Answered by, Elizabeth Weber, DVM, Ocala, FL
Courtesy of AAEP
How can a thoroughbred race horse “scope” perfectly and still have breathing problems?
Answer: There are two possible reasons why a horse may have a “clean” or “perfect” scope reported, and yet be found to have breathing issues. The first is that a scope, much like a pre-purchase exam, is only one snapshot in time. While certain types of breathing issues may be congenital (present from birth), others may develop over time, due to factors such as illness, injury, or inflammation. So, the results can change on a horse from one scope to the next. For example, a horse with a traumatic injury to the recurrent laryngeal nerve could show paralysis of one arytenoid cartilage, even if his arytenoid function was entirely normal the week prior. Likewise, a horse with an upper respiratory infection may intermittently displace his soft palate due to inflammation, even if he showed no evidence of displacement on previous scopes. Medications, especially sedatives and tranquilizers, can influence how a horse scopes as well. So, a perfect scope today does not guarantee the same result tomorrow!
The second reason is due to the difference in circumstances between when the horse is scoped and when the horse is performing. Most endoscopies are performed at rest – when the horse is standing in the stall. Most of the time this is a good approximation of the horse’s airway function during work, especially if the scope is performed fairly soon after the horse is exercised, but there are cases in which the horse’s resting scope does not reflect how he breathes during work. As the horse breathes faster, airway dynamics cause an increase in negative airway pressures, which can affect function. Issues such as pharyngeal wall collapse are generally only seen under these conditions, and so are not observable on a resting scope. Additionally, studies have shown that airway dynamics can be influenced by head carriage, meaning that the way a horse is ridden can influence his airway function during work. So, a horse that shows no abnormalities with his head and neck relaxed in the stall could paint a very different picture when ridden in a tightly controlled frame.
For horses that fall into the second category, with a normal resting endoscopy but signs of breathing issues during work (primarily, abnormal respiratory noise or poor performance), a dynamic scope may clarify the situation. During a dynamic scope, an endoscope is secured to the horse as it trains and video imaging is relayed to a screen, so that the veterinarian can evaluate the airway function under the conditions of training. This can be done with the horse on a treadmill in a clinic setting, or with the rider on board in the horse’s regular training environment, and may reveal issues that are not appreciable at rest.
I hope that helps explain why you might find the apparent contradiction of a horse that was reported to have a normal scope, and yet seems to have breathing issues. I suspect you’re not the only person to wonder how this could occur!