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Overground endoscopy is currently considered the definitive method to diagnose recurrent laryngeal neuropathy.
DIAGNOSIS OF L A R Y N G E A L P R O B L E M S: HO CU S PO CUS O R CU T T I NG - EDG E SCIEN CE? Celia M. Marr, Equine Veterinary Journal
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ecurrent laryngeal neuropathy (RLN) is the correct term for the condition better known as roaring or laryngeal hemiplegia. It is extremely common in Thoroughbreds and represents one of the major causes of poor performance or jockey-reported noise. Because it is so important, many young horses are scoped at sales looking for laryngeal asymmetry to try to identify those that may be at risk of having this condition. But scoping at rest is fraught with difficulty – the larynx may be normal at rest only to show signs of weakness during exercise, so positive cases can be missed. Conversely, tired
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young horses can have apparently poor laryngeal function at rest that in fact is of little significance. Many question whether subjecting foals and young horses to endoscopy at sales is reasonable, although in part this latter concern is reduced now that videoendoscopy is available. Furthermore, the dynamic endoscopy, overground or on a treadmill, is widely accepted as the best way to evaluate horses suspected of having upper airway disorders leading to dynamic obstruction of the airway during exercise, a population that may or may not have obviously abnormal throats when examined at rest. Nevertheless, there is a need for better tools to evaluate the larynx for clinical application and also to allow researchers to study the condition in more detail. Two recent studies published in Equine Veterinary Journal have addressed this issue.
What is recurrent laryngeal neuropathy?
RLN is actually due to damage to the nerves supplying the larynx rather than being an abnormality of the structure of the larynx itself. The larynx is a cartilage structure which is opened and closed by a collection of muscles that encircle it. Damage to the laryngeal nerves leads to atrophy of the main laryngeal adductor, the cricoarytenoid lateralis (CAL) muscle; and the main arytenoid abductor, the cricoarytenoid dorsalis (CAD). Atrophy (or wasting) of the left CAD muscle means that it can no longer open the arytenoid cartilage during inspiration. This in turn causes dynamic airway collapse at exercise, reducing the amount of air entering the lungs and, if severe, reducing the arterial oxygen concentration. Traditionally, the diagnosis of RLN has been made using endoscopy to determine the degree of arytenoid abduction or asymmetry.