Recurrent laryngeal neuropathy (RLN), also known as idiopathic laryngeal hemiplegia (ILH), is by far the most common cause of laryngeal dysfunction in horses. It causes partial or total paralysis of the muscles which open the larynx during inspiration. Affected horses make a characteristic noise when breathing in ("whistling" or "roaring") and are less able to perform strenuous work. Opinions vary as to whether the condition is present from birth or is acquired early in life, but it is usually first detected when the horse starts training. Most authorities agree that, once present, the condition does not deteriorate further.
Evidence was revealed at the World Equine Airways Symposium recently to challenge that view. Prof Dixon and his team at the Royal (Dick) School of Veterinary Studies, Edinburgh, presented the results of a study which showed that RLN can develop and progress in adult horses. They found that laryngeal function can deteriorate, sometimes slowly and progressively, but sometimes suddenly and dramatically.
They examined the records of 351 confirmed cases of RLN in older horses of mixed breeds. In 52 cases (about 15%) there was evidence that laryngeal paralysis had progressed.
All horses were examined using an endoscope on at least on occasion. The degree of laryngeal paralysis was graded on a scale of 0 (normal) to 5 (total paralysis).
In 30 cases there was both endoscopic and clinical evidence that the condition had progressed. These horses had been examined initially, and over the course of the clinical deterioration. Seventeen of these horses were normal (grade 0 or 1) on first examination. On subsequent examination the RLN had deteriorated by an average of 3 grades.
In the remaining 22 cases, endoscopy was only carried out after the deterioration had occurred. There was, therefore, only clinical evidence of progression. This included the sudden onset of respiratory "noises" (16 cases) or such sounds getting worse (6 cases) and associated poor performance. When they examined these cases endoscopically they found that 13 of the 22 horses had marked RLN (grade 4 or 5). It was apparent from the performance history of these horses that the condition had only recently developed.
They quote one notable example in a 6 year old National Hunt racehorse. The horse had an excellent racing record, with no history of respiratory "noises". Over a 5 month period it had developed progressively more noise and increasing exercise intolerance. Endoscopic examination showed total paralysis (grade 5) which was incompatible with the previous racing record. The horse could not have achieved its racing success if the laryngeal paralysis had been present at the time.
According to Prof Dixon, these findings are particularly relevant in the context of examinations for purchase. "In three of the cases the recorded deterioration occurred within 10 weeks of purchase, and in 2 cases it was the source of conflict between purchaser and vendor."
"It should be considered that differences in clinical and endoscopic findings between vets who perform examinations before and after purchase, perhaps just weeks apart, may simply reflect clinical progression of RLN, rather than inter-assessor variation or, has been alleged in some such cases, negligence by the vet who performed the pre-purchase examination."
The findings are also relevant when considering the management of less severe cases of RLN. These are often treated by being "Hobday-ed", a relatively simple procedure. However, Prof. Dixon warns that if such cases appear to suffer from progressive RLN, it may be better to treat them by laryngoplasty ("tie-back") initially, rather than have to subject them to a second general anaesthetic and surgery if the Hobday operation is not sufficient.
Source: World Equine Airways Symposium, Edinburgh. 2001