Roaring is a common name for a condition called laryngeal hemiplegia, where one side of a horse’s airway becomes partially paralyzed and collapses inward during breathing. It affects up to 18% of horses and gets its nickname from the harsh, whistling or roaring sound the horse makes when exercising at speed. The left side of the airway is affected in the vast majority of cases.
What Happens Inside the Airway
A horse’s larynx works like a gate. Two pieces of cartilage (called arytenoid cartilages) swing open when the horse breathes in, creating a wide channel for air. A long nerve called the recurrent laryngeal nerve controls the muscle responsible for pulling these cartilages open.
In roaring, that nerve gradually degenerates. As it loses function, the muscle it controls wastes away, and the cartilage on the affected side can no longer open properly. Instead of swinging wide during a breath, it sags inward along with the attached vocal fold. This narrows the airway opening, forcing the horse to work harder to breathe. The turbulent airflow over the collapsed tissue is what creates the characteristic roaring noise.
The left side is almost always the one affected, and the reason is surprisingly simple: the left recurrent laryngeal nerve takes a much longer path through the body, looping all the way down around the base of the heart before traveling back up to the larynx. That extra length makes it more vulnerable to degeneration.
Which Horses Are Most at Risk
Roaring is most common in tall, large-breed horses. Thoroughbreds, Warmbloods, and draft breeds are disproportionately affected. The logic follows the same principle as the left-versus-right nerve difference: taller horses have longer nerves, and longer nerves are more susceptible to this type of degeneration. Researchers at University College Dublin recently identified specific genes associated with the disorder, confirming a hereditary component.
The condition is progressive. A horse may start with subtle changes detectable only on veterinary examination, then develop audible noise and exercise intolerance over months or years. Some mildly affected horses never progress to a point where performance suffers, while others deteriorate enough to need surgery.
Signs You Might Notice
The most obvious sign is abnormal respiratory noise during fast work, typically a high-pitched whistle or deeper roar heard when the horse inhales. At a walk or trot, the airway demand is low enough that many horses seem perfectly normal. The noise and breathing difficulty show up at canter and gallop, when the horse needs maximum airflow.
Beyond the sound itself, you may notice your horse tiring more quickly than expected, struggling to maintain speed, or seeming reluctant to work at higher intensities. In racehorses, the first clue is often a decline in race times or finishing positions rather than any noise the rider can hear over the wind and hoofbeats.
How Roaring Is Diagnosed
Diagnosis starts with a resting endoscopic exam, where a small camera is passed through the nostril to visualize the larynx. The vet watches how the arytenoid cartilages move, looking for asymmetry or sluggishness on one side. However, resting exams can miss a lot. One large study of 291 horses found that 49% of horses with a completely normal airway at rest had abnormalities that only appeared during exercise.
For this reason, many veterinary hospitals now use high-speed treadmill endoscopy, where the horse gallops on a treadmill while the camera records airway function in real time. This reveals whether the cartilage actually collapses under the demands of hard exercise and catches cases that would look normal in the barn aisle.
The Grading Scale
Vets use a standardized four-grade scale to describe how well the larynx functions:
- Grade I: Both sides move symmetrically and open fully. Normal function.
- Grade II: Some asymmetry or delayed movement, but the affected side can still achieve and maintain full opening. Many Grade II horses perform without noticeable problems.
- Grade III: The affected side cannot achieve or maintain full opening. This grade has three sub-levels ranging from occasional partial opening to near-total loss of movement. Most horses that need surgery fall here.
- Grade IV: Complete paralysis. The cartilage and vocal fold on the affected side do not move at all.
Surgical Options
Horses with Grade III or IV function that are expected to perform athletically typically need surgery. The two main approaches work in fundamentally different ways.
Tie-Back Surgery (Prosthetic Laryngoplasty)
This is the most common procedure. The surgeon permanently sutures the paralyzed cartilage in an open position, holding the airway wider so the horse can breathe freely during exercise. It’s effective but involves a trade-off: because the cartilage is propped open at all times, the airway loses some of its ability to close during swallowing.
Overall success rates range from 50% to 90%, depending on what the horse is asked to do afterward. Pleasure horses and show horses tend to do well because their airflow demands are moderate. Racehorses have somewhat lower success rates because they push their respiratory systems to the limit. In one large follow-up study, 77% of horses raced at least once after surgery, and 56% of those that had raced before showed improved performance. A smaller, more recent review found even better numbers: 94% raced at least once, and 60% won at least one race post-surgery.
The main downside is coughing. Up to 43% of horses cough in the immediate postoperative period, and about 14% develop a chronic cough related to small amounts of food or saliva entering the airway during meals. True aspiration pneumonia, where inhaled material causes a lung infection, is thankfully rare.
Nerve Reinnervation
Rather than mechanically propping the airway open, this approach grafts a healthy nerve segment onto the wasted muscle, gradually restoring its ability to contract on its own. The advantage is that it preserves the natural architecture of the larynx, which significantly reduces the risk of swallowing-related complications like chronic coughing.
Muscle activity can return as early as eight weeks after surgery, though full recovery takes longer. Early research in Standardbred racehorses showed the procedure could return affected horses to their previous performance levels. Because this technique doesn’t permanently alter the airway’s ability to close, it avoids many of the 16-plus documented complications associated with tie-back surgery. The trade-off is a longer timeline before the horse is ready to return to full work.
Vocal Cord Removal (Ventriculocordectomy)
This procedure removes the collapsed vocal fold and the tissue pocket beside it, reducing the obstruction without repositioning the cartilage. It can be performed through an endoscope with the horse standing under sedation, which makes it less invasive than the other options.
In a study of 24 horses that underwent this procedure, 83% had their exercise intolerance resolve, and 75% had complete resolution of abnormal airway noise. However, recurrence is a concern. Among horses whose noise came back, it returned within an average of seven months. This procedure is sometimes combined with tie-back surgery for horses that need maximum airway improvement.
Living With a Roarer
Not every horse with roaring needs surgery. Horses in light work, trail riding, or lower-level competition often manage fine with a mildly compromised airway. The noise can be startling if you’re not expecting it, but Grade II horses in particular may never show performance limitations.
For horses that do need intervention, the choice of surgery depends on the horse’s job, the severity of the paralysis, and the acceptable risk of complications. A racehorse chasing prize money faces a different calculation than a dressage horse or a pleasure mount. Your vet’s grading of laryngeal function, ideally confirmed with exercise endoscopy, is the foundation for that decision.