Roaring is the term commonly used to describe an abnormal breathing noise that is caused by partial or complete paralysis of one, or rarely both, sides of the horse’s larynx (“voice box”). It’s a distinctive sound heard when the horse breathes in, rarely audible at rest but getting louder the heavier the horse breathes, e.g. with exercise. The noise is a high-pitched, whistling roar. The horse’s neigh may also change. The veterinary/ medical term for the condition is laryngeal hemiplegia.
Roaring is caused by damage to the recurrent laryngeal nerve (RLN), which supplies the muscles of the larynx responsible for opening and closing the vocal cords and cartilage inside the larynx. It’s most common in large breeds, including drafts, and in long-necked horses. The nerve is extremely long, the longest motor nerve (nerve supplying muscle) in the horse’s body. It originates in the brainstem, travels down the neck and into the chest before it changes direction and goes back up the neck to finally end on the muscles of the larynx.
The nerve on the left side travels further into the chest before backtracking, and can be as much as nine feet long. The nerve on the right side is about a foot shorter.
Anything that damages the RLN can cause roaring. Damage to the nerve cell body itself, inside the brain, can be caused by any inflammatory/infectious process in the brainstem. EPM is a common cause of this.
Damage to the nerve axon, the long tentacles that run from the nerve body in the brain to the muscles of the larynx, may be caused by tumors, abscesses, enlarged lymph nodes, accidental injection of drugs outside the jugular vein in the neck, severe guttural-pouch infections, trauma to the nerve during surgical procedures or from wounds, and by chick-pea or lead poisoning. However, these causes are most likely to be found when it is the right-side nerve that is involved.
Paralysis on the left side is far more common and in most cases is termed “idiopathic,” meaning no cause can be found. The hypothesis is that the extremely long length of the nerve makes it difficult for the parent cell to keep it properly nourished.
The roaring sound, and when it occurs in the respiratory cycle (during inspiration), is so distinctive that most veterinarians can diagnose a roarer with a high degree of certainty by that alone. It’s also often possible to feel an obvious difference in the amount of muscle along the larynx. Scoping the horse confirms the diagnosis.
Endoscopy, or scoping, also allows the degree of paralysis to be graded, which can help in making decisions regarding treatment. In early cases, the findings on scoping may not be conclusive. The exam can then be repeated in a few weeks, or in a clinic or university setting the horse can be worked on a high-speed treadmill and then scoped. Work on a high-speed treadmill, or scoping immediately after a fast work, is also often recommended for high-performance horses, since it gives a more accurate indication of how much the condition is compromising their ability to breathe.
Although even low grades of laryngeal paralysis can result in dramatic, obvious breathing noises, this doesn’t necessarily translate into performance problems. In fact, horses can continue to race with up to grade 3 (on a 5-point scale) problems. Even horses with grade 4 or 5 paralysis may do fine as pleasure, show or lower-level event horses.
The first option is to do nothing if the horse can still perform comfortably. If the problem is interfering with performance, or the noise isn’t acceptable, there are several surgical options to pick from.
Our table describes what each procedure is most likely to help and drawbacks. All surgeries come with inherent risks. Success is also proportional to the experience of the surgeon with any particular procedure. Be sure to have an in-depth discussion with the surgeon and get a second opinion.