The Equine Upper Airway: How It Works and What Goes Wrong
Diseases and abnormalities of the upper airway are some of the main reasons for poor performance in sport horses. As such, a good understanding of the upper airway and its possible problems are crucial to making the correct diagnosis and returning the horse to function.
How the Airway Functions
Horses are obligate nasal breathers, meaning air for gas exchange can only pass through their nostrils and upper airway. But, due to the airway’s dynamics, there’s a substantial amount of airflow resistance. And that negative airway pressure during inspiration increases during exercise. Inspiration is an active process (which means it requires muscle contraction), while expiration is a passive process (it does not require muscle contraction), except during exercise. Minor changes can lead to decreased oxygen uptake, greatly inhibiting the horse’s performance. Oxygen is like gasoline—just as most vehicles can’t function without gas, the horse can’t function without oxygen.
At rest the horse takes 10 to 18 breaths per minute. Each time the horse breathes in, he inhales between 5 to 8 liters of air, which amounts to 75 to 100 liters of air per minute. During work, however, the horse’s respiration rate can reach as high as 120 to 140 breaths per minute, and he can inhale 10 to 12 liters of air per breath. That amounts to taking in 1,200 to 1,500 liters of air per minute.
Veterinarians diagnose most upper airway disorders using a flexible endoscope while the horse is at rest. Many horses tolerate an endoscopic examination without sedation, but some require a sedative. In these cases, it’s important for the veterinarian to remember that sedation with detomidine or acepromazine changes the endoscopic picture of the upper airways and can lead to incorrect diagnosis or a missed diagnosis, especially of recurrent laryngeal neuropathy (roaring).
Some disorders, however, can’t be identified when the horse is at rest. In these cases, veterinarians can use a newer diagnostic tool—the dynamic or overground endoscope—to examine the horse’s upper airway during exercise and under the horse’s normal working conditions (Figure 2). Abnormalities best diagnosed during exercise include displacement of the soft palate and pharyngeal collapse. Veterinarians can also better gauge the severity of laryngeal neuropathy during exercise.
Recurrent Laryngeal Neuropathy (RLN) RLN is caused by dysfunction of the recurrent laryngeal nerve resulting in paralysis, either partial or complete, of the arytenoid cartilage. In 98% of cases, the horse is affected on the left side. The most important muscle involved is the dorsal cricoarytenoid muscle, which is the only abductor muscle to the larynx. The etiology (or cause) is still unclear but many researchers and veterinarians believe it’s idiopathic (of unknown cause). Some also believe RLN could be hereditary.
The most common clinical sign is a “roaring” sound during inspiration. The veterinarian can make a presumptive diagnose based on the horse’s clinical signs, but must use endoscopy to make a conclusive diagnose. Veterinarians can also grade the degree of severity via endoscopy.
The most common corrective surgery for RLN is laryngoplasty (often called “tie-back” surgery) alone or in combination with ventriculectomy (removing the ventricle mucosa—the lining of the soft tissue structures of the larynx that are affected by RLN—to form a scar between the vocal fold, thyroid, and arytenoid cartilages). This surgery’s success is well-documented and can return upper airflow to baseline values.
Another treatment option is laryngeal reinnervation, which involves transferring part(s) of a normal, innervated (supplied with nerves) muscle, or a nerve, into the nonfunctioning or poorly functioning cricoarytenoideus dorsalis muscle of the larynx. Success depends on the desired use of the horse after surgery—for racehorses the success rate is reported to be 50 to 70%, while for pleasure horses it can reach 80%.
Epiglottic Entrapment (EAE) EAE is characterized by an entrapment of epiglottis in the glossoepiglottic fold, which is located under the epiglottis (Figure 4). The etiology is often unknown, but possible causes include primary inflammation of the fold or a hypoplastic (underdeveloped) epiglottis. This condition is often seen in conjunction with dorsal displacement of the soft palate.
The clinical signs of EAE can vary from none to respiratory sounds during performance, coughing, and regurgitation. Veterinarians can use endoscopy and/or other imaging modalities to diagnose EAE. Treatment is generally surgical and involves cutting the fold through the horse’s mouth with at sharp hook or with a laser. The prognosis is good.
Dorsal Displacement of the Soft Palate (DDSP) This condition is characterized by an intermittent or persistent dislocation of the soft palate over the epiglottis (Figure 5). This leads to an inspiratory and/or expiratory “snoring” sound and labored or difficult breathing. Although the etiology and pathogenesis (how the condition develops) is still unclear, there is evidence that neuromuscular dysfunction of the palatal musculature plays an important role in some cases.
DDSP is very difficult to confirm at rest, so the best way to diagnose it is by overground endoscopy. The most frequently used treatment method is the “tie-forward” surgical technique, with a reported success rate of up to 80%.
Pharyngeal Wall Collapse (PWC) Pharyngeal diameter affects airway resistance—if the diameter decreases, airway resistance increases greatly. Severe pharyngeal wall collapse can, in some horses, cause a decrease in arterial oxygenation, leading to poor performance.
Horses with PWC usually appear normal at rest during an endoscopic examination, but sometimes nasal occlusion can induce collapse of the nasopharynx. The pathophysiology of PWC is not yet fully understood, and some affected horses do not show clinical signs. Veterinarians have tried various treatments—including local or systemic steroids and non-steroidal anti-inflammatory drugs—but, to date, they have not identified an efficient treatment.
About the Author Julie Fjeldborg, DVM, PhD, is an associate professor in the University of Copenhagen Department of Animal Sciences, in Denmark. Fjeldborg’s research program explores a variety of topics related to poor performance in horses due to the airways. She is the WEVA regional ambassador for the Nordic countries and heads the organizing committee for the World Equine Airway Symposium 2017, which will be in Copenhagen.
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