Early detection and treatment of laryngeal neuropathy

John Cheetham, VetMB, Dipl. ACVS, Department of Clinical Sciences, Cornell University College of Veterinary Medicine, has recently received a grant from the Grayson-Jockey Club Research Foundation to study recurrent laryngeal neuropathy in horses.

Recurrent laryngeal neuropathy, also referred to as roaring, is a common disease in horses. Affected horses exhibit upper airway obstruction. It occurs, and is especially critical in, exercising horses since the nerves and muscles (cricoarytenoid dorsalis muscle) that control the movement of the arytenoid cartilages of the larynx do not function correctly, causing improper opening of the larynx during inspiration. This may lead to inadequate ventilation and poor performance during exercise.

The grant description notes: "Laryngeal neuropathy, or 'roaring,' is estimated by the author to affect 8 percent of race horses and a higher percentage of sport horses. This researcher over the last three years has developed two non-invasive procedures to assess the cricoarytenoid dorsalis (CAD) muscle and the nerve that supplies it. The CAD muscle is the only muscle that opens the larynx during exercise. The present proposal will validate two diagnostic tests for early detection of roaring in horses. The ability to identify young horses which are predisposed to become roarers would enable surgical intervention to restore the nerve supply before atrophy and fibrosis of the CAD muscle occurs."

"What we're trying to do is work out which horses go on to develop laryngeal problems," says Cheetham. In most cases, recurrent laryngeal neuropathy is seen in racing horses at 2, 3 or 4 years of age with obstructed airway and poor performance. Tie-back, or laryngoplasty, which is the current form of surgical intervention, has a good success rate. "But there is room for improvement, and there are some possible complications associated with it," Cheetham says.

One of the recurrent laryngeal neuropathy research goals is to try to detect which horses will go on to develop the disease. Cheetham is investigating whether a diagnostic test could be applied to a weanling population that will tell us which horses are going to develop disease during their 2-, 3- or 4-year-old season.

The impact of the Grayson-Jockey Club Research Foundation grant will be twofold.

"First, we could intervene much earlier," says Cheetham. "So if we knew that there was a 90 percent chance that a particular horse was going to develop disease, then perhaps we could do a reinnervation technique or some other technique that would give us a better success rate."

Thus, it would mean that a horse would not experience time out of training or a break in its career and perhaps not be exposed to some of the complications of the tie-back procedure. "Although tie-back is a good procedure, and I certainly wouldn't discourage it, we're just trying to improve things all the time," says Cheetham.

"Second, it would also give trainers an idea of which horses to focus their efforts on and which young horses would perhaps be better doing a different job—possibly discontinuing training," says Cheetham.

The research

Although he has recently received the Grayson-Jockey Club Research Foundation grant, Cheetham has been working on recurrent laryngeal neuropathy diagnosis and treatment options for a while. He started with laryngeal ultrasonography a few years ago, which involves looking at the lateral aspect of the larynx to evaluate one of the muscles (cricoarytenoideus lateralis) affected by laryngeal neuropathy. That procedure continues to be used at Cornell, Rood and Riddle Equine Hospital in Kentucky and Guelph University.

"During the last 12 months, we've developed a new technique, transesophageal ultrasound," says Cheetham. "We place a small esophageal probe about 15 mm in diameter up the nose and pass that into the esophagus, just over the top of the larynx. Using that, we can look down on the larynx itself and examine the two muscles that control the opening of the arytenoids—or flappers, as they are commonly called in the racing industry—the dorsal cricoarytenoid muscle," says Cheetham. "We can look at them directly and determine their geometry quite precisely, such as how thick they are, what the cross-section is and, to some degree, what the fiber pattern looks like within the muscle."

The other tool developed at Cornell is a way of directly measuring the speed of conduction of the nerve that innervates those muscles—the recurrent laryngeal nerve.

Using the Grayson-Jockey Club Research Foundation grant, over the next two years. Cheetham and colleagues at Cornell will look at these two techniques—nerve conduction velocity (measuring nerve health) and esophageal ultrasound (measuring the thickness of the two muscles responsible for opening the airway during exercise). They'll also investigate how these two techniques relate to exercising function. "This is the most important thing clinically," says Cheetham.

Once those data are determined, the next step will be to try to apply those to a population of young horses, probably weanlings. "That is probably a few years away," says Cheetham. "We are working on some new reinnervation techniques, so if we could pick up disease at an early stage, it would allow us to restore function using a nerve graft, a neuromuscular pedicle graft or some sort of nerve anastomosis. There are a whole slew of those techniques being developed."

The benefits

"With the two testing modalities Cheetham and colleagues will use—nerve conduction velocity and transesophageal ultrasound to look at dorsal cricoarytenoid muscle structure and anatomy—their goal is to determine nerve health and really try to relate those two diagnostic tests to the exercising arytenoid or laryngeal function," says Brett Woodie, DVM, MS, Dipl. ACVS, Rood and Riddle Equine Hospital.

"They're going to start out with adult horses and try to validate these techniques and then determine the sensitivity and specificity of those techniques as it relates to the diagnosis of recurrent laryngeal neuropathy," Woodie says. "With that in mind, they will then be able to take that information and use it in a younger group of horses to see if we can predict which horses will develop recurrent laryngeal neuropathy. And we'll have the capability to intervene at an earlier stage—to detect the disease early before they exhibit clinical signs of an abnormality."

Once Cheetham's research is completed, it is hoped that the new diagnosis and treatment methods will improve the plight of older horses on the racetrack, compared with current therapies.

"Everyone realizes that a laryngoplasty is not really a physiological solution to the problem as compared with the horse without a laryngoplasty," says Woodie. As also noted by Cheetham, laryngoplasty does have some complications associated with it. "But right now that is our best technique—along with some reinnervation procedures such as the neuromuscular pedicle graft—to deal with a horse that has laryngeal neuropathy," Woodie says.

The problem with the neuromuscular pedicle graft is that often the timeline, as far as the length of time it takes to determine if you've been successful or not, can affect a race horse's training schedule.

"A lot of the horses, because of their age, don't have the luxury of waiting an extra year before pursuing their racing career," Woodie says. "Being able to determine which horses will be affected at a later time may allow us to find other means to deal with this before we would need to use a laryngoplasty. I think that is really where it is headed—to stop the progression of the loss of those large myelinated nerve fibers before we get to the typical recurrent laryngeal neuropathy."

The future

"As we look at benefits as to the future diagnosis and treatment of the disease, a lot of it will hinge on the validation of these two tests and how well those tests work to diagnose recurrent laryngeal neuropathy," Woodie says. "We will take a group of horses that we know have the issue and see how well those two techniques prove that they have the problem." The next step should be to look at a younger group of horses and try to follow them along.

"Even when you're tracking and following those younger horses, if you're looking at the natural progression of disease, a lot of times the disease condition is not evident until they are 2 years of age or older," says Woodie. "And we could follow a horse two, three years or more, and it may be a normal horse."

"I think right now we don't really know what sort of timeline the progress of the research we're looking at will take," says Woodie. "Stage one, validating the two diagnostic tests, will be the first critical step. Depending on how that goes, the rest of it will have to follow suit."

Cheetham and colleagues at Cornell have a great opportunity to solve a problem that has plagued racing horses for years. If they can assess the early detection of recurrent laryngeal neuropathy and perfect the new diagnosis and treatment modalities, it will surely benefit racehorses in the future.

Kane is a researcher and consultant in animal nutrition. He is an author and educator on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. He is based in Seattle.