How to perform and interpret equine upper airway endoscopy (Proceedings)

October 1, 2011
Melissa R. Mazan, DVM, DACVIM

For any practitioner engaged in sport horse practice, a familiarity with upper airway endoscopy is extremely useful.

For any practitioner engaged in sport horse practice, a familiarity with upper airway endoscopy is extremely useful.  It is important to discuss the capabilities of the endoscopy system purchased with the vendor, and to make sure that its capabilities work for you.  For in-house work, a large, clear video monitor is a necessity.  Most practitioners use a 1-meter scope, which is more than sufficient for upper respiratory examinations. 

Although a 2.5 – 3.0 meter scope can be used for both upper and lower respiratory work, as well as gastroscopy, the long length can be quite awkward, and it is preferable to have a separate, longer endoscope for gastroscopies and bronchoscopies.  It is also very useful to have a system that can record video and make hard copy pictures.  Video is extremely important, because we all have imperfect memories – and a video says a thousand words.  Hard copies are very nice to have because owners very much appreciate having the documentation of what was seen. 

During the initial examination, it is best to forgo sedation, as sedation will interfere with upper airway function, and can give the impression of pharyngeal collapse, and dysfunction of the upper airway.  A twitch is useful both to keep the horse in place and to control the movement of the head.  It is important to examine both nasal passages, even though the majority of the examination is done on one side, however, the caudal portion of the pharynx/larynx should be examined first, as it causes the least resentment from the horse. 

It is also important to perform the majority of the exam from the same side in each horse, as the view will always look a bit ‘skewed' from one side or the other.  When having your assistant pass the scope, make sure that they do not occlude the nostrils. The author has the assistant ‘pinch' the scope between the nostril and the thumb, as this will hold the scope securely in place, but will not occlude the horse's breathing. 

The larynx is examined first.  The larynx should be examined for integrity of the structure, to ensure that there is no swelling or anatomical abnormalitiy, and to ensure that moveable structures are in their correct place.  The epiglottis should have a scalloped edge, and its vascular pattern should be clearly apparent – failure to visualize the epiglottis at all indicates dorsal displacement of the soft palate, and ability to visualize the outlines of the epiglottis but failure to see its distinguishing features usually indicates epiglottic entrapment. 

The palatopharyngeal arch should not descend into the rima glottidis.  The vocal chords and saccules should be intact and clearly visible – otherwise, this may indicate that there has been a previous surgery.  The nose should be briefly occluded to assess whether the induced increasingly negative pressures result in displacement of the soft palate or pharyngeal collapse. 

A small amount of clean, distilled water should be discharged onto the pharynx, to assess swallowing function. Backing away from the larynx slightly, and aiming somewhat dorsally, the ethmoids are easily visualized. After examining the larynx, it is easy to briefly pass the scope into the trachea. The rings of the trachea should be clearly visible, there should be no mucus or pus, and there should be no evidence of edema or hyperemia. 

A ‘slap test' – which is performed by lightly slapping the horse on both sides of the whithers  to induce movement of the arytenoids – can be performed at this point.  It is important to remember, however, that this test is not adequate for grading laryngeal hemiplegia, rather it confirms merely whether the animal's arytenoids are capable of movement.  Arytenoids that are permanently in the open position, for instance, indicate prior surgery.

After examining the larynx and trachea, the guttural pouches should be inspected.  This should be done with sedation – there are a large number of neurovascular structures within the GP, and it is best to avoid damage. It is not sufficient to note discharge apparently emanating from the guttural pouches – miscellaneous mucus/pus often gets caught in this area, and is an unreliable indicator of whether GP disease is present.  The guttural pouches are easily entered by using either a closed biopsy instrument or the stilette from a stallion catheter as a guide. 

The endoscope is brought to the opening of the GP, and the guide is inserted at the dorsal-most aspect of the pouch.  After it has been advanced several centimeters into the pouch, the assistant rotates the endoscope to assist in opening the flaps and the endoscope is easily guided into the opening. If you have difficulties with this maneuver, change the angle of the horse's head.  It is easiest to enter the right GP from the right nasal passage and vice versa, but both can be approached form one side in many horses.  Within the guttural pouch, assessment of the internal and external carotid and maxillary arteries, retropharyneal lymph nodes, stylohyoid bone, stylohyoid-petrous temporal bone articulation, and surrounding musculature should be made.

 

As you exit the pharynx, a full assessment of the nasal passages should be made, especially looking for evidence of discharge from the nasomaxillary opening, seen with sinus disease.

Although many abnormalities can be diagnosed on resting endoscopy, most functional abnormalities require dynamic endoscopy (assessment of the upper airway using the treadmill).  Nasal occlusion will give an idea of functional abnormalities, as the pharyngeal pressures this achieves mimic those of peak exercise, nonetheless, there is not a clear correlation between DDSP during nasal occlusion, for instance, and that seen at exercise.  Nonetheless, horses that displace easily during nasal occlusion are at high risk for having displacement at exercise.   

Some disorders of the pharynx and larynx

Nasopharyngeal collapse

The nasopharynx is a muscular tube that is essentially without support, and relies on perfect innervation to prevent collapse.  Any inflammation, trauma, or neurologic disease that impairs this muscular function can, therefore, lead to nasopharyngeal collapse.  This is best assessed with dynamic endoscopy, but the nasal occlusion test can lead to suspicion of the disorder.  Severe lymphoid hyperplasia can also lead to this disorder. 

Dorsal displacement of the soft palate

This problem can occur because of abnormalities of innervation, or because of mechanical abnormalities, such as cysts, masses, or inflammation due to trauma or infection, or because of abnormal epiglottic anatomy, which then fails to remain dorsal to the soft palate.  The horse that has DDSP on resting endoscopy usually has severe neurologic dysfunction.  More commonly, DDSP is only seen on dynamic endoscopy, although horses that easily displace with nasal occlusion, or after being induced to swallow, are highly suspicious for this problem.  Interestingly, and contrary to intuition, airflow obstruction with soft palate displacement is during expiration, not during inspiration.

If a horse displaces easily on standing endoscopy, and has a history of making a snoring noise during exercise, with profound exercise intolerance in concurrence with this noise, then DDSP is highly likely.  In cases of suspected DDSP, it is useful to examine the subepiglottic tissue, as in chronic cases, ulceration may exist.  It may be necessary to use local anesthesia and a biopsy tool to displace the epiglottis to achieve a reasonable view of this tissue.  Finally, the guttural pouches should always be examined to look for concurrent inflammation, as the pharyngeal branch of the vagus nerve, which runs through the ventral aspect of the medial pouch, is responsible for proper function of the pharynx. 

Laryngeal disorders

The most common of the laryngeal disorders include laryngeal hemiplegia and arytenoid chondritis. 

Left laryngeal hemiplegia is by far more common than right-sided or bilateral conditions.   Although we know that this is due to a demyelination of the recurrent laryngeal nerve, it is not known why the left side is preferentially affected.  As this primarily affects large, long-necked horses, it may be due to increased length of the left recurrent v. the right recurrent nerve.  Disease on the right side is highly suspicious of trauma due to intravenous injection. 

Studies have shown that LH does not affect performance except in horses performing peak exercise, such as racehorses, thus it is often the noise rather than the function of the larynx that is a problem.  Many horses have a somewhat ‘lazy' left arytenoid at rest; treadmill endoscopy often shows this to resolve at exercise.  This is considered a grade II.  Grade III is distinguished by asymmetrical movements, but the horse is not able fully to abduct the artytenoids with exercise.  Nasal occlusion and swallowing can also help to reveal this defect, and high speed treadmill is probably not necessary to conclude that this is a problem in a high-speed performance horse. 

Arytenoid Chondritis is an inflammation/infection of the arytenoid cartilages, which can present as severe respiratory embarrassment.  Endoscopy reveals a misshapen, enlarged arytenoid.  Surrounding tissues often appear inflamed.