Old-time horsemen called it "roaring." The common scientific term is laryngeal hemiplegia. However you describe it, the condition involving larynx dysfunction can severely compromise a horse’s ability to breathe, especially during exercise.
The problem often is first discovered in growing, tall male horses. It should be remembered, however, that the condition is permanent. They don’t grow out of it. The majority of horses which develop laryngeal hemiplegia will be 16 hands or more when mature. The Thoroughbred seems to be the breed most often afflicted. However, other breeds that have the genetic tendency to be tall also are affected. Young horses often are first diagnosed when they begin undertaking serious exercise for an athletic career and make a noise, or they are exercise intolerant when training or performing over a distance.
The incidence of the affliction in the type of horses described above runs from 3%-8%, according to published data.
In simplistic terms, the condition involves the inability of muscles of the larynx to function properly, primarily because of nerve disfunction. As the horse tries to breathe, the arytenoid cartilage on one or both sides of the throat cannot be kept out of the airway. This causes a whistling or roaring sound as air is drawn through the obstructed larynx. (See illustrations page 67.)
The condition almost always is manifested on the left side of the larynx, perhaps because of the more unusual nerve course around the aorta on that side of the head. The muscles responsible for opening the larynx are innervated (the supplying of a nerve stimulus to a part of the body) by the recurrent laryngeal nerve. When this nerve is unable to stimulate the muscle, laryngeal hemiplegia is the result.
The condition is what scientists call a distal axonopathy of the recurrent laryngeal nerve. That is when the nerve starts dying at the tip, and the death progresses along the nerve.
Heredity has been implicated in laryngeal hemiplegia. And while plant and environmental toxins and metabolic disorders have been suggested as causes, only lead poisoning definitively has been shown to cause the condition.
What is known is that when the nerve no longer can provide stimulus, the muscle ceases to function, begins to atrophy or wither away, and the horse develops a breathing problem. There is some question whether laryngeal hemiplegia is progressive, moving from a partial paralysis to complete paralysis. (For information on the anatomy of the laryngeal area, see sidebar page 73.)
First of all, the condition is characterized by atrophy (shrinking or wasting away) of the intrinsic muscles of the larynx. As was mentioned earlier, the condition almost always manifests itself on the left side. If it occurs on the right side, it usually is the result of some form of injury, such as trauma from perivascular (situated around a vessel) injections associated with the right jugular vein.
The muscle atrophy in left laryngeal hemiplegia appears to be the result of damage to the nerves that signal the muscle to function during abduction and adduction. Without the nerve signals, the muscles become inactive and atrophy is the ultimate result.
The afflicted horse will demonstrate exercise intolerance and will produce whistling or roaring noises when breathing because of the turbulent air flow through the obstructed larynx. The inability of the laryngeal muscles to abduct the arytenoid cartilage results in the left vocal fold’s everting into the lumen or passageway of the larynx, thus obstructing airflow and creating the characteristic "roaring" sound. Affected horses often will have a history of declining performance over weeks to months.
In most horses, this history of noise and exercise intolerance following by endoscopic examination of the resting horse is all that is needed to make a diagnosis.
The combination of the endoscope and the high-speed paced treadmill has made it easier for practitioners to diagnose more subtle cases of laryngeal hemiplegia. The larynx will be examined endoscopically at rest and during strenuous exercise on the treadmill.
Howard J. Seeherman, DVM, MS, Dipl. ACVS, formerly of Tufts University and now a senior scientist at the Genetics Institute, did some of the original investigations on laryngeal problems. In Current Therapy in Equine Medicine 4 (edited by N. Edward Robinson, BVetMed, PhD, MRCVS), Seeherman said, "Diagnosis of left recurrent laryngeal neuropathy (LRLN) can be made based on a variable combination of presenting signs. These signs include palpable atrophy of the left cricoarytenoideus dorsalis muscle (CAD), abnormal upper airway inspiratory noise during exercise, variable exercise intolerance, and, of course, resting endoscopy.
"Palpation of the muscular process of the arytenoid cartilage represents a quick and reliable method for assessing atrophy of the CAD muscle. This technique is easily performed in the standing, awake horse. The degree of atrophy can be graded from 1 to 4, corresponding to normal, mild, moderate, and severe atrophy. Horses with significant atrophy have a distinct knob or knuckle-like projection of the left side created by the exposed muscular process with minimal overlying muscle."
Two Australians, Reuben J. Rose, DVSc, PhD, FRCVS, DipVetAn, MACVSc, and David Hodgson, BVSc, PhD, FACSM, Dipl. ADVIM, both affiliated with the University of Sydney, wrote the following in their book, Manual of Equine Practice:
"If there is any indication of more prominence of the left muscular process of the arytenoid cartilage, a laryngeal adductor test ("slap test") should be performed while palpating the muscular process on left and right sides. Slapping the left thorax (the part of the body between the neck and the respiratory diaphragm—chest) gently with the open hand should result in the right muscular process adducting, and this can be felt as a ‘flicking’ or movement of the process. Similarly, slapping of the right thorax should result in the muscular process of the left cartilage ‘flicking.’
"In horses with idiopathic laryngeal hemiplegia, the ‘flicking’ of the left muscular process does not occur or is reduced in response to the ‘slap test.’ "
Preferably, this test should be done while the horse is being examined endoscopically. When there is doubt, endoscopy during exercise is the "gold standard."
Seeherman explains that as the horse begins to exercise, the arytenoid cartilages alternately open and close in synchrony with breathing. With increased exercise intensity and respiratory effort, he said, the arytenoid cartilages are maintained in the fully open position throughout the respiratory cycle in the normal horse.
If the horse cannot keep the cartilages open during exercise, the cartilage is pulled into the airway during inhalation and exhalation and blocks airflow, thus the noise heard by the rider. Horses sometimes look normal while standing, and can maintain open airways until the CAD muscle gets tired; then fatigue keeps the cartilage from being held out of the airway.
There are degrees of seriousness when laryngeal hemiplegia is involved. Researchers have placed the condition into four categories or grades, ranging from normal to being in need of treatment.
Following are the four grades:
1. Synchronous full abduction and adduction of the left and right arytenoid cartilages.
2. Asynchronous movement of the left arytenoid cartilage during any phase of respiration. Full abduction of the left arytenoid cartilage inducible by nasal occlusion (closure) or swallowing.
3. Asynchronous movement of the left arytenoid cartilage during any phase of respiration. Full abduction of the left arytenoid cartilage is not inducible or maintained by nasal occlusion or swallowing.
4. Marked asymmetry of the larynx at rest and no substantial movement of the left arytenoid cartilage during any phase of respiration.
Although it will vary from horse to horse, the textbook rule of thumb is as follows:
Grade 1 horses do not require surgical treatment.
Grade 2 horses usually do not need to be treated, with the rare exception of dynamic laryngeal collapse with exercise.
Grade 3 horses need to be examined endoscopically during exercise. These are considered borderline horses as far as surgery is concerned. Some Grade 3 horses will need it; others won’t.
Grade 4 horses usually require surgical treatment before they are able to return to exercise.
The good news is that there is a surgical treatment available that can help many horses with laryngeal hemiplegia.
At the 1998 American Association of Equine Practitioners meeting in Baltimore, Md., John A. Stick, DVM, Dipl. ACVS, of Michigan State University, presented a paper on "What to Expect Following Surgery of Obstructive Lesions of the Upper Respiratory Tract." Included in the paper was a report on surgery for laryngeal hemiplegia. Both Stick and his colleague in the study, Susan J. Holcomb, VMD, MS, PhD, are at the Veterinary Medical Center at Michigan State.
"The definitive diagnosis of laryngeal hemiplegia," Stick reported, "is made on endoscopic examination of the upper airway. The affected arytenoid cartilage assumes a paramedian (midline or midplane) position within the rima glottis and has limited to no movement.
"When horses are affected by Grade 4 laryngeal movements at rest, the laryngeal aperture is reduced in size by an inability to abduct the affected arytenoid cartilage. Airflow reduction is exacerbated during maximal exercise by the dynamic collapse of the unsupported arytenoid cartilage into the airway during inspiration."
Horses afflicted with Grade 4 laryngeal hemiplegia, said Stick, are candidates for a surgical procedure known as laryngoplasty. Simply put, the procedure involves the placement of prosthetic sutures between the cricoid and arytenoid cartilages, which, when tied, permanently abduct the affected arytenoid cartilage.
Horses with Grade 3 laryngeal movements which suffer collapse of the arytenoid cartilage during exercise will benefit from surgery as well, Stick reported.
"Prosthetic laryngoplasty returns upper airway function to near normal," Stick said. "Combining ventriculocordectomy (to be described shortly) does not further improve upper airway function in horses. Therefore, prosthetic laryngoplasty alone remains the primary treatment option for idiopathic laryngeal hemiplegia, especially when the athletic endeavors of the horse involve speed.
"Hence, racehorses, three-day event horses, and other horses that perform at maximum speed should have this surgical procedure performed in preference to others. Additionally, horses that work with their head and neck in flexion, such as dressage horses, will be compromised by laryngeal hemiplegia during neck flexion because head position does alter airway mechanics, so that laryngoplasty will be needed. Other types of sport horses may benefit from less invasive surgery."
It would be well if we could rest the case there, but it isn’t quite that simple. There is the matter of complications.
"Numerous complications have been reported following the laryngoplasty technique," Stick said, "with coughing (26%) and nasal discharge (16%) the most commonly encountered. These signs are not indications for suture removal and usually resolve over two to three months. Postoperative incisional infection and incisional seroma (a tumor-like collection of serum in the tissues) are rare. Incisional infections generally can be resolved by providing ventral drainage and antimicrobial and anti-inflammatory therapy."
The good news is that once the post-surgery complications have been worked through, the prognosis for return to athletic endeavors is quite good.
"For horses that are not used for athletic speed events," Stick reported, "a prognosis for successful outcome after laryngoplasty is estimated to be 80% to 90%. However, in racehorses, the prognosis for a successful outcome has been estimated to be between 48% and 85%, depending on the criteria used to evaluate success.
"Laryngoplasty eliminated noise in 65% of horses and improved postoperative racing performance in 69%.
"Laryngoplasty failure is unusual, but when it does occur, repeat laryngoplasty can be performed. Moderate abduction can be achieved with repair laryngoplasty in most horses; however, 50% of racehorses will return to usefulness after a second laryngoplasty while 80% of non-racing horses improve sufficiently to be useful.
"Laryngoplasty is the preferred treatment for Grade 4 laryngeal hemiplegia in equine athletes expected to perform at speed."
Stick also described other forms of surgery that can be used in the treatment of laryngeal hemiplegia. They include the following:
Ventriculectomy—The removal of laryngeal saccules.
Ventriculocordectomy—Excising the entire ventricular floor anterior to the vocal process and anteroexternal (situated on the front and situated to the outer side) surface of the arytenoid.
Arytenoidectomy—Surgical removal of an arytenoid cartilage.
Stick had this to say about ventriculectomy and ventriculocordectomy:
"The removal of laryngeal saccules (sacculectomy, ventriculectomy) with (ventriculocordectomy) or without the removal of a section of the vocal fold has been advocated as a method to treat laryngeal hemiplegia. We know that a ventriculectomy alone fails to improve upper airway flow mechanics and obstructions induced by laryngeal hemiplegia in strenuously exercising horses. However, ventriculectomy or ventriculocordectomy alone does have a place in treating certain sport horses with laryngeal hemiplegia.
"For example, ventriculectomy will reduce the noise and fatigue induced by laryngeal hemiplegia in draft breeds used for both show and pulling contests. Additionally, show horses (western pleasure, hunters, etc.), especially with Grade 3 laryngeal movements in which abduction can be maintained during strenuous exercise, but the vocal folds collapse, may be candidates for this technique. Complications are minimal with no reports of dysphagia (labored breathing) following this procedure."
Stick reiterated that when performance speeds are required, ventriculectomy is not recommended as the sole treatment for Grade 4 laryngeal hemiplegia.
The final surgical procedure that he discussed was arytenoidectomy—removal of a portion of the arytenoid cartilage.
He divided the procedure into two methods—subtotal and partial.
In the subtotal procedure, the muscular process and rim of the corniculate process are left intact. In the partial procedure, only the muscular process is left and all else is removed.
"Airflow mechanics studies show that subtotal arytenoidectomy is not effective in reducing upper airway resistance created by laryngeal obstruction," Stick reported. "However, partial arytenoidectomy is an effective treatment for airway obstruction involving the arytenoid cartilage. Partial arytenoidectomy should be combined with bilateral ventriculocordectomy, and all redundant loose tissue should be removed from the airway.
"Analysis of this surgery shows that the procedure does not completely restore the upper airway to normal, but that partial arytenoidectomy is an effective treatment option for upper airway obstruction caused by arytenoid disorders."
There can be complications when this type of surgery is performed, Stick said. They include labored breathing and chronic coughing.
"Some of this," he said, "occurs from damage to the pharyngeal nerve plexus during surgery. As nerve function returns over several months, this complication becomes self-limiting. However, horses should not be expected to return to training when coughing persists because aspiration pneumonia is a common complication. Owners are encouraged to feed the animal on the ground, and pasture feeding may be beneficial with this complication.
"An arytenoidectomy does not completely restore upper airway function to normal, but it is an effective treatment for arytenoid problems when all of the other techniques are unsuccessful."
How Many Are Out There?
In an effort to determine how many horses are afflicted with upper airway problems, including laryngeal hemiplegia, veterinarians and researchers at New Bolton Center documented their findings when examining Thoroughbreds and Standardbreds at the Center over a particular time frame. Eric J. Parente, DVM, gave a report on the results of that study at the 1994 AAEP meeting in Vancouver. Some 350 horses were involved in the study.
"All horses were racing Thoroughbreds or Standardbreds that were referred to the Jeffords High Speed Treadmill Facility at New Bolton Center between July of 1992 and March of 1994 for evaluation of poor performance," Parente reported. "Racing history and any history of previous surgical intervention were obtained from the trainer.
"A standing endoscopic examination of the upper respiratory tract was performed as well as the endoscopic examination during high speed exercise on a treadmill. After a thorough schooling, the horses were allowed to warm up for 1,600 meters. The endoscope was passed up the animal’s right nostril and held in place with velcro strips between the halter and endoscope.
"The high-speed examination followed, and the protocol consisted of an incremental increase in speed from 11 meters per second to 14 meters per second every 800 meters at a three degree incline for Thoroughbreds, and zero degrees for Standardbreds.
"There was some variation in the protocol, depending on the animal’s fitness."
The results of the tests revealed that 23 of the horses had a Grade 3 laryngeal hemiplegia on standing endoscopic evaluation. This meant that full abduction of the arytenoid was not achieved after swallowing or nasal occlusion. Seventeen of the above 23 horses were unable to maintain adequate abduction of the arytenoid in question during the exercise protocol.
Parente gave this summary of the findings relative to laryngeal hemiplegia:
"Our experience with laryngeal hemiplegia indicates that a significant proportion of the horses determined to be Grade 3 left laryngeal hemiplegia are unable to maintain adequate upper respiratory patency (the condition of being wide open). This is in contrast with a previous study which indicated that 83% of horses with Grade 3 left laryngeal hemiplegia were able to maintain adequate abduction.
"The discrepancy may be attributed to the larger number of clinically fit racehorses in the present study traveling at faster speeds than in the earlier study."
The information outlined above from some top researchers in the field conveys at least four messages for the horse owner.
First, laryngeal hemiplegia is not an uncommon occurrence in the general horse population. When it is severe, it compromises the horse’s performance potential.
Second, it seems to afflict a certain type of horse—the tall, long-necked horse, with Thoroughbreds being afflicted more than most other breeds.
Third, the endoscope and treadmill make a definitive diagnosis possible.
Fourth, and most importantly, there are surgical procedures that can treat the condition.
For Further Information see the web site from Michigan State University on laryngeal hemiplegia that includes a video of the larynx during exercise. The address is www.cvm.msu.edu/research/pulmon/laryn2.htm.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.