Equine Laryngeal Paralysis Research

Equine Laryngeal Paralysis Research Recent research sheds light on the cause of laryngeal paralysis. Recurrent laryngeal neuropathy (RLN) is a common problem of larger horses. It can have a significant effect on performance. Paralysis of the larynx, usually on the left side, results in interference to airflow. The arytenoid cartilage and vocal fold are sucked into the airway as the horse breathes in, often accompanied by a whistling or roaring noise. The recurrent laryngeal nerve is so named because of its lengthy course from the base of the brain to the larynx. Unlike other nerves that travel directly to the muscles they supply, it first passes down the neck, turns round the great blood vessels at the base of the heart, and travels back up the neck to reach the larynx. In an adult Thoroughbred it is about 250cm long. The left recurrent laryngeal nerve is longer than the right. What causes the damage to the recurrent laryngeal nerve? Is it a specific problem of that nerve - perhaps relating to its great length? Or is it part of a more generalised degenerative condition affecting other nerves as well?

Scientists at the University of Edinburgh looked for signs of damage to the recurrent laryngeal nerves and various other long nerves (phrenic, median and peroneal nerves) in three horses clinically affected with RLN.

Dr Caroline Hahn and her colleagues found significant damage to the left recurrent laryngeal nerve.  There was also atrophy of the muscle supplied by the nerve (neurogenic atrophy).

Mild to moderate degenerative changes were found in the right recurrent laryngeal nerve in two of the three horses.

On the other hand, none of the other long nerves that were sampled showed signs of damage. Neither was there any evidence of neurogenic atrophy in the muscles they innervated.

The recurrent laryngeal nerves are the longest nerves in the body and this study showed that the degeneration is specific for this nerve and not one that affects all nerves but is expressed more in the RLN because of its longer length.

Why does it matter whether the damage is confined to the recurrent laryngeal nerve, or is more widespread? If other nerves were affected  as part of a more generalised condition, then factors such as nutritional deficiencies and poisoning could be involved. But if no other nerves are affected it is unlikely that such factors play a part and it is more likely to be due to something specifically affecting the RLN, such as the nerve being stretched by the (excessively?) long neck of modern horses.

These findings "provide convincing evidence that RLN is not a polyneuropathy" the scientists report in the November 2008 edition of the Equine Veterinary Journal. Strictly speaking, they say, RLN should be categorised as a bilateral mononeuropathy - as both the left and right recurrent laryngeal nerves are affected.

Further work now needs to be done to identify the genetic and local factors that may influence the recurrent laryngeal nerves.

For more details see:

Histological and ultrastructural evidence that recurrent laryngeal neuropathy is a bilateral mononeuropathy limited to recurrent laryngeal nerves.
CN Hahn, K Matiasek, PM Dixon, V Molony, K Rodenacker, IG Mayhew.
Equine Vet Journal (2008) 40, 666 - 672

Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews  - Equine Science Update 2008

More Articles by Mark Andrews BVM&S CertEP MRCVS

Email this article to a friend or share

---

Voices for Horses offers news, articles and videos from third party providers unless otherwise stated. We do not necessarily endorse the views expressed or content and make it available to you only as a service. All news items, articles and videos are copyright of the authors, film makers and original publications.

<< Back to Advice and Information