It is estimated that 3-5% of young Thoroughbreds have left laryngeal hemiplegia, also known as roaring. In laymen’s terms this means that the nerve controlling the opening of the horse’s arytenoid cartilage is damaged. The arytenoid cartilages usually close over the trachea when a horse swallows, but they should quickly pop back open to allow the animal to breathe easily.

With laryngeal hemiplegia, one of the cartilages obstructs a part of the windpipe. Because of this, “owners usually complain of a roaring noise when riding at high speeds and progressive exercise intolerance,” said Michael Karlin, DVM, an equine surgery resident at the University of Illinois Veterinary Teaching Hospital in Urbana. He goes on to explain that the noise, which sometimes sounds like a whistle, occurs during inspiration as the cartilaginous obstruction causes turbulent airflow.

Veterinarians have not yet determined why the left side is more frequently affected than the right. Some hypothesize that the left recurrent laryngeal nerve that supplies the left arytenoid cartilage is longer, has a more tortuous route, and is thus more prone to damage. But according to Karlin, more than 90% of cases are idiopathic, meaning clinicians are unable to determine the cause of the nerve to malfunction.

In rare instances, a horse will present with a right-sided laryngeal hemiplegia. In contrast to the left-sided version, a disorder of the right nerve is usually related to an actual disease process, resulting in infection or inflammation. For example, equine protozoal myeloencephalitis (EPM), could cause transient laryngeal hemiplegia