapse under strenuous exercise," Parente reported.

Intermittent dorsal displacement of the soft palate--This common problem occurs when the soft palate (the rear roof of the mouth) moves up and over the epiglottis (a triangle-shaped cartilage that prevents food from entering the windpipe) and partially blocks the airway. It is the one problem that is not always noted even with HSTM endoscopy, he said.

Intermittent epiglottic entrapment--This occurs when the epiglottis becomes trapped by the aryepiglottic folds of tissue and cannot move. Airway turbulence occurs when the folds balloon with high air movement in the airway.

Pharyngeal collapse--This can occur in various areas of the pharynx, with lateral wall collapse being the most common. "The noise that is created is dependent on the severity of the collapse, but is often a softer, lower-pitched inspiratory sound than that which is created by horses with LH," he said. "Some degree of pharyngeal collapse during nasal occlusion (blocking of nostrils) is not pathologic and not correlated with collapse during exercise."

Axial (inward) deviation of the aryepiglottic folds--This problem occurs when folds of tissue to either side of the larynx move in toward the center of the airway, obstructing airflow. This diagnosis can only be made during HSTM endoscopy, Parente said. "It is almost exclusively a racehorse problem and can occur unilaterally or bilaterally (in one or both sides)," he noted. "Most (affected) horses are younger and show a noise similar to a horse suffering from LH. Occasionally head flexion (at the poll) can also exacerbate this abnormality."

Epiglottic retroversion--"Epiglottic retroversion is a rare condition that creates a dramatic abnormal respiratory noise and is seen clinically and can be reproduced experimentally," he noted. "Because of dysfunction of the hyoepiglottis muscle, the epiglottis cannot maintain its position on the palate, and the epiglottis is sucked into the larynx during inspiration. The disorder has been seen clinically in horses that have had severe upper respiratory infections or surgery that may have affected the hypoglossal nerve (which controls the hyoepiglottis muscle)."

Vocal cord collapse--This problem "is often considered a passive secondary result of decreased abduction (suboptimal opening) of the ipsilateral arytenoid (on the same side as the vocal cord collapse)," Parente said.
"Sound analysis while the horse is exercising in its home environment may be a viable alternative to HSTM endoscopy in the future," Parente said. "There is growing evidence of its value, but there is limited information on its ability to definitively diagnose all the abnormalities described here."

Fixing Problems

After describing all these respiratory problems, Parente went on to discuss various surgical options, success rates, and complications, using many case images and videos to illustrate his points.

"Upper respiratory surgery has improved significantly over the last decade, and transendoscopic laser surgery has allowed us to access and effectively treat disorders that previously required a more invasive approach," he said. "But there's a big learning curve with laser surgery; it's not for everyone." He discussed the following surgical success rates:

• About 75% of horses should be able to return to racing after laryngoplasty for recurrent laryngeal neuropathy, which causes LH, and at least one-half have a significant improvement in performance.

• There is no evidence to support the theory that waiting until a horse is completely paralyzed (from LH) before pursuing surgery will improve the surgical outcome.

• In general, success rates for draft and sport horses are better than those reported for racehorses and range from 86% to 93%.
• Sixty percent to 80% of the horses return to racing after partial arytenoidectomy.

• Furthermore, the transendoscopic laser approach seems to have the lowest re-entrapment rate of 5%.

"There have been great advancements (in studies of equine respiratory function) with improved imaging," he concluded. "We can see things we couldn't before, so we have a better understanding of dynamics, an improved ability to treat problems effectively, and a further understanding of physiologic principles and development of new strategies."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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