Roaring – laryngeal hemiplegia
Roaring in horses can cause reduced performance. The official term is ‘laryngeal hemiplegia’, meaning total or partial paralysis of one side of the larynx.
The trachea of the horse doesn’t open into the back of the mouth as it does in humans. Instead, it opens into the back of the nasal passages (nasopharynx). In figure 1 a camera has been advanced up into the nasal passages to show the normal anatomy of the larynx.
During breathing it is very important that the airways are opened wide to facilitate as much airflow as possible. In horses with laryngeal hemiplegia, also called “roaring”, the left arytenoid cartilage and vocal cord aren’t able to open fully due to paresis or paralysis. This causes these structures to hang down and vibrate as air passes through, producing a typical roaring sound. Because they don’t open properly, airflow is partially obstructed, which causes exercise intolerance. Affected horses are asymptomatic at rest, but may have an unusual whinny.
Diagnosis is confirmed by endoscopy. The severity of laryngeal hemiplegia varies. The left arytenoid cartilage and vocal cord are positioned further towards the midline to different degrees, and/or reduced mobility may be observed. With complete laryngeal hemiplegia, the arytenoid cartilage and vocal fold are located in a median position within the laryngeal lumen and are immobile. If a definite diagnosis can’t be made in the standing horse, their laryngeal function can be evaluated endoscopically during exercise to confirm laryngeal dysfunction.
The two most common surgeries are a prosthetic laryngoplasty (a “tie-back surgery”), and/or a laryngeal ventriculectomy/ventriculocordectomy.
Prosthetic laryngoplasty (a “tie-back surgery”), in which the paralyzed cartilage is “tied back” into an open position through an incision in the throat latch area. This stabilizes the affected side of the larynx during inspiration.
Laryngeal ventriculectomy/ventriculocordectomy, in which the ventricle and vocal cord underneath the arytenoid cartilage are removed to widen the airway. 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 speed. The procedure may also be combined with a tie-back surgery.
Prosthetic laryngoplasty is commonly done in racing horses and is the only technique that satisfactorily reduces the impedance to inspiratory flow. Postoperative complications include chronic cough, aspiration of feed, implant failure, and implant infection. Athletic performance will improve after surgery; however, horses are more likely to experience inflammatory airway disease and exercise-induced pulmonary hemorrhage, have fewer race starts, and are unlikely to develop their predicted performance potential.