URBANA, Ill. — It is estimated 3-5 percent of young thoroughbreds have left laryngeal hemiplegia, also known as roaring.
In laymen’s terms this means 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 breath 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, an equine surgery resident at the University of Illinois Veterinary Teaching Hospital in Urbana.
Sounds like a whistle
He goes on to explain 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 the left recurrent laryngeal nerve that supplies the left arytenoid cartilage is longer, has a more tortuous route and thus is more prone to damage.
But in reality, Karlin said that, “more than 90 percent 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, the much-feared Equine Protozoal Myeloencephalitis, could cause transient laryngeal hemiplegia.
Other unusual sounds
While roaring is a classic noise to most equine veterinarians, there are other unusual sounds horses may make that can be confused with other disorders.
“To diagnose laryngeal hemiplegia we need to use an endoscope to conduct an upper airway exam,” noted Karlin.
With a snake-like video camera a veterinarian can visualize the opening to the trachea and see if one of the cartilages is not fully abducted, meaning it is partially covering the entrance to the windpipe.
In most cases, horses only make the roaring noise at high speeds. But since the disease is progressive, the problem will become worse with time.
Few different procedures
To correct the position of the misplaced cartilage, surgeons can try a few different procedures.
“We most commonly do what is called a ‘tie-back,'” said Karlin.
This surgery entails placing a suture into a certain part of the cartilage so the airway remains open as the horse exercises.
It requires general anesthesia, but there are several other types of procedures available depending on the severity of the condition and what is the intended use of the horse.