Most people who have been around horses long enough have heard a few make abnormal respiratory noises while exercising. These horses usually draw attention to themselves by the sheer volume of noise they create as they go around the show jumping course or trot and canter around the show ring. Other times, the noise is more subtle and the rider will complain about a horse which just runs out of energy and tends to stop. In other horses, the noise isn't loud until the rider asks the horse for more collection and bending at the poll. All of these horses have one thing in common--laryngeal hemiplegia.

In this article, we will explain the condition laryngeal hemiplegia (otherwise known as roaring), what the known causes are, how a diagnosis is made, and what the treatment options are for horses with this disorder. Although there are other abnormal conditions of the horse's upper airway that can cause respiratory noise, laryngeal hemiplegia is one of the more common disorders.

A Normal Larynx

Before we begin a description of this abnormal condition, you must have an understanding of the normal larynx. We will confine this description to the anatomy pertaining to laryngeal hemiplegia.

The larynx of the horse is a structure composed of cartilage and muscle tissue, and it makes the transition between the pharynx (food tube) and trachea (windpipe) of the horse. The larynx serves a multitude of very important functions in the horse. During exercise, its cartilages (sometimes called flappers) open as wide as possible to allow a greater amount of air to enter the trachea and reach the lungs. These arytenoid cartilages are paired, with one on each side of the larynx. During swallowing, the arytenoid cartilages close to cover the opening of the trachea and, along with the epiglottis (another cartilaginous structure of the larynx), protect the airway while food or water passes from the pharynx to the esophagus, the opening of which lies just above the opening of the trachea. The larynx also functions in vocalization.

So when the larynx is working properly, it serves a vital function--but what happens when it doesn't work properly?

What is Laryngeal Hemiplegia?

Laryngeal hemiplegia comes from the root words "hemi," meaning half, and "plegia," meaning paralyzed. In this disorder, the muscles of one side of the larynx are partially or fully paralyzed. More specifically, laryngeal hemiplegia develops when the nerve that controls movement of the cricoarytenoideus dorsalis (CAD) muscle does not function properly. The CAD muscle controls the movement of the arytenoid cartilages. Therefore, when this nerve (the recurrent laryngeal nerve) does not function properly, the larynx does not function properly, resulting in one of the arytenoid cartilages being weakened or completely paralyzed. When this happens, the weakened or paralyzed cartilage cannot move out of the airway. This not only results in air turbulence and the characteristic noise, but also greatly decreases the amount of air that can enter the trachea. This can result in varying degrees of exercise intolerance or fatigue. The noise produced varies from a whistle to a roaring sound, thus the term "roarer."

In a horse with complete paralysis of the arytenoid cartilage, negative pressure is generated as he breathes in during heavy exercise. The negative pressure sucks the paralyzed arytenoid cartilage into the airway (dynamic collapse), leading to further (often almost complete) airway obstruction. In other words, the harder he has to breathe, the worse the obstruction becomes. He has to stop exercising to breathe normally.

What Causes Laryngeal Hemiplegia?

Laryngeal hemiplegia, although most commonly occurring on only one side of the larynx, can occur on both sides (bilateral). However, it usually is seen only on the left side. In most horses, there is no distinct cause, so it is known as idiopathic (having no known cause) laryngeal hemiplegia.

This problem has been reported in 2-8% of the equine population. Most veterinarians believe there is a hereditary basis to the disease, and it is most often seen in larger breeds of horses. Small horse breeds are rarely affected.

Clinical signs of laryngeal hemiplegia are usually seen before five years of age. Racing Thoroughbreds might be diagnosed earlier because of the routine use of endoscopy during sales of these young horses. Although the disease most often occurs "naturally," there are some other known causes.

Direct damage to the recurrent laryngeal nerve (right or left side) can occur during either jugular vein injections, poor jugular catheter placement, or failed jugular vein injections where a solution was injected around the vein instead of in the vein (perivascular injection). Diseases such as guttural pouch mycosis (fungal infection of the guttural pouch--the fist-sized cavity high in the skull that is an outpouching of the auditory, or ear, tube), strangles, or even equine protozoal myeloencephalitis (EPM) can result in laryngeal hemiplegia as the infection can affect the recurrent laryngeal nerve.

Trauma from a kick or other accident can also affect the nerve. The nerve might be damaged during surgery of the head or neck, but this is rare.

Another disease called arytenoid chondritis can be confused with idiopathic laryngeal hemiplegia. Arytenoid chondritis is an inflammatory condition, not a nerve problem, of one or both of the arytenoid cartilages that usually results in paralysis of the cartilage. The disease results in an enlarged arytenoid cartilage, and often there is granulation tissue present on the cartilage. The clinical signs are exactly the same as idiopathic laryngeal hemiplegia--respiratory noise and exercise intolerance. The exact etiology of this condition is unknown, and it is very important to distinguish between the two conditions as the treatments are very different.

Grades of Laryngeal Hemiplegia

The severity of laryngeal hemiplegia is graded on a four-point scale, based on the movement of the laryngeal cartilages. Grade 1 is considered normal and grade 4 represents complete paralysis of the affected arytenoid cartilage. Grade 2 denotes laryngeal movement that is asynchronous (the cartilage movement is not the same as on the other side), but the arytenoid cartilage can open fully. Grade 3 represents asynchronous movement, but the arytenoid cartilage cannot open fully during maximal exercise.

Horses with grade 4 laryngeal hemiplegia usually require surgery to improve performance and help reduce the noise made during exercise, but it depends on what is required of your horse. For example, if your horse is only performing at low levels--such as low-level jumping, dressage, or trail riding--then no treatment might be necessary. However, upper-level jumpers, three-day eventers, or racehorses will require surgery to be able to perform up to their other physical abilities.

Diagnosing Laryngeal Hemiplegia

If your horse is making a noise during exercise or even at rest, with or without exercise intolerance or fatigue, then he should be examined by your veterinarian. There are many causes of respiratory noises in horses, so a complete examination is necessary.

A horse with laryngeal hemiplegia will make a noise when he inhales; other causes of respiratory noise will occur during exhalation or even occur while breathing both in and out. Horses with laryngeal hemiplegia have an atrophied CAD muscle, which can be palpated over the larynx. The definitive diagnosis is made during endoscopy. A flexible fiberoptic camera can be placed in your horse's nasal passage and upper airway in order to observe the larynx cartilages.

The horse with laryngeal hemiplegia will have one arytenoid cartilage that does not move in sync with the other side, or might not move at all. During endoscopy, horses are often encouraged to swallow and/or have their nostrils covered briefly so that they will take deep breaths.

Horses with a grade 4 paralysis can be easily diagnosed solely with standing or resting endoscopy. Horses with grade 3 laryngeal hemiplegia at rest might require endoscopy during high-speed treadmill work to determine if treatment is required. For this, the horse is first accustomed to running on a high-speed treadmill, then he is outfitted with the flexible endoscope positioned up one nostril so that the larynx can be viewed at maximal exercise. Grade 2 involves asynchronous movement of the left arytenoid cartilage during any phase of respiration. Full abduction of the left arytenoid cartilage, if the horse is capable of it, is inducible by nasal occlusion (covering) or swallowing.

Treatment Options

There are a few surgical treatment options for horses with laryngeal hemiplegia, including laryngoplasty (reparative surgery of the larynx), laryngeal reinnervation (restoration of nerve control), and arytenoidectomy (cutting out arytenoid cartilage to improve breathing). They all have pros and cons, as discussed below.

Laryngoplasty--The most common surgical procedure performed for laryngeal hemiplegia is laryngoplasty, often called the "tie-back" procedure. This procedure involves placing a large suture(s) on the outside of the laryngeal cartilages in order to pull the arytenoids out of the way. Stabilizing the cartilage out of the airway prevents dynamic collapse of the cartilage.

Studies have shown that stabilizing the cartilage is more important than the amount of opening obtained. As a result, inhalation noise and exercise intolerance are greatly improved with laryngoplasty. The success rate of this procedure can be quite high (up to 95%), and it is more effective at improving performance in non-racehorses. This procedure is not recommended in young horses (less than two years of age) as the laryngeal cartilage might be weak and prone to pull out from the suture. Noise reduction can be variable, so the procedure might not be effective in cases where only noise elimination is desired.

Although laryngoplasty is the most successful procedure for improving performance in horses with laryngeal hemiplegia, it can have several complications. The most common complication is coughing. Many horses will cough in the early postoperative period, but a small percentage will cough chronically and might develop a lung infection due to food particles or water entering the trachea during swallowing (aspiration). In these cases, the arytenoid cartilage was pulled back too far. To remedy the problem, the suture(s) will have to be removed. Other complications can include pulling out of the suture (laryngoplasty failure), and local infection (suture sinus), all which can compromise the success of the surgery.

Laryngeal Reinnervation--More precisely known as a neuromuscular pedicle graft, this procedure involves transferring part(s) of a normal, innervated (supplied with nerves) muscle into the non-functioning or poorly functioning CAD muscle. This will allow for reinnervation of the CAD muscle with the transferred nerve supply. Although the reinnervated CAD muscle doesn't function as a normal CAD muscle would (the first cervical nerve does not fire at rest), the results can be quite good during exercise. This procedure is often reserved for very young horses--such as yearlings--or horses with grade 3 hemiplegia.

This procedure does not carry the risks of coughing or aspiration of food or water as with the laryngoplasty. However, the neuromuscular pedicle graft can take six to 12 months for reinnervation to be complete in grade 4 hemiplegic horses. The time for complete reinnervation might be shorter in grade 3 hemiplegic horses. As a result, this procedure is not for horses which need to resume exercise soon after surgery. The success rate of this procedure from one study performed at Michigan State University was 76%. If the procedure not successful, then a laryngoplasty can be performed.

Arytenoidectomy--An arytenoidectomy is the removal of all or a portion of the arytenoid cartilage. This is an extreme procedure, and it is usually reserved for horses in which laryngoplasty has failed. It is also performed in horses with arytenoid chondritis or other abnormalities with the arytenoid cartilages. This procedure requires the use of a tracheostomy (surgically creating an opening into the trachea via the outside of the throat) during anesthesia to gain access to the inside of the larynx. Complications with this procedure can be severe, including difficulty swallowing (dysphagia) and difficulty in breathing due to swollen laryngeal tissues postoperatively. Coughing from aspirated food or water can be a chronic problem.

Special Considerations for Draft Horses

Draft horses are known to develop laryngeal hemiplegia more often than horses of the lighter breeds. Surgery for these horses warrants special consideration, as they are much more prone to developing myositis (inflammation of the muscles) postoperatively. This myositis can be mild--resulting in transient weakness--to severe--resulting in a horse which cannot stand for hours to days following surgery and requires very intensive care. As a result, many surgeons are reluctant to anesthetize a draft horse for any significant length of time.

A surgical procedure called a ventriculectomy can be performed in a very short period of time and has been shown to be effective in reducing the noise and improving performance for draft horses with laryngeal hemiplegia without having to perform a laryngoplasty (which can be a time-consuming surgery). The ventricles are soft tissue structures of the larynx that are affected by laryngeal hemiplegia. During this surgery the ventricle mucosa (lining) is removed. Although this procedure alone is not successful for high-performance horses, it can improve hemiplegia and the shorter anesthesia time greatly reduces the risk of postoperative myositis in draft horses.

The decision to perform surgery on a horse with laryngeal hemiplegia will require good communication between you and your veterinarian so that all risks, potential complications, outcomes, and costs are thoroughly discussed before the surgery is performed. Although with any surgery there are inherent risks, surgery for laryngeal hemiplegia can be very successful, turning your horse from a respiratory cripple into a star.


REFERENCES

Bohannon, T.; Beard, W.; Robertson, J. Laryngeal hemiplegia in draft horses; a review of 27 cases. Veterinary Surgery, 19, 456-459, 1990.

Stick, J.A.; et al. Larynx. Equine Surgery, eds. Auer and Stick. 2nd ed., WB Saunders Co. 349-366, 1999.

Stick, J.A. Instrumentation and Techniques in Respiratory Surgery. The Veterinary Clinics of North America, Equine Practice. New Surgical Techniques and Instrumentation, 1996.

Tetens, J.; Derksen, F.J.; Hillman, D.J. Idiopathic laryngeal hemiplegia. Compendium on Continuing Education, Vol. 23 (1), 2001.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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