Laryngeal Hemiplegia  is the most common cause of respiratory noise during exercise. It is by paralysis of one or both of the arytenoid cartilages due to lack of innervation which causes atrophy to the muscle that moves the arytenoid cartilage. The left arytenoid cartilage is the most common side affected (up to 95%). In a normal horse, the arytenoids allow maximal airflow into the trachea during abduction (the outward movement of the arytenoid cartilages to open the entrance into the trachea). Horses with laryngeal hemiplegia have paralysis of the arytenoid cartilage, which prevents them from abducting, thereby decreasing airflow into the lungs, because the arytenoid cartilage is hanging in the airway and physically impeding airflow, resulting in respiratory noise and exercise intolerance.  

The prosthetic Laryngoplasty is the most common treatment. The paralyzed cartilage is “tied back” into an open/abducted position through an incision in the throat latch area. This procedure can be performed under general anesthesia or on the standing sedated horse (see videos underneath).

Ventriculectomy/Cordectomy: The ventricle and the vocal cord (located under the arytenoid cartilage) is removed to widen the airway that is performed alone or along with a prosthetic laryngoplasty . This procedure alone can improve performance and decrease respiratory noise in draft breeds or in show horses that do not need to perform at high rates of speed. It is done under anesthesia through an incision under the jaw into the airway (known as a laryngotomy) or by using a laser passed through an endoscope (or “scope”) up the nostril. The standing laser technique is ideal for draft breeds that may have difficulty recovering from general anesthesia.

You can find more information about this pathology and its treatment in the attached article :

Recurrent laryngeal hemiplegia