Endoscopy and respiratory disease in horses

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Unlike in small animal practice, endoscopy is commonly used in equine practice. Although endoscopy as a diagnostic tool is often thought of for upper airway dysfunction (eg, laryngeal paralysis, dorsal displacement of the soft palate) only, it can be a useful adjunct in a number of other respiratory conditions of both the upper and lower respiratory tracts.

Many clinical signs in horses warrant endoscopy of the respiratory tract. The most common include nasal discharge and abnormal respiratory noise during work. Diagnostic differentials for nasal discharge are often determined by the character of the discharge. Once a differential diagnosis is developed, endoscopy can be performed. Common differentials for unilateral nasal discharge include guttural pouch empyema (often associated with Streptococcus equi subsp equi infection), guttural pouch mycosis, ethmoid hematomas, masses of the sinuses or nasal passages, and dental disease. When bilateral nasal discharge is noted, conditions of the lower airway (eg, pneumonia or equine asthma) become more likely, although strangles and guttural pouch empyema can also cause bilateral mucopurulent discharge. Viral infections of the upper respiratory tract, such as equine influenza and herpesviruses, can also lead to bilateral nasal discharge.

The endoscopic exam

A complete endoscopic examination of the upper airway includes visualization of the nasal passages, nasopharynx, larynx (including arytenoid cartilages, aryepiglottic folds, and epiglottis), soft palate, dorsal pharyngeal recess, ethmoids, both guttural pouches, and proximal trachea. Portions of the exam can be tailored to the specific condition suspected. With unilateral nasal discharge, for example, it is probably not necessary to examine both nasal passages.

Upper airway evaluation

If upper airway function is the primary reason for endoscopy, then examine the horse without sedation. Sedation can decrease abduction of the arytenoid cartilages and lead to dorsal displacement of the soft palate, so definitive diagnoses involving the function of these structures should be made only in an awake patient. Many patients will allow endoscopy without sedation with just the use of a nose twitch and potentially application of topical anesthetic (2% lidocaine on a gauze square often works well) in the nasal passages.

In cases in which exercise intolerance or excessive respiratory noise is the presenting complaint, then dynamic endoscopy during exercise may be the best modality if obvious abnormalities cannot be identified with the horse at rest.

Conditions commonly identified with dynamic endoscopy include dorsal displacement of the soft palate and recurrent laryngeal neuropathy (RLN). Multiple grading scales can help objectively assess the severity of RLN while at rest and during exercise, ranging from a 4-point scale to a 6-point scale with a variety of subcategories.¹ Dynamic endoscopy can be performed as part of a complete treadmill workup for exercise intolerance or as an isolated diagnostic on the farm.

When using endoscopy to find the source of nasal discharge, considering the character of the discharge (unilateral vs bilateral, serous vs mucopurulent vs feed material vs epistaxis) is crucial to ensure that the appropriate structures are visualized. In an ideal world, every exam would include all the structures described above, but patient compliance sometime limits the extent of an exam. In patients with unilateral epistaxis, an ethmoid hematoma and guttural pouch mycosis are the most likely diagnoses. While these could cause bilateral epistaxis, that would more likely be caused by exercise-induced pulmonary hemorrhage, in which case examination of the lower airway may also be warranted. Similarly, unilateral mucopurulent nasal discharge may result from sinusitis (either primary or secondary to dental disease), a mass in the nasal passage, or guttural pouch empyema; thus it would be prudent to perform the exam from the affected nostril.

In any horse suspected of having strangles, examine both guttural pouches thoroughly as this is the site where chronic shedders harbor infection.

Lower airway evaluation

Endoscopy of the airway does not need to be limited to the upper respiratory tract. Not only can endoscopy provide valuable information about conditions in the lower airways, it can also be used to facilitate airway sampling. While a 1-m endoscope will not reach past the carina, it can be used to perform tracheal washes and to inspect the trachea for the presence of purulent material, blood, or feed material that could contribute to clinical signs. Occasionally, the upper airway examination can provide information about lower airway disease. For example, the guttural pouches are important sites to examine in horses suspected of having aspiration pneumonia secondary to dysphagia. Among the critical structures inside the guttural pouches are cranial nerves IX (glossopharyngeal nerve), X (vagus nerve), and XII (hypoglossal nerve), which together control swallowing.

Common noninfectious causes of lower-airway disease, such as exercise-induced pulmonary hemorrhage (EIPH) and equine asthma syndrome (formerly known as inflammatory airway disease, recurrent airway obstruction, heaves, and chronic obstructive pulmonary disease), are marked by the accumulation of blood (EIPH) or mucus (equine asthma syndrome) in the trachea, both of which can be visualized endoscopically.^2,3

Scoring systems have been developed for objective quantification of blood or mucus in the airways in cases of EIPH and equine asthma syndrome, respectively. A common scale for grading blood in the airway for EIPH typically ranges from 1 to 5:

• Grade 1: Flecks of blood are present in the trachea.
• Grade 2: Blood flecks and a small continuous stream of blood are present in the trachea.
• Grade 3: The stream of blood is less than half the total tracheal diameter.
• Grade 4: The stream of blood is larger than half the tracheal diameter.
• Grade 5: The trachea is filled completely with blood.^4,5

In some cases of EIPH, blood may not always be visible in the trachea, depending on the amount of time that has passed since exercise.

Similar to EIPH, there is no standard scale for grading mucous in the trachea, but 1 commonly used scale grades the amount of mucus from 0 to 5:

• Grade 0: No mucus is visible.
• Grade 1: Single to multiple blobs of mucus are visible; this grade is commonly seen in young racehorses without overt disease.
• Grade 2: Multiple blobs of mucus, larger than in grade 1, are visible, but they are still nonconfluent.
• Grade 3: The mucus becomes confluent or forms a stream.
• Grade 4: Pool-forming mucus is visible.
• Grade 5: Profuse amounts of mucus are visible in
  the trachea.⁶

In addition to being used to identify excessive mucus in the airways, 3-m endoscopes can also be used to perform bronchoalveolar lavages for definitive diagnosis of asthma.

Kate L. Hepworth-Warren, DVM, DACVIM, is an equine internal medicine specialist who works as a clinical assistant professor of equine internal medicine at North Carolina State University College of Veterinary Medicine in Raleigh. Outside work, she enjoys traveling, cooking, reading, running, and the beach.

References

1. Smith KJ, Dixon P. Recurrent laryngeal neuropathy: grading of recurrent laryngeal neuropathy. In: Hawkins J, ed. Advances in Equine Upper Respiratory Surgery. Wiley Blackwell; 2015:3-8.
2. Coutëil LL, Hoffman AM, Hodgson J, et al. Inflammatory airway disease of horses. J Vet Intern Med. 2007;21(2):356-361. doi:10.1892/0891-6640(2007) 21[356:iadoh]2.0.co;2
3. Sullivan S, Hinchcliff K. Update on exercise-induced pulmonary hemorrhage. Vet Clin Equine. 2015;31(1):187-198. doi:10.1016/j. cveq.2014.11.011
4. Barakzai S. Trachea and bronchi. In: Barakzai S, ed. Handbook of Equine Respiratory Endoscopy. Saunders Elsevier; 2007:89.
5. Pascoe JR, Ferraro GL, Cannon JH, RM Arthur, Wheat JD. Exercise-induced pulmonary hemorrhage in racing thorough- breds: a preliminary study. Am J Vet Res. 1981;42(5):703-707.
6. Couëtil LL, Cardwell MJ, Gerber V, Lavoie J-P, Leguillette R, Richard EA. Inflammatory airway disease of horses-revised consensus statement. J Vet Intern Med 2016;30(2):503-515. doi:10.1111/jvim.13824