Section: Information In-depth
Laryngeal hemiplegia is typically "idiopathic" meaning no precise cause is evident. In most cases the condition only affects the left side of the horses larynx. The left recurrent laryngeal nerve which innervates the cricoarytenoideus dorsalis muscle (CAD muscle), the muscle responsible for opening the left aspect of the larynx during breathing, undergoes spontaneous, nerve fiber loss. Initially this results in muscle weakness but as the nerve fiber loss progresses there is total loss of muscle function.
Although most often thought to have a genetic basis, the nerve can also be injured from inappropriately placed injections in the neck, infection within the guttural pouch, trauma to the neck, strangles abscesses, tumors, certain toxins, and central nervous system diseases. Any of these problems can result in injury to the right recurrent laryngeal nerve as well as to the left.
The larynx is comprised of several paired cartilages that fit together to form a hollow tube in which air can pass through. At the leading edge of this tube are the paired, right and left arytenoid cartilages. (Figure 1) These cartilages close completely together (adduct) when the animal swallows to protect the horse from aspirating feed or water. When the horse is exercising, the cartilages maximally open (abduct) to provide the largest diameter tube in which air can be transported through. Loss of nerve function, followed by loss of the CAD muscle, results in failure of the arytenoid cartilage on that side to perform normally. The arytenoid cartilage, and the vocal cord attached to it, collapse into the airway, thus causing a major obstruction. The reason why obstruction occurs during inspiration relates to the negative pressure in the airway that rises during inspiration to pull air in from the nose.
The symptoms that the condition causes include the following:
1) respiratory noise – soft whistle to "roar" as air moves past the collapsed arytenoid cartilage and vocal cord.
2) poor performance – exercise tolerance progressively declines as the function of the nerve and muscle progressively deteriorates. This may occur over several weeks to months.
3) impaired vocalization – affected horses may have an abnormal whinny, as their ability to tense the vocal cord is lost.
