Left recurrent laryngeal hemiplegia is a disease of horses which makes breathing more difficult and therefore impairs performance. During exercise, horses with left recurrent laryngeal hemiplegia make loud breathing noises that are described as "roaring" or "whistling."
To get more air into their lungs during exercise, horses dilate their nostrils, nasopharynx, and larynx. Two nerves, the right recurrent laryngeal nerve and the left recurrent laryngeal nerve, cause muscle contraction, which leads to the dilation of the larynx to facilitate breathing during exercise. Laryngeal hemiplegia is caused by degeneration (paralysis) of typically the left recurrent laryngeal nerve. With the loss of the nerve, the muscle on the left side of the larynx wastes away and is no longer capable of dilating the larynx. With the right recurrent laryngeal nerve still functioning horses can still breath, but they fatigue more quickly as compared to normal horses.
The condition has been reported in horses as young as two months of age but is most commonly found in horses between ages two and seven that are beginning their performance careers.
There is a genetic tendency for the development of the condition with large, long-necked horses (Belgians, Thoroughbreds) being at highest risk. In these horses no reason has been found for the nerve degeneration, although some theories exist. Less commonly trauma, such as that sustained by a kick or a laceration, can injure the nerve and result in laryngeal hemiplegia.
A presumptive diagnosis of laryngeal hemiplegia can be made based on the performance history of the horse and the classic noise that they make while breathing during exercise. Affected horses may also have an abnormal whinny, because the loss of the nerve also affects the vocal cord on the same side.
To make a definitive diagnosis, your veterinarian will use a fiberoptic scope to view the larynx as it opens and closes during breathing.
Treatment is only required when the condition adversely affects the athletic performance of the horse. Most horses have sufficient breathing capacity to take in enough oxygen to perform at rather rigorous levels of work. High-speed distance horses (racehorses, advanced three day event horses, grand prix level jumpers) are typically the only ones in which oxygen intake is affected. To eliminate the abnormal respiratory noise, the "roar," represents the reason why most other show ring horses are treated.
A prosthetic laryngoplasty surgical procedure, known as a tieback surgery, is the most common method used to treat the condition. A permanent suture is placed surgically to mimic the action of the atrophied muscle and keep the larynx dilated on the affected side. There are no medical treatments.
To decrease the abnormal respiratory noise, removal of the vocal cord on the affected side is often done in combination with the tieback procedure.
Horses recovering from surgery require four weeks of stall rest before they are permitted to exercise. During this period of rest the tissues surrounding the suture heal, providing further strength to the repair.
Prognosis for success is good, ranging from 50 to 90 percent, depending on the horse's occupation. Horses can be returned to their regular work by eight weeks following surgery.
Major complications include failure of the suture, infection, coughing, and aspiration of feed material while eating.
As most cases develop without a predisposing cause, there is little in the way of prevention. Injections into the neck area (including veins) should only be administered by qualified personnel as reaction to a drug placed inappropriately can result in nerve injury.
When purchasing a horse for high level or racing competition, even if the horse is a yearling, an endoscopic examination of the larynx should be performed.
Laryngeal hemiplegia is typically "idiopathic" meaning no precise cause is evident. In most cases the condition only affects the left side of the horses larynx. The left recurrent laryngeal nerve which innervates the cricoarytenoideus dorsalis muscle (CAD muscle), the muscle responsible for opening the left aspect of the larynx during breathing, undergoes spontaneous, nerve fiber loss. Initially this results in muscle weakness but as the nerve fiber loss progresses there is total loss of muscle function.
Although most often thought to have a genetic basis, the nerve can also be injured from inappropriately placed injections in the neck, infection within the guttural pouch, trauma to the neck, strangles abscesses, tumors, certain toxins, and central nervous system diseases. Any of these problems can result in injury to the right recurrent laryngeal nerve as well as to the left.
The larynx is comprised of several paired cartilages that fit together to form a hollow tube in which air can pass through. At the leading edge of this tube are the paired, right and left arytenoid cartilages. (Figure 1) These cartilages close completely together (adduct) when the animal swallows to protect the horse from aspirating feed or water. When the horse is exercising, the cartilages maximally open (abduct) to provide the largest diameter tube in which air can be transported through. Loss of nerve function, followed by loss of the CAD muscle, results in failure of the arytenoid cartilage on that side to perform normally. The arytenoid cartilage, and the vocal cord attached to it, collapse into the airway, thus causing a major obstruction. The reason why obstruction occurs during inspiration relates to the negative pressure in the airway that rises during inspiration to pull air in from the nose.
The symptoms that the condition causes include the following:
1) respiratory noise – soft whistle to "roar" as air moves past the collapsed arytenoid cartilage and vocal cord.
2) poor performance – exercise tolerance progressively declines as the function of the nerve and muscle progressively deteriorates. This may occur over several weeks to months.
3) impaired vocalization – affected horses may have an abnormal whinny, as their ability to tense the vocal cord is lost.
The diagnosis of laryngeal hemiplegia is suspected based on the characteristic respiratory noise. Palpation of the musculature surrounding the right and left sides of the larynx can also lend to a presumptive diagnosis of laryngeal hemiplegia. Atrophy of the left CAD muscle facilitates palpation of the laryngeal cartilages in comparison to the normal side. An endoscopic (fiber optic) examination of the larynx is necessary, however, to confirm the diagnosis.
The degree to which the arytenoid cartilages abduct during the exam is assessed. In quietly standing horses there will be little movement from either the right or left arytenoid cartilage. If the horse is stimulated to swallow both arytenoids can be seen to close to prevent aspiration and then open rapidly again to allow breathing. If the horse is stimulated to take a deep breath by partially holding off the airflow through the nostrils, both arytenoids should fully abduct (open to their maximum diameter). Horses will also fully abduct when they are slapped on either side of their withers (known as the slap test). Likewise during trotting, cantering, galloping, or pacing both arytenoids should fully abduct.
A grading system, consisting of four grades, exists for documenting arytenoid movement in the non – exercising horse:
Grade I – Arytenoids are in normal resting position and when stimulated to open have simultaneous complete abduction.
Grade II – Arytenoids are in normal resting position but when stimulated to open the left responds more slowly than the right but will completely abduct.
Grade III – The left arytenoid is in a more closed position than normal, opens more slowly than the right side, and does not abduct fully.
Grade IV – The left arytenoid is in a closed (adducted) position and fails to show any movement when stimulated to abduct.
Horses with Grade III function should be further evaluated either immediately following maximal exercise or during an exercise treadmill test. In the latter, the horse is asked to gallop (trot or pace if a Standardbred) on a treadmill, with the endoscope held in place to allow real-time assessment of the arytenoid function. About 80% of the horses with resting Grade III function will have normal function during exercise. In the remaining 20%, the CAD muscle is too weak to overcome the strong negative pressures created in the airway to draw in air during inspiration.
Horses with Grade IV function have complete loss of the left recurrent laryngeal nerve and CAD muscle. These horses require no further diagnostic tests.
Prosthetic laryngoplasty (Tie Back Procedure) is the treatment of choice for racehorses and high-level competition horses to enable them to continue to perform competitively. This technically difficult surgical procedure, which requires that the horse be under general anesthesia, places a permanent suture to replace the function of the CAD muscle. The suture is tied tightly to result in just under maximum abduction of the affected arytenoid cartilage. In this position the horse is assured near maximum airflow when it requires it.
A drawback to the prosthetic laryngoplasty procedure is the permanent abduction of the arytenoid cartilage; a great benefit to the horse while it is exercising but not when it is eating. The inability to close the airway during swallowing can lead to aspiration of feed and water and chronic coughing. In a few individuals this may lead to pneumonia. Treatment in part may require removal of the permanent suture.
For those horses that have laryngeal hemiplegia but do not have careers that require them to compete at maximum speed and distance, research has shown that the hemiplegia does not affect their ability to breath. They receive more than enough oxygen. The noise that is made during exercise however can negatively impact the horse. Under certain horse show rules noise production is grounds for disqualification due to "unsoundness of wind". For this group of horses a prosthetic laryngoplasty will be performed in addition to either a sacculectomy (also known as ventriculectomy ) and/or a vocal cordectomy.
The sacculectomy does not alter the position of the affected arytenoid but does remove the tissue beneath the affected vocal cord, which acts to tighten the vocal cord when the wound heals. By removing this excess tissue and tightening the vocal cord there is less flaccid tissue to flutter in the airway as air rushes past during breathing. This will diminish the abnormal respiratory noise but will not always eliminate it unless it is combined with the prosthetic laryngoplasty. This is the procedure that is most frequently being performed on working draft horses that have laryngeal hemiplegia.
The vocal cordectomy involves removing the vocal cord as well as the tissue beneath it, essentially combining the sacculectomy with vocal cord removal. Some surgeons believe that this offers the best chance to eliminate the unwanted noise. It is also a procedure that is now commonly performed in adjunct to the tie-back procedure in racing horses. It does not improve the horse's ability to breathe; it only decreases the chance that there will be a respiratory noise.
The sacculectomy and the vocal cordectomy procedures can be performed in the standing, sedated horse or with the horse under general anesthesia, using a Laser guided by the endoscope placed through the horse's nasal passage. Alternatively the surgeries, with the horse anesthetized, can be performed with surgical instruments through a small incision made below the jaw into the throat. In either case, healing is quick and there are few complications. These procedures do not put the horse at risk for aspiration of feed or water during swallowing.
Nerve Muscle Pedicle Graft Transfer is a procedure that is being done on a limited basis to correct laryngeal hemiplegia. The goal of the surgery is to restore the function of the atrophied CAD muscle by transplanting a nerve to replace the failed left recurrent laryngeal nerve. The nerve that is transplanted is the 1st cervical nerve. This nerve is capable of transmitting signals to instruct the CAD muscle to contract and abduct the arytenoid during respiration. It takes 6 to 12 months for the muscle to regain its strength to be able to function normally during exercise. It is this time period that usually limits the usefulness of the procedure in horses that are in the midst of their athletic careers. It is a viable option for horses that can be given the time away from competition. There are no known complications associated with the procedure and the option exists if it fails, to have the prosthetic laryngoplasty performed. A horse that has already had a prosthetic laryngoplasty performed is not a candidate for the procedure.
Horses that have had a prosthetic laryngoplasty performed will require antibiotics and anti-inflammatories (usually phenylbutazone) for several days following surgery. They will also require strict stall rest. It is important that they be given 4 weeks of rest to allow the muscles around the permanent suture to heal. The healed muscles will in part, along with the permanent suture, help to maintain the left arytenoid in its new abducted position. Early return to exercise jeopardizes the surgical results. Horses should have an endoscopic examination of the larynx prior to returning to exercise. Sutures will be removed at 2 weeks following surgery.
Initially after surgery, horses will be fed carefully to minimize the chance of aspiration of feed material. Their diet will consist of mashed grain and wet hay fed from the ground. Small volumes of food will be offered frequently at first, then gradual return to their normal diet once it is clear that they are chewing and swallowing without difficulty. For those that experience difficulty, food may be with held until the swelling and pain associated with the surgery subsides. In most instances these individuals can be returned to eating in 24 to 48 hours with no further problems.
Horses will also have their incision sites monitored for abnormal swelling and early signs of infection. If infection develops at the surgical site the permanent suture as a last step, may have to be removed to be able to resolve the infection.
Horses undergoing only a sacculectomy or vocal cordectomy will also be treated with antibiotics following surgery. The risk of infection and the consequences of infection though are much less. They will also be treated with phenylbutazone to minimize pain. Horses can be fed but again their ability to swallow is monitored. Once the pain and swelling has resolved in a few days horses are returned to their normal diet without further problems.
Horses undergoing only a sacculectomy or vocal cordectomy can be returned to light exercise or paddock turnout within 2 weeks and full work in 4 weeks.
Wound care for 3 to 4 weeks, consisting of cleaning the wound 2 to 3 times a day, will be necessary in those horses that undergo sacculectomy or vocal cordectomy through a surgical wound into the throat rather than by endoscopic Laser surgery. Care is not difficult and the surgical wound heals quickly leaving no visible scar. Some surgeons will suture the surgical wound closed at the time of the original surgery which eliminates postoperative surgical wound care.
Horses that have a nerve muscle pedicle graft transplant require a short course of antibiotics and phenylbutazone and two weeks of stall rest to allow the incision site to heal. Once the skin sutures are removed they can begin paddock turnout. The horse can be re-evaluated endoscopically for arytenoid function as early as 3 months but as mentioned it may take as long as 1 year for full function to return.
Prosthetic laryngoplasty is associated with a 48 – 90% chance of restoring normal airway function. The success rate is highest in non-racing individuals. In racehorses the loss of even 1-second in time can be the difference between winning a race or losing it. And therefore standards for success are more stringent.
There is a 60 to 80% chance of eliminating the respiratory noise.
Ten percent of the horses following prosthetic laryngoplasty will develop a chronic cough.
One percent of the horses following prosthetic laryngoplasty will die secondary to aspiration pneumonia.
Other complications associated with the prosthetic laryngoplasty procedure include failure of the repair by either breakage of the permanent suture or the suture pulling through the cartilages of the larynx it was placed through. Repair by performing a second prosthetic laryngoplasty surgery is generally not successful.