Endoscopic examination of the normal equine upper airway at rest (Proceedings)

This paper will focus on the normal anatomy of the upper airway and manipulations of the endoscope to allow a thorough examination of the region in question. Adequate restraint of the horse allows for a controlled and complete endoscopic examination. This is not always achievable. When dynamic functional collapse of the upper airway is suspected from the horse's presenting history ideally the resting endoscopic examination is performed without use of sedation. Sedatives may alter nasopharyngeal and laryngeal movements and consequently affect the assessment of the airway. Twitch restraint will temporarily control most horses resistant to passage of the endoscope, however there are the infrequent horses where sedation is deemed necessary and should be used and taken into account when assessment of laryngeal and nasopharyngeal function is being made. There are many endoscopes available - a 100cm long, 8-10 mm diameter endoscope is suitable for the upper airway.

The operator of the endoscope should be standing in a comfortable position to readily look into the eye piece while the endoscope is being passed or to easily look at the television monitor and the horse in the case of videoendoscopy. One option is to stand diagonally off to the right front of the horse's head during videoendoscopy, with the person passing the scope positioned on the immediate right or left side of the horse's head. Most horses react to some degree to at least the initial passage of the scope. Use of local anesthetic sprays up the nasal passage or local anesthetic gels on the scope and applied to the entrance of the nasal passage may help limit movements. It is important to not "choke down" excessively on the end of the endoscope when directing it into the ventral nasal meatus. Allow at least 6 inches of length that can be readily passed into the nasal passage to get the endoscope in far enough in one movement. If the horse moves rapidly during insertion there is more chance of the endoscope staying in the nasal passage and not being flipped out if a reasonable length can be inserted quickly. The guiding hand should be positioned over the bridge of the nose with the thumb of that hand directing the endoscope medially and ventrally to enter the ventral nasal meatus. Do not inadvertently clamp down on the contralateral nares and obstruct airflow on that side. Hooking a finger or two in the contralateral false nostril can ensure that the airway remains open on that side. The endoscope should be able to be passed and guided equally well with left and right hands, according to the side of the horse's head one is standing. On the right side, pass with the right hand and guide with the left. On the left side, pass with the left hand and guide with the right. Before passing the endoscope it is worth reminding one's self of the positioning of the light source, the camera and the instrument channel at the end of the endoscope (Figure 1). The eccentric positioning of the instrument channel facilitates manipulations to enter the guttural pouch. In addition, turning the steering wheels to their maximal extents before passing the endoscope helps subsequently with knowing how to change directions once in the upper airway.

Figure 1. End of endoscope showing positioning, clockwise from top, of camera, flush nozzle, light, and instrument channel.

Nasal cavity

The endoscope should initially be directed into the ventral nasal meatus, similar to passing a nasogastric tube, unless pathology obstructs this passage way. If the endoscope is not directed ventrally and medially to enter this space, then it may end up in the nasal diverticulum (false nostril) or in the middle nasal meatus. The endoscope will pass ventral to the ventral nasal concha and move freely through to the nasopharynx in the normal 450 kg horse. Relatively smooth mucosal surfaces should be present with slight, fine dimpling or a porous-looking surface not uncommon. The nasal septum should lie vertically and not deviate one way or the other. This may be confirmed by not appreciating any subjective narrowing of the nasal passages. The transition from caudal nasal passage to the nasopharynx is anatomically referred to as the nasal choana. To fully examine the nasal passages the endoscope should be directed to look dorsally after assessment of the ventral nasal cavity. A "Y" shaped appearance marks the junction of the common nasal meatus with the middle nasal meatus, bound by the ventral and dorsal nasal conchae, and the nasal septum. Lying most dorsally is the dorsal nasal meatus, usually too narrow to pass a standard endoscope without significant mucosal trauma. Additional examination of the nasal cavity is achieved by passing the endoscope in the middle nasal meatus. This often happens anyway with failed attempts to direct the endoscope into the ventral nasal meatus. Following the middle nasal meatus is simple and rarely is there too much resistance to do so. The nasomaxillary opening is located at the caudal end of the middle nasal meatus, between the ventral and dorsal nasal conchae, and it is inspected for any signs of discharge arising from the paranasal sinuses.

Before passing into the nasopharynx the ethmoid turbinates are visualized in the caudodorsal nasal cavity. The prominent bulbous structure seen at the forefront of the ethmoid turbinates is the middle nasal concha, a much shortened version of its dorsal and ventral counterparts, and also referred to as the greater ethmoturbinate. When examining the ethmoid turbinate region, rotating the scope left and right and up and down to adjust the incident light pathway will allow deeper visualization of the recesses compared to only looking straight on. Early development of ethmoid hematomas can be missed when they are still in the caudal recesses of the turbinates unless creative angles are used to take full advantage of the endoscope's flexibility. Left and right nasal cavities should be examined in a standard upper respiratory track endoscopic examination. Often the history or clinical signs will suggest a problem more caudal than the nasal cavity and the one nasal passage used to pass the endoscope is barely looked at. Clinical conditions could easily be missed. Frequently it is easier to assess the rostral nasal passages on the way out rather than on the way in, due to typical horse resistance when passing the endoscope in.

Nasopharynx

The nasopharynx can be readily examined after passing the endoscope up the left or right nasal passage. This musculomembranous tube is a dynamic structure, even at rest, with changes in inspiratory and expiratory airway pressures and movements related to swallowing. When viewed from a rostral position in the nasopharynx, the white leading edge of the fibrocartilaginous flaps marking the pharyngeal opening of the auditory tubes (entrances to the guttural pouches) are readily seen on the left and right dorsolateral nasopharyngeal walls. During swallowing these flaps spring open in an axial direction to facilitate dependent drainage from the guttural pouches. Evidence of any discharge from the pouch openings should be ascertained and normally there should be none. The blind-ending pouch dorsocaudally on midline is the dorsal pharyngeal recess (where a stomach tube may get jammed). Pharyngeal lymphoid tissue may be prominently enlarged depending on the age of the horse or existent pathology. It is identified as a mucosal cobblestone or pimpled appearance, up to substantial grape-sized clusters of submucosal swellings, concentrated in the dorsal pharyngeal wall and spreading down the lateral walls.

The soft palate is examined and if possible the caudal free border is viewed for any evidence of ulceration or thickness. Passing the endoscope into the trachea often causes temporary dorsal displacement of the soft palate and a few seconds are available to assess the caudal border before the horse swallows and positions are normalized. Transient dorsal displacement of the soft palate is not uncommon during examination of the upper airway and in many instances is normal and should not be over-interpreted. Significance is attached to how long it takes a horse to correct the displacement by swallowing and how many swallows it may take, in light of any history suggestive of displacement occurring.

To visualize the caudal end of the nasal cavity and nasal septum, the endoscope can be passed into the nasopharynx and turned 180 degrees in the dorsal midline plane in one maneuver. This is useful when examining pathology located in the caudal region of one nasal passage and also when determining the primary side of a caudal nasal mass if it appears to be growing into both nasal passages (e.g. ethmoid hematoma). When doing the 180 degree turn, care is taken to go dorsally, because going ventrally could direct the endoscope into the oropharynx and risk damage from teeth.

Guttural Pouches

The endoscope is passed up each ipsilateral nasal passage for guttural pouch endoscopy, although with increased skill both pouches can be accessed from the one side. The pharyngeal openings are assessed for evidence of discharge before the endoscope is inserted into each pouch. Entrance is most easily obtained using a blunt-ended "guide wire" (e.g. an endoscopic biopsy instrument works very well) passed down the biopsy channel of the endoscope. To enter the left pouch, the endoscope is directed to lie against the left pharyngeal wall looking from a ventral to slightly dorsal aspect to direct the biopsy instrument under the fibrocartilage flap, heading in a dorsocaudal direction along the funnel shaped pathway to the pouch. The instrument will pass with minimal resistance if directed correctly and there is no obstructive pathology present (e.g. scarring or a mass). If it is passed too ventrally the instrument will snag on the ventral fold of the tunnel or deflect out into the caudal nasopharynx. Once the instrument has been inserted about 6 inches the endoscope is passed along the wire guide, and into the pouch, again directing the endoscope slightly dorsocaudally to follow the pathway into the guttural pouch. With the endoscope in the upright position on the left side the eccentrically positioned instrument is maximally opening the flap to facilitate passage of the endoscope. However, if there is difficulty passing the instrument into the left pharyngeal opening, the endoscope can be rotated 180 degrees (turned upside down) so the instrument passes intimately along the left nasopharyngeal wall and under the cartilage flap. Once the instrument is in the pouch, the endoscope is rotated back to normal upright position to abduct the flap, and the pouch is entered. The instrument ideally is withdrawn as the endoscope is passed along its length into the pouch, however, frequently the endoscope is passed in and the flexible instrument will partially curl inside the pouch with minimal concern for harm to structures in the area. Once in the pouch the instrument is immediately withdrawn back into the endoscope. At no time should the endoscope be forced into the guttural pouch.

Entrance to the right guttural pouch follows a similar technique with one significant difference: once the instrument has been inserted into the pouch with the endoscope positioned upright and against the right nasopharyngeal wall, the endoscope should then be rotated 180 degrees clockwise to allow for the eccentrically positioned instrument to medially abduct the cartilage flap. This allows for easier entrance of the endoscope. So, insert instrument, rotate the endoscope 180 degrees, see the cartilage flap pushed open by the instrument and pass the endoscope in, heading dorsocaudally, upside down. Once in the guttural pouch, withdraw the instrument and rotate the endoscope back 180 degrees to the normal dorsoventral viewing position.

If a biopsy instrument is not available to act as a guide wire transendoscopically, a Chambers mare catheter or uterine pipette (21" long) with a slight angle to its last 2-3 inches, can be inserted up the contralateral nostril, directed into the pouch opening under visualization and used to abduct the flap for endoscope insertion. Blind catheterization of the guttural pouches with these rigid instruments is also possible. The external landmarks to gauge the location of the pouch opening are the intersection of a tangential line from the medial canthus of the eye with a rostrocaudal line along the middle of the nostril. Usually sedation is necessary to minimize head tossing and allow atraumatic insertion of rigid instruments into the horse's nasal passage.

The guttural pouches are examined methodically. The main anatomical feature to work from is the stylohyoid bone dividing the pouch into smaller lateral and larger medial compartments. Laterally, the maxillary artery and vein are identified. Caudodorsally the stylohyoid articulation with the petrous temporal bone, i.e. the temporohyoid joint, is examined for size and shape. On the medial caudal wall lies the internal carotid artery and cranial nerves IX, X, XI, and XII and large branches. On the medial floor the retropharyngeal lymph nodes may be apparent beneath the mucosa and normally should not be bulging up into the pouch floor. On the ventrolateral wall of the medial compartment the external carotid artery is visible. Left and right pouches are separated on midline by a thin septum rostroventrally and the insertions of the rectus capitis and dorsal capitis muscles dorsally. The endoscope should be withdrawn slightly and turned ventrally to completely examine the floor of both compartments for masses or fluid accumulation. The normal pouches are lined by a glistening, clear, thin, pseudostratified ciliated epithelium containing goblet cells, with all the above mentioned structures visible through the membrane.

Larynx

The larynx is a dynamic structure best evaluated for complete function in real time when stressed by high intensity exercise (i.e. a high speed treadmill videoendoscopy or overground mobile endoscopic examination). However, at rest, information can be obtained from studying the movements of the larynx with normal airflow. The larynx is assessed for symmetry, form, and function, starting with the epiglottis and progressing to the corniculate processes of the arytenoid cartilages, and then evaluating the vocal cords, entrances to the laryngeal ventricles, and the inner surface of the laryngeal lumen. The dorsal palatopharyngeal arch is usually tucked out of view behind the corniculate processes but may be visible in some normal horses. The epiglottis sits dorsal to the soft palate and has varying "tone" and a scalloped or serrated margin with an easily visible vascular pattern on its dorsal surface. The shape of the epiglottis can be affected by sedatives. The aryepiglottic fold is ventral to the epiglottis and not immediately visible except for the triangular portion bridging between the caudal epiglottis and corniculate process of the arytenoid cartilage on each side.

Techniques to Test Pharyngeal and Laryngeal Function At Rest

Laryngeal and pharyngeal movements are assessed at rest by stimulating the horse to swallow and by simulating intense negative upper airway inspiratory pressures using nasal occlusion. The horse is stimulated to swallow by touching the epiglottis or adjacent pharyngeal wall with the endoscope or infusing water into the larynx via the endoscope. Full abduction of the normal larynx occurs directly after swallowing and this degree of abduction is similar to what is seen at maximal exercise intensities. Abduction movements of the arytenoid cartilages are graded from I to IV (with subclassifications for grades II and III). Grade I describes normal synchronous and symmetrical movements, with full abduction of the cartilages noted. Grade II describes full abduction on each side but asynchronous movements. For grade III, full abduction of the arytenoids is not seen on one or both sides and movements are asymmetrical and asynchronous, and with grade IV dysfunction there is no perceptible movement of the affected cartilage.

Nasal occlusion is performed for a variable period of time, often dependent on the temperament of the horse. The negative upper respiratory pressures generated by nasal occlusion are similar to those recorded when the horse is exercising at maximal heart rate. Movements of the larynx and nasopharynx are assessed during nasal occlusion. Importantly, it should be remembered, that correlation between what is seen at rest during nasal occlusion testing and what may occur during exercise for conditions like dorsal displacement of the soft palate and dynamic nasopharyngeal collapse can be weak.

A third simple test of laryngeal function is the "slap test" or thoraco-laryngeal reflex, which is dependent on intact spinal reflexes. When the dorsal chest wall is firmly slapped below the withers the contralateral arytenoid cartilage will adduct (flick axially). A lack of adduction on a particular side is suggestive of laryngeal hemiplegia however the slap test result is not correlated to the degree of laryngeal adductor myopathy.

Cervical Trachea

A one meter endoscope can pass readily into the cervical trachea. The trachea is assessed for symmetry along its lumen (any masses, any deviations, any narrowing?) and for the presence of any exudate/transudate. Most horses cough vigorously when the endoscope enters the trachea. Many will allow an effective exam without need for sedation, and the coughing can subside quickly. Whether the trachea is examined before or after the guttural pouches is a matter of preference and is based on evidence of abnormal discharge coming from either site. The endoscope should be passed into the area suspected to be most likely contaminated or infectious last of all.

References

Ducharme NG, Hackett RP, Fubini SL, Erb HN. The reliability of endoscopic examination in assessment of arytenoid cartilage movement in horses. Part II. Influence of side of examination, reexamination, and sedation. Vet Surg, 1991;20:180-4.

Lane JG, Bladon B, Little DR, Naylor JR, Franklin SH. Dynamic obstructions of the equine upper respiratory tract. Part 2: comparison of endoscopic findings at rest and during high-speed treadmill exercise of 600 Thoroughbred racehorses. Equine Vet J, 2006;38:401-7.

Holcombe SJ, Ducharme NG. Upper airway function of normal horses during exercise. In, Equine sports medicine and surgery: basic and clinical sciences of the equine athlete, Saunders Elsevier, Philadelphia, 2004, pp541-558.