Laryngeal hemiplegia is the paralysis of the larynx, specifically affecting the left arytenoid cartilage and vocal fold. This is caused by damage to or degeneration of the left recurrent laryngeal nerve, the longest nerve in the equine body, which courses through the left side of the neck and is responsible for muscle function.

This condition is more common in large breed horses, especially draft breeds, Warmbloods and Thoroughbreds. It is more common in male horses. It probably has a genetic component.

Damage to the nerve results in paralysis of the muscles that lift the left arytenoid cartilage, allowing the vocal fold (a flap of tissue within the larynx aka "flapper") to drape over and obstruct airflow through the windpipe. This reduces airflow into the lungs and creates turbulent airflow during exercise.

This results in upper airway obstruction in race and performance horses that can result in a roaring, whistling or snoring sound when exercising (especially at the canter and faster gaits), exercise intolerance and poor performance.

Paralysis of the right side or both sides is uncommon. Paralysis on the right side is usually due to trauma to the right recurrent laryngeal nerve, rather than degeneration.

Diagnosis is typically made with upper airway endoscopy. The severity of the condition is graded on a scale Graded from I (normal) to IV (lack of movement of one arytenoids). Subtle cases must be diagnosed with dynamic (treadmill) endoscopy.

Mild cases may not require treatment. More severe cases in performance horses, especially flat racing horses, may require surgery. There are different types of laryngeal surgical approaches, all of which are intended to remove the obstructing tissue and allow free airflow at inspiration, without causing the horse to aspirate feed or water into the airway.