Treatment options for RLN (Recurrent Laryngeal Neuropathy)

Recurrent Laryngeal Neuropathy (RLN) remains a major cause of poor athletic performance. RLN is a distal axonopathy that predominantly affects the left recurrent laryngeal nerve¹ . RLN causes axonal loss, demyelination, and subsequent loss of cricoarytenoideus dorsalis (CAD) muscle volume. This results in a loss of arytenoid abduction particularly during fast exercise. Several studies have identified RLN associated neuropathological changes in both left and right recurrent laryngeal nerves in thoroughbred horses that are clinically unaffected, indicating that many horses are subclinical cases^2-6. The current standard treatment for RLN is the prosthetic laryngoplasty or tie-back as described by Marks et al (1970) with the goal of permanently abducting the left arytenoid cartilage. This is commonly performed in combination with a vocalcordectomy or ventriculocordectomy. In horses that perform at lower exercise intensities where the main complaint is an upper respiratory noise vocalcordectomy or ventriculocordectomy alone may be sufficient. Several modifications to the laryngoplasty procedure have been made in recent decades in an attempt to reduce the adverse effects on upper airway health. The discovery of the close anatomic relationship between the vestibulum oesophagi and the muscular process of the arytenoid cartilage has made surgeons aware of the risk of penetrating the oesophageal lumen when placing the prosthesis through the muscular process⁷ . In many clinics prosthetic laryngoplasty is now performed in the standing horse avoiding the risk of general anaesthesia. The ability to tighten the suture with the larynx in its normal anatomical position under endoscopic visualization may be the most important advantage of performing laryngoplasty in the standing horse and may reduce the likelihood of inappropriate abduction⁸.

Laryngeal reinnervation using the first and second cervical nerves⁹ remains a more physiologically viable treatment option for horses with RLN. This technique is not suitable for all horses with RLN, particularly those with advanced disease where minimal CAD muscle remains. Recently the laryngeal reinnervation procedure has been modified to include transplantation of the ventral branch of the spinal accessory nerve into the CAD muscle. In many horses this procedure can be performed standing. Evidence of reinnervation has been seen as early as 3 months with most horses returning to their previous use by 9 months. The ventral branch of the spinal accessory nerve provides motor innervation to the sternomandibularis muscle and has the advantage of being activated both at inspiration when horses are galloping, and when horses are grazing¹⁰. This means that exercise is not mandatory for the rehabilitation period and that the horse can maintain muscle volume at rest even in the absence of training¹⁰.