Recurrent laryngeal neuropathy (RLN, aka “roaring” or laryngeal hemiplegia) is a respiratory abnormality that can obstruct a horse’s airflow during exercise, resulting in decreased performance and upper airway noise. Veterinarians often address the condition in athletic horses by performing a tieback (laryngoplasty) procedure under general anesthesia or while horses are standing and sedated. Not all affected horses, however, need to undergo the expense and hassle of surgery to live and perform with the condition. So how do veterinarians determine whether a horse is a suitable candidate?
During the 2020 American Association of Equine Practitioners’ Convention, held virtually, Rolf Embertson, DVM, Dipl. ACVS, a surgeon at Rood & Riddle Equine Hospital, reviewed the factors he and his colleagues consider when recommending tieback surgery.
What Is RLN?
First, let’s review the causative condition. Recurrent laryngeal neuropathy generally refers to a slight to complete lack of abduction (opening) of an arytenoid cartilage (usually the left) in the horse’s larynx. It results most commonly from a degenerative condition of the recurrent laryngeal nerve but can also occur due to acute nerve trauma, a congenital anomaly, or inflammation of the cartilage.
When the nerve degenerates, it causes paralysis of the cricoarytenoideus dorsalis (CAD) muscle, which is responsible for abducting the arytenoid cartilage and opening the larynx during respiration. This is a progressive condition that starts with incomplete abduction of the left arytenoid cartilage and eventually results in complete paralysis with no movement of the arytenoid cartilage. As a result, the larynx can’t open enough to allow adequate airflow during exercise. This not only limits performance and horses’ ability to breathe when working but also causes an abnormal respiratory sound due to the restriction in airway diameter near the vocal cords. During tieback surgery, the surgeon uses sutures to reposition the arytenoid cartilage and allow for better airflow.
Indications for Surgery
Embertson said he considers a variety of factors when assessing a horse for tieback surgery:
- The horse’s performance history.
- The primary complaint. “Is it upper-airway noise? If that’s true, and there’s no problem with performance, then a ventriculocordectomy (removing the vocal cords and adjacent ventricular mucosa) may be all that’s needed to significantly decrease noise production,” he said. “Or is it decreased performance? If that’s true, then you need more airflow, so a tieback will help you with that.”
- The horse’s intended use. Racehorses and horses participating in other strenuous disciplines, such as eventing or polo, need a tieback to do what they do, said Embertson.
- “In horses that are less than 2 years of age, the cartilage does not hold the suture as well, so there’s more likely a chance for a tieback to fail,” he said. For young horses, which likely aren’t in training yet anyway, he recommends doing a nerve transplant or waiting until they’re at least 2 years of age before doing a tieback.
- Diagnostic test results. Embertson recommends performing a physical exam, palpating the larynx, and doing an upper-airway endoscopy at rest initially. If further diagnostics are needed, he said a laryngeal ultrasound is helpful. An endoscopic exam during strenuous exercise (dynamic endoscopy) is the gold standard diagnostic test for determining upper-airway abnormalities. These steps help the practitioner grade the severity of airway collapse.
- Possible, albeit uncommon, complications of tieback surgery, such as surgical site infections, aspiration, coughing, inadequate or loss of abduction (in other words, clinical signs return), and chondritis (inflammation of the arytenoid cartilage).
- Cost, if it’s a factor for the owner.
Practitioners grade arytenoid movement using a well-accepted scale based on arytenoid movement during a resting endoscopic exam. The scale ranges from I to IV, including subgrades, with IV indicating complete paralysis. Embertson reviewed each grade and how it affects his decision to recommend tieback surgery:
- I: Rarely abnormal on ultrasound exam and rarely collapse during exercise. The cartilages are synchronous and symmetrical at rest. This is considered normal.
- II.1: Rarely abnormal on ultrasound exam and rarely collapse during exercise. The cartilages are asynchronous (the movement doesn’t occur at the same time) and asymmetrical, but abduct fully at rest. This is considered normal.
- II.2: Infrequently abnormal on ultrasound exam and infrequently collapse during exercise. The cartilages are asynchronous and asymmetrical and have difficulty achieving and maintaining full abduction at rest.
- III.11: Many abnormal on ultrasound exam and many collapse during exercise. The cartilage does not fully abduct or does not maintain abduction at rest.
- III.2: Most abnormal and most collapse. The cartilage does not fully abduct.
- III.33: Almost all abnormal and almost all collapse.
- IV: All abnormal and all collapse. No cartilage movement.
Embertson said if he finds Grade I or II arytenoid movement during resting endoscopy of a racehorse with a history of poor performance and upper-airway noise, he performs an ultrasound exam and dynamic endoscopy before recommending tieback surgery.
On the Grade III.1 horses, he always does an ultrasound exam and, ideally, dynamic endoscopy before recommending a tieback surgery, “but I’ve started to feel comfortable enough to do a tieback with the ultrasound findings and without the dynamic endoscopy information if the history fits and the trainer is convinced the arytenoid is adversely affecting performance.” He also recommends doing ultrasounds on Grade III.2 horses, whereas this imaging modality is generally not necessary once the horse gets to a Grade III.3 or IV.
If the diagnostics support arytenoid collapse with Grade II.2 to III.3 arytenoid movement, said Embertson, don’t wait for the cartilage to become even more paralyzed to perform a tieback surgery. “They do just as well if you do them then as if you wait and do them when they’re a Grade III.3 or IV,” he said.
Historically, many horsemen have believed tieback sutures hold better if there’s less movement (more paralysis) of the arytenoid, but Embertson said this isn’t necessarily true. Studies have shown that Grade III horses with a fair amount of arytenoid movement have better outcomes if you pursue surgery before they’re completely paralyzed than after. “So don’t wait on those horses to do a tieback surgery,” he said.
Going forward, Embertson said nerve transplants into the CAD muscle are showing promise for some horses. It makes sense to perform this surgery in yearling Thoroughbreds that aren’t yet suitable candidates for tiebacks and young sport horses with long careers ahead of them.
“If this procedure works, you don’t have the risk of some of the complications that might occur with the tieback,” he said, noting the downside is it takes at least six to eight months to confirm whether the transplant is effective.