“Roaring” is a term used to describe the observed respiratory symptoms in a horse afflicted by the medical condition called laryngeal hemiplegia. Somewhat rare, it affects mainly thoroughbreds and other highly exercised horses. Laryngeal hemiplegia is caused by nerve damage (paralysis) that results in one of the arytenoid cartilages failing to open and thus obstructing the airway. Arytenoid cartilage controls the function of the vocal cords. The problem is more common in the left cartilage, although it is not understood exactly why. Owners complain of a roaring or whistling sound from the animal when ridden at high speeds. When severe, the horse will become gradually more intolerant of exercise.
Almost all cases of left laryngeal hemiplegia are idiopathic in nature, the specific cause of the nerve damage eluding veterinarians. Rarer still, right laryngeal hemiplegia is almost always attributable to a secondary complication of an inflammatory disease or infection. Treatment for the former (left LH) may require surgery, while treating the latter (right LH) involves addressing the underlying disease mechanism.
All other causative disease possibilities ruled out, laryngeal hemiplegia is diagnosed by observation of clinical signs, a history of exercise intolerance, and laryngeal endoscopy. An endoscope is a rigid or flexible tube with a lens at the tip and an eyepiece or camera at the handle. The horse will require restraint in order to place the endoscope into the pharynx (back of the throat) and observe the laryngeal cartilages functioning during respiration. In a healthy animal without laryngeal paralysis, both sides of the cartilaginous “voice box” will open and close fully and at the same time. In laryngeal hemiplegia, one side will hesitate to open and obstruct the trachea during inspiration. In some cases, the endoscope will be passed through the nostril, into the pharynx, and fastened to the bridle. A digital video camera will be attached to capture the function of the laryngeal cartilage while the horse is conscious and at heavy respiration during exercise. This can be accomplished on a treadmill in the hospital or in the field by a wireless connection to an observation monitor.
Horses diagnosed with mild laryngeal hemiplegia can simply be retired from heavy exercise, and should remain comfortable. These animals should be especially protected from subsequent respiratory disease by appropriate vaccinations, avoiding exposure to known infections, feeding low dust diets, and providing good ventilation in stables. For more severe respiratory obstruction, surgical intervention may be considered. There are three procedures that may provide relief from symptoms. The first is called a Hobday operation wherein the obstructive part of the laryngeal tissue is removed, causing scarring which holds the larynx open. More modernly, a “tie-back” procedure can be done (usually in addition to a Hobday operation), using sutures to tack the larynx open. The scarring from the Hobday operation will help to hold the airway open should the tie-back sutures fail. Finally, a tracheostomy tube can be surgically and permanently implanted to circumvent the larynx entirely. This is rarely performed, as the wound in the neck through which the tube is permanently sutured will be constantly susceptible to infection and suture failure. Humane euthanasia should be considered in the most severe cases to prevent unnecessary suffering.
Differential diagnoses for symptoms of roaring include infections such as equine protozoal myeloencephalitis (EPM), laryngeal cysts and tumors, lymphoid hyperplasia (inflammatory lymph tissue enlargement of the pharynx), and Epiglottic entrapment, whereby the epiglottis’ normal movement is restricted by excess tissue (usually the soft palate).