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Laryngeal Hemiplegia in Horses

Next to the vocal cord area of the larynx are structures called the arytenoids (pronounced ah-ret-e-noids). The arytenoids open and close in a vertical manner and are at the window to the windpipe. During inspiration, the arytenoids open wide and allow for maximal air flow through the laryngeal area into the trachea. The arytenoids are moved by a series of small muscles surrounding them under control of the laryngeal nerves. The right laryngeal nerve has a relatively straight shot from the brain to the larynx, but, due to some fluke of nature, the left laryngeal nerve travels from the brain all the way down the neck, flips around one of the major blood vessels near the heart, and travels all the way back up the neck before finally connecting to the muscle responsible for opening the left arytenoid.

A) Normal image of the larynx, horse. B) Laryngeal hemiplegia, horse. Illustration by Dr. Gheorghe Constantinescu. Drawn, with permission, from a slide courtesy of Dr. D. A. Wilson, College of Veterinary Medicine, University of Missouri.

Additional information

At laryngeal hemiplegia a neurologic disorder of the laryngeal nerve causes for a decreased ability for abduction at inspiration. Laryngeal hemiplegia, also known as roaring, is a condition of the upper respiratory tract that results in respiratory noise and decreased performance in the horse. This condition is most commonly reported in the racing Thoroughbred, but also occurs in other large breed performance horses including Warmbloods, Draft horses, and Standardbreds. It is estimated that about 8% of large sport horses are affected with laryngeal hemiplegia.

Risk Factors
Often horses diagnosed with laryngeal hemiplegia have no known cause for developing this condition. Although the exact cause of recurrent laryngeal nerve degeneration is unknown, several factors have been implicated in some cases. These include trauma to the cervical (neck) region, prior neck surgery, perivascular (around, but not into the vein) injection of an irritating drug, esophageal obstruction/rupture, guttural pouch infections, organophosphate intoxication, lead poisoning, Streptococcus equi infection (strangles), various diseases of the central nervous system, and thiamine deficiency. Involvement of the right arytenoid is usually associated with direct trauma to the nerve, including perivascular injection on the right side of the neck.

Idiopathic Laryngeal Hemiplegia
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