Many horses of all types and ages have been observed to make an abnormal noise upon exercise. These horses and ponies are often referred to as whistlers or roarers. The noise produced can vary from barely audible to a loud roaring noise and is due to turbulent airflow through an abnormal airway. This finding is common in racing Throroughbreds and can be a cause of poor performance in athletic horses. In less severe cases, the problem may go undetected and may have no effect on the average horse’s athletic capabilities.
The larynx forms an essential part of the upper respiratory tract. The main function of the larynx is to prevent inhalation of food into the lower airway. Its opening comprises a pair of arytenoid cartilages that support the vocal folds and the epiglottis which provides a protective flap over the airway during swallowing.
The abnormal inspiratory noise which can be heard when affected animals are exercised is due to narrowing or asymmetry of the larynx. The correct term for this phenomenon is recurrent laryngeal neuropathy (RLN).
This upper respiratory disease occurs due to injury to the left recurrent laryngeal nerve (the nerve responsible for innervating the left side of the larynx). Causes of injury to this nerve are multiple and may include trauma to the neck, guttural pouch disease, strangles and neoplasia. In the majority of cases however, the cause remains unknown. Damage to this nerve results in the failure of abduction (opening) of the left arytenoid cartilage; this causes a decrease in airway diameter and disturbance to the flow of air along the upper respiratory tract.
The picture at the start of this factsheet shows an endoscopic view of a horse with severe RLN. The cartilage on the right side of the image (left side of the horse) will not open fully, resulting in a dramatic reduction in airway diameter.
RLN affected horses tend to have a reduced exercise capacity and make an abnormal respiratory noise on exercise. Your vet will palpate the larynx externally to see if he/she can feel any asymmetry of the muscles of the larynx. A slap test may be performed over the withers which reveals failure of contraction of the muscle responsible for opening the left side of the larynx in horses suffering from RLN.
The gold-standard diagnostic technique is endoscopic examination of the larynx. Endoscopy involves passing a fibre-optic camera via the nostrils and along the upper airway so that the larynx can be visualised directly. Asymmetry of the larynx and failure of opening of the arytenoids cartilages confirms the diagnosis of RLN. In mild cases, endoscopy during exercise may be necessary to appreciate the reduction in airway diameter, this can be performed either on a treadmill or in some institutes ridden endoscopic examination can be performed.
The only method of treatment available for RLN is surgery. It is important to understand that not all cases of RLN require treatment. Treatment is only indicated if the condition is severe and/or affects the horses performance, ie the horse has reduced exercise tolerance.
Many horses used for general purpose riding, dressage and showjumping suffer from RLN with no ill-effect as they do not need to exercise at maximal intensity. In contrast, RLN can severely affect those horses used for racing or endurance riding.
Surgery can involve a hobday (ventriculectomy) and/or a laryngeal tie-back procedure depending on the type and severity of laryngeal paralysis. Recently, some institutions have started to perform standing laser surgery of the larynx which avoids the risks associated with general anaesthesia.