"Endoscopy is the primary diagnostic tool for evaluation of the upper airway of the horse," said Dr. Brett Woodie of Rood and Riddle Equine Hospital, at the clinic's annual client education seminar held Jan. 5 in Lexington. He reviewed the history of endoscopy and its current use at rest and the advantages of dynamic examination of the upper airway of horses that are exercising on a high-speed treadmill.
Reasons a horse owner would want to have his horse's upper respiratory tract scoped by a veterinarian include exercise intolerance, abnormal respiratory noise, nasal discharge, nosebleeds, coughing, abnormal swelling of the head or neck, or as a screening test. Endoscopic exams are performed at rest on Thoroughbred yearling sales prospects.
Fiberoptic endoscopy was available in the early 1970s that allowed veterinarians to view internal structures of the airway. However, video endoscopy became available in the early 1980s and allowed veterinarians to keep records of what they saw and compare images over time and to other horses, as well as showing the images to clients.
It is extremely important to have a capable horse handler during an endoscopic exam, said Woodie. He recommended using a twitch or gum chain for restraint. It is best to try to avoid using sedation prior to an endoscopic exam because it can affect the normal function of the larynx and pharynx.
When performing endoscopy at rest, Woodie said he looks for structural abnormalities through both of the nasal passages, and watches the horse swallow and respond to nasal occlusion (achieved by covering the horse's nostrils). "A horse can generate a tremendous amount of negative pressure (during occlusion)," he said, which can reveal how the larynx and pharynx might behave during exercise.
Woodie showed attendees examples of endoscopic examinations of normal and abnormal airways. He presented examples of epiglottic entrapment, laryngeal hemiplegia (paralysis of one side of the larynx, also called "roaring"), dorsal displacement of the soft palate (DDSP), and how a granuloma of the arytenoid (a chronic inflammatory lesion) limits airflow.
In horses when it is not known if partial paralysis of one side of the larynx will be performance-limiting, in those which show poor performance or make respiratory noise but have a normal endoscopic exam at rest, or in those that have an abnormality at rest that might affect airflow, a dynamic (at exercise) exam is suggested. While it can be difficult to see problems with an untrained eye on an endoscopic examination performed on a high-speed treadmill, valuable information can be gathered from the exam by an experienced veterinarian about how the horse's airway responds during exercise.
The horse can wear full tack during the dynamic exam, and a Standardbred can be in full harness and bridle with a trainer or owner holding the lines. Proper patient preparation is necessary. The value of the exam will be limited unless the horse is physically fit. The horse should be wearing protective equipment (shoes and protective bandages, bell boots, or wraps), and he must be acclimated to running on the treadmill.
"This takes a little time," said Woodie, "Most horses seem to pick it up right away."
A cooling source (fan) near the treadmill must be supplied since horses don't have air rushing past their bodies for cooling as they would during normal exercise, and their body temperatures can soar to 108°F during maximal exercise.
Woodie said that diseases such as epiglottic reversion (an epiglottis that is flipping backwards) and axial deviation of the aryepiglottic folds (tissue spanning from the arytenoid to the epiglottis that collapses across the opening to the trachea) cannot be seen during an at-rest endoscopic exam. Some abnormalities like DDSP, can cause a terrible upper airway obstruction and this disease, since it is intermittent, can be difficult to accurately diagnose at rest.
"Endoscopy on a high-speed treadmill is the gold standard for identifying dynamic abnormalities of the upper airway, and to determine if intervention is necessary, and for the selection of the proper intervention," said Woodie. "The fatigue factor (of airway structures) is not covered with at-rest exams. You may not see any upper airway abnormalities at rest, but that might not be an indicator of what the airway is doing during high-speed exercise.
"The resting exam can push us toward a dynamic exam to learn more about how the upper airway is functioning at exercise."