26 Jun 2017
Update on recurrent laryngeal neuropathy research in horses
Safia Barakzai looks at latest advances in treatment of this equine neurological condition.

A laryngoplasty pre-operatively, before tie-back (left), and postoperatively, after tie-back and cordectomy.
Research into recurrent laryngeal neuropathy (RLN) is moving at a fast pace, although much recent work has not translated into equine practice yet. Information about the likely genetic components of RLN, new methods for diagnosis of the disorder, better evaluation of existing treatments and development of novel treatments are discussed.
Research into recurrent laryngeal neuropathy (RLN) is moving forward at a fast pace, yet much has not translated into clinical practice.
This article aims to outline recently published and ongoing research in this field.
Aetiology
For a long time, a genetic component to RLN has been suspected, but with the completion of the horse genome sequence, several groups have been making progress in this field. However, difficulties exist in defining the RLN phenotype because of a large proportion of horses that are affected sub-clinically, and also because of progression of the disease, which can occur at any age.
Histopathological studies have shown a 100 per cent prevalence of left cricoarytenoideus dorsalis muscle changes in horses1. Whole genome scan approaches have found loci that have protective effects for RLN in warmbloods2, and a duplication on equine chromosome 10, which was noted in 10 out of 234 cases and 0 out of 228 breed-matched controls3.
A further study4 found a significant association between a locus on equine chromosome 3 and RLN-affected horses – this gene has previously been shown to affect body size in horses and it is already well documented RLN preferentially affects horses more than 15 hands high.
Improved understanding of the relationship between genetics, equine growth rate and RLN prevalence may significantly advance our understanding and management of this disease.
Diagnosis
Diagnosis of RLN in the clinical setting is still principally based on resting and exercising endoscopy, and large-scale studies5,6 have given us a good indicator of the likelihood of vocal fold and arytenoid collapse in horses with various resting grades of RLN.
Trans-cutaneous laryngeal ultrasonography7,8 has been shown to be a useful predictor to differentiate between horses with grade A laryngeal function at exercise compared to those with grades B and C.
The author will often use it pre-operatively to ensure a horse does not have a congenital abnormality of the larynx, or an endoscopically silent arytenoid chondritis, which may be mimicking a true idiopathic RLN.
Trans-oesophageal ultrasound9 is also reported to be quite reliable for predicting laryngeal function at exercise, although the equipment required is not available in most equine practices.
Update on existing treatments

Ventriculocordectomy with or without laryngoplasty (LP or “tie-back”) remains the mainstay of treatment for RLN in clinical cases. Some small adjustments in surgical technique have been described, but the simple anatomic description of the oesophageal diverticulum10 is probably the most groundbreaking revelation, which should decrease the incidence of postoperative dysphagia significantly.
Brandenberger et al‘s paper10 showed placement of the suture prosthesis at the muscular process for laryngoplasty very frequently (21 out of 25 specimens) resulted in penetration of the oesophageal adventitia and occasionally (4 out of 25) resulted in penetration of the lumen of this diverticulum if the surgeon had no specific knowledge of this structure.
With education about the anatomy of the upper oesophageal diverticulum, it is easy to avoid this structure and thus drastically reduce postoperative dysphagia and surgical site infection. The first papers11-14 showing results for exercising endoscopy of horses after laryngoplasty have been eye opening, and indicate other dynamic abnormalities are often present in horses after LP.
Similar findings have been shown for horses having unilateral ventriculocordectomy.
Novel treatments for RLN
Research groups15-18 are searching for a more physiologic method of restoring arytenoid abduction in horses with RLN, which may negate the many complications associated with LP surgery. Using an electronic pacemaker (functional electrical stimulation; FES) to stimulate the abductor branch of the recurrent laryngeal nerve has been shown to be successful in small numbers of experimental cases, but unresolved issues exist with regulation of racehorses that may potentially have these implants.
Additionally, effectiveness of FES in horses with severe neuropathy (grades 3.3 and 4) is reduced. A presentation of laryngeal re-innervation with a direct C1/C2 transposition18 (as opposed to nerve-muscle pedicle grafting, which has proven technically difficult in the hands of many equine surgeons) was given at the American College of Veterinary Surgeons meeting in 2016, and has shown good results in some cases.
When re-innervation and pacing are combined, results are thought to be more reliable (Perkins, personal communication).