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.