A precise cause for unilateral laryngeal dysfunction is only found in a minority of horses, for example due to damage to the recurrent laryngeal nerve by guttural pouch mycosis, perivascular irritant injections or nerve trauma during neck surgery. Mechanical reasons for unilateral laryngeal dysfunction can also be identified in horses with laryngeal dysplasia (4-BAD syndrome). The aetiology of bilateral laryngeal dysfunction can usually be found, such as when it occurs secondary to hepatic encephalopathy or immediately following general anaesthesia. However, the vast majority of cases of unilateral laryngeal dysfunction, either partial paralysis (hemiparesis) or total paralysis (hemiplegia), have no obvious aetiology and consequently these cases areusually diagnosed as suffering from recurrent laryngeal neuropathy (RLN) (synonyms: idiopathic laryngeal hemiplegia, 'roaring', 'whistling'), which is one of the most important upper airway disorders of horses.
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