The Larynx.qxd:Jerkins feature.qxd
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EQUINE LARYNX
Left is an endoscopic photograph of a normal horse’s larynx at rest. Note the symmetrical arytenoids and vocal folds leading into the horse’s windpipe and the tongue-like epiglottis sitting in front of the larynx. Below is an endoscopic photograph of a horse at rest showing obvious laryngeal hemiplegia. Note the left side of the horse’s larynx (right as we view it) is hanging into midline and thus it will obstruct airflow when the horse is at exercise
yet only a prototype of this ‘over-ground’ endoscope exists but could this be the future of accurate diagnosis of equine wind problems? By far the most common condition that causes an abnormal inspiratory sound, and possibly the most common cause of any abnormal respiratory sound in the thoroughbred racehorse, is idiopathic left laryngeal hemiplegia (paralysis of the left side of the larynx). This condition is caused by degeneration of the nerve that supplies the left side of the larynx so that that it ‘hangs’ into midline causing an inspiratory ‘whistling’ or ‘roaring’ sound during cantering or galloping and thus obstructing airflow to the lungs. The cause of this nervous degeneration is not known but this again leads me onto yet another poor anatomical design point of the horse. The right laryngeal nerve has a simple route, branching off from the vagus nerve (which comes from the brain) travelling directly to the larynx. However, God decided that the left laryngeal nerve shouldn’t have it so easy and instead it must travel all the way to the heart, where it wraps around a large pulsing artery, before coming all the way back to the larynx. The left laryngeal nerve is also the longest nerve in the body and so it stands to reason that it is commonly damaged and perhaps unsurprisingly, there is also data to suggest that the bigger the horse, the greater its chance of developing laryngeal hemiplegia.
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