Performing tieback surgery on a standing sedated horse is faster, easier, and more precise than the traditional surgery under general anesthesia. | Photo: Courtesy Steven E. Garib

Airway collapse during exercise, known as dynamic collapse of the airway, can affect performance horses of all disciplines, limiting their ability to breathe. Better known as laryngeal hemiplegia, or colloquially as “roaring” because of the abnormal sound it can cause, this condition most commonly affects Thoroughbred and Standardbred racehorses and working Draft horses.

Paralysis of the left arytenoid cartilage causes the vocal folds to deviate inward and hamper air flow in and out of the lungs during work. The result is poor performance from inadequate air exchange and tissue oxygenation during exercise. The restriction in airway diameter near the vocal cords is what causes the airway noise. This paralysis of the arytenoid cartilage results from denervation to the cricoarytenoideus dorsalis (CAD) muscle, which is responsible for opening the larynx. Without appropriate laryngeal musculature, the larynx cannot open sufficiently. As such, the increased inspiratory pressure causes further airway collapse.

To diagnose laryngeal hemiplegia, veterinarians use an endoscope (a slender tubular instrument with a video camera passed through the horse’s nostril) to examine the larynx. There are two types of endoscopy: static and dynamic. Static endoscopy occurs in the standing unsedated horse. Dynamic endoscopy involves working the horse either on a treadmill or on a track or arena with the endoscope in place so the veterinarian can see laryngeal action in real time during work