LEFT RECURRENT LARYNGEAL HEMIPLEGIA
"ROARING"

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 (see Figure 1), nasopharynx, and larynx (these structures are illustrated in Figure 2 below).  The large decrease in air pressures created in the airways during inhalation would cause the larynx to collapse were it not for the abductor muscle which dilates this structure (the dorsal cricoarytenoid muscle). Contraction of the dorsal cricoarytenoid muscle dilates the larynx by pulling the arytenoid cartilage and vocal cord out of the airstream. In left recurrent laryngeal hemiplegia, the dorsal cricoarytenoid muscle is paralyzed so that the arytenoid cartilage and vocal cord on the affected side collapse into the larynx during inhalation and obstruct the airways. Air flowing over the obstruction is most likely responsible for the inspiratory sound known as "roaring."

Left recurrent laryngeal hemiplegia is most commonly seen in horses between ages two and seven. There appears to be a greater incidence in large breed horses (greater than seventeen hands) and there is some evidence that left recurrent laryngeal hemiplegia may be hereditary. Horses affected with left recurrent laryngeal hemiplegia often exhibit progressively declining performance levels over a period of weeks or months. Some horses cough and others may have changes in their neigh.

In order to appreciate how left recurrent laryngeal hemiplegia causes airway obstruction in the horse, it is necessary to understand the anatomy and function of the larynx. The larynx is a short section of the upper airway between the pharynx and the trachea (see Figure 2). The larynx closes during swallowing to prevent aspiration of food and opens widely during exercise to facilitate breathing. The larynx is the primary structure involved in vocalization (it is the "voice box"). Specific muscles regulate opening and closing of the larynx.

The larynx is comprised of several cartilages of different shapes that form a semi-rigid framework (see Figure 3). These are the epiglottis, the paired arytenoid cartilages, the thyroid cartilage, and the cricoid cartilage. The left arytenoid cartilage is involved in left recurrent laryngeal hemiplegia. The arytenoid cartilage has a lateral process called the muscular process, a dorsally located corniculate process, and a ventrally located vocal process. The vocal folds (vocal cords) attach to the vocal processes of the arytenoid cartilages and to the midventral portion of the thyroid cartilage. The rima glottidis is the space between the vocal cords in the middle of the larynx (see Figure 4).

There are several laryngeal muscles which close the rima glottidis. The dorsal cricoarytenoid muscle of the larynx is the only muscle that opens the rima glottidis (see Figure 4). It inserts on the muscular process of the arytenoid cartilage. When the dorsal cricoarytenoid muscles contract, they rotate the arytenoid cartilages and pull the vocal processes (to which the vocal cords are attached) laterally, thus opening the rima glottidis. The dorsal cricoarytenoid muscles are innervated by the recurrent laryngeal nerves (which branch off cranial nerve X, the vagus nerve). Although laryngeal hemiplegia may occur on either side, 95% of cases occur on the left side.

In the resting horse, the rima glottidis is opened and closed in concert with respiration. As exercise intensifies, the arytenoid cartilages are kept maximally abducted throughout both inspiration and expiration. This requires that the dorsal cricoarytenoid muscles be continuously stimulated to contract by the recurrent laryngeal nerves.

Left recurrent laryngeal hemiplegia is a motor neuron disease wherein the nerve cell axons die back. It has therefore been referred to as a distal axonopathy (disease of the axons) or left recurrent laryngeal neuropathy. When the left recurrent laryngeal nerve stops stimulating the dorsal cricoarytenoid muscle of the larynx to contract, the muscle atrophies (shrinks or wastes). Figure 5 is a line drawing of an equine larynx with left recurrent laryngeal hemiplegia. Note that the left dorsal cricoarytenoid muscle is shrunken and that the left corniculate process of the arytenoid cartilage and the vocal cord remain in the midline of the larynx when the right side is abducted (pulled away from midline). In horses with left recurrent laryngeal hemiplegia, the muscular process of the arytenoid cartilage can be palpated above the atrophied dorsal cricoarytenoid muscle. Left recurrent laryngeal hemiplegia may also be associated with trauma to the left recurrent laryngeal nerve, localized infections, and certain toxins.

Diagnosis of left recurrent laryngeal hemiplegia usually begins when owners or trainers seek veterinary advice because they notice that a horse has some degree of exercise intolerance and that it is making "roaring" or "whistling" noises during strenuous exercise. The veterinarian will conduct a complete physical examination to rule out other causes of poor performance. Palpation of the larynx may reveal wasting of the dorsal cricoarytenoid muscle and prominence of the muscular process. Passage of an endoscope tube through the nostril into the pharynx of the resting horse allows the veterinarian to examine the upper airway, especially the larynx.  The veterinarian will then determine if the arytenoid cartilages are moving synchronously and symmetrically during breathing. Occluding the nares (nostrils) will cause exaggerated movements of the arytenoid cartilages and facilitate the diagnosis of hemiplegia. Swallowing can be induced to further evaluate the mobility of the arytenoid cartilages.   Figure 6 is an endoscopic view of a normal larynx with symmetrically abducted vocal cords while Figure 7 shows the laryngeal asymmetry of a horse with left recurrent laryngeal hemiplegia.

A grading system for laryngeal function in resting horses has been developed as follows:

Grade I: Both left and right arytenoid cartilages abduct completely and synchronously during respiration.

Grade II: Left arytenoid cartilage abducts asynchronously during respiration. Full abduction of the left arytenoid cartilage can be induced by occluding the nares.

Grade III: Left arytenoid cartilage abducts asynchronously during respiration. Full abduction of the left arytenoid cartilage cannot be induced by occluding the nares.

Grade IV: Left arytenoid cartilage does not abduct during respiration and stays at or near the midline of the larynx when the right arytenoid cartilage abducts.

Grade I and II horses are considered physiologically normal. When a horse has been diagnosed as Grade III, it's airway should be examined during exercise.

Evaluation During Treadmill Exercise

Examination of the equine airway during exercise has been made possible by the advent of the high speed treadmill and equipment for videoendoscopy.  Horses brought to an equine hospital for endoscopic examination during exercise on a treadmill will need to be kept at the hospital for a couple of days for treadmill training prior to the examination.  On the day of the evaluation, an endoscope tube is inserted through the nostril and positioned so that the opening of the larynx is visible on a video monitor.  The speed of the treadmill is gradually increased until the horse is exercising strenuously.   The veterinarian will then observe the movements of the arytenoid cartilages and vocal cords while the horse is running at high speed.  If the left arytenoid cartilage and vocal cord collapse and occlude the airway during inhalation, left recurrent laryngeal hemiplegia is confirmed (see Figure 8. To download video, click on image. After video has downloaded, click on video image to start).

During intense exercise, the partial obstruction of the upper airway by the collapsed arytenoid cartilage and vocal cord makes it harder for horses with left recurrent laryngeal hemiplegia to breathe. These horses cannot get enough oxygen into their lungs to supply their needs during strenuous exercise. The exercise intolerance observed in horses with left recurrent laryngeal hemiplegia is due, therefore, to the increased work of breathing and the insufficient delivery of oxygen to the lungs during exercise. 

The collapse of the arytenoid cartilage and vocal cord in horses with left recurrent laryngeal hemiplegia affects inspiratory airflow and impedance (resistance to airflow). In respiratory function testing of horses with left recurrent laryngeal hemiplegia at Michigan State University Equine Pulmonary Laboratory, horses exercised at treadmill speeds corresponding to maximum heartrate (approximately 11.0 meters/second) had significantly decreased peak inspiratory airflow and significantly increased inspiratory impedance (resistance to airflow).  

Decreased inspiratory airflow during intense exercise in horses with left recurrent laryngeal hemiplegia has been shown to significantly decrease peak oxygen consumption. Horses with left recurrent laryngeal hemiplegia also have significantly increased partial pressure of carbon dioxide in arterial blood during strenuous exercise.  During strenuous exercise, it is vitally important to increase oxygen delivery to working skeletal and cardiac muscles and to remove metabolic waste products such as carbon dioxide. Compromise of these functions contributes to the declining exercise performance seen in horses with left recurrent laryngeal hemiplegia.

Surgical treatments for horses with Grades III or IV left recurrent laryngeal hemiplegia include prosthetic laryngoplasty ("tie-back"), ventriculectomy, ventriculocordectomy (ventriculectomy and vocal cordectomy), partial arytenoidectomy, subtotal arytenoidectomy, and dorsal cricoarytenoid reinnervation using neuromuscular pedicle grafts. Prosthetic laryngoplasty is the preferred surgical procedure at this time.

Prosthetic Laryngoplasty:

Prosthetic laryngoplasty is a procedure whereby a suture replaces the dorsal cricoarytenoid muscle and permanently abducts the arytenoid cartilage to a normal resting position. This also abducts the vocal cord attached to the vocal process of the arytenoid cartilage and prevents collapse of these structures into the airway during exercise. Complications that may occur after prosthetic laryngoplasty include failure of the suture to keep the arytenoid cartilage abducted, infection of the suture site, aspiration of ingested food into the airways, or coughing.

Ventriculectomy and Ventriculocordectomy:

The laryngeal ventricles are outpouchings of mucosa bounded medially by the arytenoid cartilage and vocal cord (refer back to Figure 4 for an illustration of the lateral ventricle). In ventriculectomies, the ventricle mucosa is removed. This causes the ventricle to collapse and the vocal cords to adhere to the laryngeal walls due to the formation of scar tissue. This procedure increases the diameter of the rima glottidis but does not prevent collapse of the arytenoid cartilage into the upper airway during inspiration in exercising horses with left recurrent laryngeal hemiplegia. In ventriculocordectomy, portions of the vocal cords as well as the ventricle mucosa are removed. The free edges of what remains of the vocal cords are sutured to the vestibular fold (the soft tissue at the entrance to the larynx immediately preceding the lateral ventricle; refer back to Figure 4).

Many equine surgeons perform both prosthetic laryngoplasty and ventriculocordectomy simultaneously to correct left recurrent laryngeal hemiplegia but this may be unnecessary based on a study conducted at Michigan State University Equine Pulmonary Laboratory. This study found that there was no significant difference in airway function while exercising at maximum heart rate between horses that received either prosthetic laryngoplasty or both prosthetic laryngoplasty and bilateral ventriculocordectomy.

Partial Arytenoidectomy and Subtotal Arytenoidectomy:

In partial arytenoidectomy, the arytenoid cartilage and corniculate cartilage are removed to increase the diameter of the rima glottidis. The muscular process of the arytenoid cartilage is left intact. In subtotal arytenoidectomy, only the arytenoid cartilage is removed. The corniculate cartilage and muscular process are left in place. Studies evaluating respiratory indices in exercising horses after these surgeries indicate that neither of these procedures is effective in fully restoring upper airway function in horses with left recurrent laryngeal hemiplegia.

Neuromuscular Pedicle Grafts:

In this procedure, a narrow strip of muscle with an intact nerve (the neuromuscular pedicle graft) is removed from the omohyoid muscle and inserted into the paralyzed dorsal cricoarytenoid muscle.  Over the subsequent months, the dorsal cricoarytenoid muscle becomes reinnervated and is stimulated to contract during exercise. Dorsal cricoarytenoid reinnervation using neuromuscular pedicle grafts effectively restores peak inspiratory airflow and airflow impedance to normal values in exercising horses with left recurrent laryngeal hemiplegia. A significant drawback to this procedure is that approximately one year is required before normal function is restored.

For more information on any of the studies described above, please see References

This article was prepared by Rachele J. Baker under the direction of the faculty and staff of the Equine Pulmonary Laboratory.