Disorders of the Larynx in Horses
The larynx is the part of the throat often called the “voice box” in humans. It is located near the top of the trachea. The larynx is composed of muscles and cartilage, and it includes the vocal cords. Several disorders of the larynx are seen in horses.
In horses, fluid buildup and swelling of the larynx can lead to arytenoid chondritis, a condition that causes swelling in the cartilage in the larynx. It is believed to result from microbial infection, often following the inhalation of irritants. Initially, there is often severe inflammation. Later, there is progressive enlargement of the cartilages that results in a fixed upper airway obstruction; this causes high-pitched breathing sounds and reduced exercise tolerance. Arytenoid chondritis occurs most often in young male horses. Thoroughbred horses in race training are more likely than other breeds to develop this condition. Removal of some of the cartilage is an effective remedy, although a competitive horse may not be able to return to full athletic capacity.
Laryngeal hemiplegia is a condition in which paralysis of the larynx occurs. For reasons that are not completely understood, this occurs more commonly on the left side than on the right side or on both sides. The nerve that controls the left side of the larynx is the longest nerve in the body. Paralysis is caused by a gradual deterioration of nerve fibers in the larynx, leading to deterioration of the associated muscles. Because muscle control is lost, the cartilage in the larynx can collapse, reducing the size of the airway through which the horse breathes. The increased resistance to airflow makes breathing more difficult and noisy when the horse is breathing air in. When the horse inhales during strenuous exercise, the airway can be essentially blocked.
The condition develops in young horses and is familial in most cases. Less common causes include injury to the nerve, accidental injection of irritating substances, and plant (for example, Cicer arietinum [chick peas] and Lathyrus subspecies) or chemical poisoning. If a horse has laryngeal paralysis on both sides, lead poisoning may be the cause. Although all breeds are affected, there is a higher prevalence in males, long-necked breeds, and larger breeds.
The most common signs of laryngeal hemiplegia are noise when inhaling during exercise and intolerance to exercise. Affected horses show no signs when at rest except for an unusual whinny in many cases. Using an endoscope, the veterinarian can observe reduced movement (or no movement) of specific tissues in the larynx.
Surgery can be done to stabilize the affected side of the larynx, which will prevent collapse of the airway during exercise. This is commonly done in racing horses and is the only technique that reduces the blockage during inhalation. Potential complications after surgery include chronic cough, chronic inhalation of feed, implant failure, and implant infection. Athletic performance will improve after surgery; however, horses may not reach (or return to) their full performance potential.
Fourth branchial arch defect is a condition in which one or more structures of the larynx have not fully developed. Incomplete development may occur on one or both sides. Right-sided defects are more common than left-sided defects or defects of both sides. The severity of clinical signs is based on the extent of the defect. The most common sign is respiratory noise, although coughing and mild difficulty swallowing have been reported. Your veterinarian may be able to feel the defect when examining your horse’s neck, or detect it using radiography or endoscopic examination. Affected horses are unlikely to become effective athletes.