Common Respiratory Problems
The main goal of the respiratory system is to transfer oxygen from the air we breathe to the red blood cells, where the oxygen will be transported throughout the body and be available for all organs and tissues. In addition, carbon dioxide, a waste product of metabolism, is eliminated from the body via the lungs. While this seems a simple task, it is one that, if compromised in any way, will severely affect the athletic ability of a horse, if not his very life.
A horse with a sinus infection with an indwelling drain for flushing.
Average room air (at sea level) contains about 21% oxygen, 70% (or thereabouts) nitrogen, and the remainder is made up of a variety of gases, including pollution. So, in fact, the air we typically breathe is not very rich in oxygen. As the altitude gets higher, the percentage of oxygen in the air becomes less (the air becomes "thinner"). The body can do a few things to compensate for the reduced concentration of oxygen in the air of higher altitudes, but it takes several weeks for the adjustment. That is the reason careful training and conditioning are required when athletes (human and horse alike) work at higher-than-normal altitudes.
Within the red blood cell there is an iron-rich protein called hemoglobin to which the oxygen molecule binds when the red cell is in the lungs. There are a number of factors that can affect the red blood cells' ability to bind or release oxygen. For example, iron deficiency can lead to anemia and a reduced oxygen-carrying capacity of the blood. Red maple leaf poisoning causes a change in the hemoglobin that decreases its binding to oxygen, and carbon monoxide toxicity prevents the release of oxygen from hemoglobin. There are numerous other toxins that can have an effect on the binding affinity of oxygen for hemoglobin.
The oxygen content of the blood is typically measured as a "partial pressure." This is somewhat relative to the atmospheric pressure (in addition to how well the lungs are working), which is 760 millimeters of mercury (mmHg) on average. If the lungs are normal, the partial pressure of oxygen in arterial blood is generally above 90 mmHg. This is measured by a machine called a blood gas analyzer.
An arterial sample of blood is obtained from one of the arteries in the head area of an adult horse (or one of the leg arteries in a foal) for analysis. Another, more subjective way to evaluate blood oxygen content is to look at the mucous membrane (gum) color. If the gum color has become blue, the membranes are said to be hypoxic (low in oxygen). The only problem with this assessment is that it generally requires a substantial drop in blood oxygen content before this condition is observed--so it is a very bad sign.
Other ways to evaluate the respiratory system include respiratory rate and character of breathing, bearing in mind that there are other things that can have an effect on these parameters. For example, pain can increase the respiratory rate and cause shallow breathing. A good physical examination--from the nose on back--is required to assess adequately the respiratory system. Is there a nasal discharge, and if so, what is the content of it--clear thin fluid, thick green discharge, thick yellow discharge, blood content, foul smelling, one nostril or both? Is there normal air flow out of both nostrils? Is there any asymmetry to the head? Has the horse been coughing, and if so what is the character of the cough (dry/wet)? Can a cough be induced by squeezing the windpipe (trachea)? Is the horse extending its head and neck to breathe? Are there any painful swellings around the head? Does the horse make a noise while breathing (at rest? at work? on inspiration? on expiration?)? Does the horse have extra abdominal movement while breathing? Is there a fever? Are there any other horses on the property with similar signs?
All of these things must be assessed and can give clues to the specific problem. Other important factors include travel history, geography of the travel, age of the horse, vaccination status of the horse, time of year, and prior illness.
Respiratory problems typically are classified as upper or lower respiratory. The anatomy of the upper airway is complex, but a good understanding of it is necessary to aid in diagnosis and understanding the disease processes common to the horse. I will go through the anatomy here, then discuss specific diseases later on in this article and in part two next month.
We will start with the nostrils, as these are the entry point for air and that ever-so-precious oxygen. The nostrils have numerous muscles that permit their flaring (dilating) to allow for air movement with the least resistance at that point. Occasionally, some horses will have small nostrils that don't open well while exercising to the point of affecting airflow and therefore performance--there are minor surgical procedures and other interventions to help correct that problem.
Going up the nostrils, there are three passages stacked on top of each other that lead back toward the throat. It is possible for these delicate passageways to become inflamed and lead to nasal passage obstruction. Occasionally, foreign bodies (plant material, wood, etc.) can become lodged in a nasal passage and cause inflammation/obstruction. Foreign objects, such as sponges, have purposefully been inserted into these passages.
The horse's head has extremely large sinus cavities that drain into the nasal passages. Horses have a long nasal septum, so the sinuses drain into the nasal passages on the same side they are located--a nasal discharge only coming from one nostril is sinus drainage.
The next stop on the way toward the back of the throat is the openings to the guttural pouches. A guttural pouch is a structure unique to horses, rhinos, and tapirs. This structure is an outpouching of the eustachian tube. Whereas our eustachian tube drains from the inner ear to the back of our throat, the horse has a pouch that is fairly large (it holds several hundred milliliters) with the opening draining just behind the nasal septum on each side. A discharge from the guttural pouch can drip down the outer wall of the throat and out the nostril on the same side as the affected pouch, or, if the discharge is great and flowing rapidly, it can come from both nostrils.
The guttural pouch is a unique, but somewhat scary structure for several reasons. The interior of the guttural pouch contains some very important physiologic features that can be affected by guttural pouch disease; there are four major nerves coming directly out of the brain (cranial nerves) that travel on the inside of the pouch, as well as two major arteries (the internal carotid artery on the way to the brain and the maxillary artery supplying much of the blood to the head). Guttural pouch disease will be discussed later in this article.
As we continue traveling the respiratory route, we come to the voice box or "larynx." The laryngeal area is a very important area with several common disease processes affecting airflow at this point in the anatomy. In the normal horse, this structure coordinates swallowing and protects the trachea and hence the lungs from the inhalation of feed material. It should be noted at this point that the normal horse cannot breathe through his mouth; horses are obligate nasal breathers, so anything that occludes the upper airway will cause an airway obstruction and respiratory distress.
The laryngeal area is where the soft palate ends (remember that the soft palate separates the oral cavity from the nasal cavity). The larynx is the junction between the nasal cavity and the trachea (windpipe). A triangle-shaped structure called the epiglottis juts out with the point facing forward and sits on top of the soft palate. As we go farther back, just before entering the trachea, there is a V-shaped structure with small bulges on each side called the arytenoids (these are adjacent to the vocal cords and often called "the flappers"). It is the arytenoids that are pulled open wide during inspiration to allow for maximal unobstructed airflow. There are several problems that occur in this area which will be discussed later.
Just after going past the arytenoids, we are in the trachea and on the way down to the lungs. Before leaving the head, I want to mention that there are many lymph nodes in the head area that can cause significant problems if they become enlarged.
The windpipe of an adult horse is about three inches in diameter and travels down the center of the neck into the thoracic (or chest) cavity, where it splits into the two main branches just above the heart. The branches of the trachea, now called bronchi, branch off many times until all the little (microscopic) branches connect to what's called an "airsac." The airsac is where oxygen exchange happens. The structure at this level becomes so fine that there is nothing but a single thin membrane separating a single red blood cell from the oxygen in the airsac. It is at this level that the oxygen diffuses across the membrane into the red blood cell and the carbon dioxide diffuses out of the red blood cell and into the airsac to be eliminated.
The thoracic cavity can be called the pleural cavity. The "pleura" is the thin cellular membrane lining the chest cavity (inside of the ribs) and covering the surface of the lungs. Within this cavity, the lungs are fully expanded and in intimate contact with the chest cavity; there is only a very small amount of fluid present within the chest cavity when everything is normal.
Further Evaluation of the Respiratory System
In addition to the thorough physical examination mentioned above, there are several other procedures that can be performed to evaluate the respiratory system further. First, we need to listen to the lungs. With many horses, especially those with an extra inch of fat over their ribs, the lungs can be difficult to hear. It is very useful, since we just can't ask them to take a deep breath, to place a plastic trash bag over a horse's muzzle, allowing the horse to rebreathe its expired air for a few minutes. This will serve several purposes: 1) He will take a deep breath so we can listen for any abnormal noises that might indicate certain disease processes; 2) the lack of ability to tolerate this can be an indication of more subtle disease; 3) coughing while the bag is on or after it is removed can be an indication of disease; and 4) if the horse takes a long time to recover from the bag, it also is an indicator of disease.
The use of a fiberoptic endoscope can be of great assistance in evaluating respiratory disease. The upper airway can be directly evaluated, including the sinuses, guttural pouch, and the structures of the larynx. Sometimes it is necessary to evaluate the larynx while the horse is exercising (a task accomplished with the help of a high-speed treadmill). The scope also can be taken deep into the lungs to look for abnormal fluid and obtain samples for evaluation and culture.
Should pneumonia or pleuritis be suspected, a transtracheal wash (see The Horse of March 1997) can be performed to collect material for evaluation and culture
Radiology and ultrasonography can be of great benefit. The head, as well as the lungs, can be radiographed and ultrasounded. Radiographs of an adult horse's lungs usually only can be accomplished at a referral center with a high-powered machine, but ultrasound examination easily can be performed in the field. Ultrasonography can be of significant benefit in evaluating respiratory disease in the horse. In addition, routine blood work also can be of benefit.
Now, with all the background out of the way, I'd like to go through some of the more common respiratory diseases.
The Upper Airway: Nasal Passages
The nasal passages can be affected by foreign body-induced abscesses. There has been a variety of foreign bodies, ranging from small plant awns to pieces of wood several inches long, found in the nasal passages. The clinical signs associated with such problems range from obstruction and reduced air flow in one nostril to the presence of a variety of nasal discharges (clear, yellow/white pus, bloody, etc.). The diagnosis is made using endoscopic examination.
More invasive problems in the nasal passages include cancer and fungal infections. The type of cancer called squamous cell carcinoma has been observed in the nasal passages and sinuses and can cause a variety of nasal discharges. One of the common characteristics of this cancer is that the discharge often is foul/fetid smelling and the tissue is ulcerated and bleeds easily.
There are several species of fungus that can invade the nasal passages and nasal septum. These fungi cause invasive fungal granulomas (masses of abnormal tissue made up of a certain cell type). The diagnosis of fungal granulomas typically is made by obtaining a biopsy of the affected tissue. The prognosis for either squamous cell carcinoma or invasive fungal granulomas in the nasal passages is quite poor; their treatment is extremely difficult.
The horse's head is about 50% sinus cavity (a fact that brings a new definition to the term "air-head" for some horses). The main sinuses are called the "frontal" and the "maxillary." The frontal sinus is in the area between the eyes and back, and the maxillary sinus is from the eyes down and associated with the teeth. The sinuses mostly are interconnected via a complicated network of passages within the bones that create them. As said before, the main connection to the nasal passage is via the maxillary sinus by way of the naso-maxillary opening; the right maxillary sinus drains into the right nasal passage and the left maxillary sinus drains into the left nasal passage. The naso-maxillary opening is halfway from the nostril to the end of the nasal septum, so drainage from a diseased sinus comes from the nostril on the same side as the affected sinus.
The most common cause of a sinus infection in the horse results from an infected tooth root. The average horse has six pairs of cheek teeth on each side (a pair is one top and one bottom). There are three premolars and three molars, with an occasional horse having an additional premolar often referred to as a "wolf-tooth" as the first cheek tooth. Any injured tooth (broken or cracked from trauma) can develop an infected root, but the third and fourth cheek teeth (the fourth premolar and first molar) are predisposed to the development of infected roots without any history of trauma.
The classic presentation of a horse with a tooth root abscess is one with an incredibly putrid-smelling nasal discharge consistently coming from one nostril. The horse generally is bright, alert, responsive, has no fever, usually eats normally, and is normal in every respect with the exception of the stinky nasal discharge. The diagnosis of a tooth root abscess generally is made by taking radiographs of the head; the tooth itself usually is normal in appearance from inside the mouth. Treatment of a tooth root abscess typically involves removal of the affected tooth and systemic antibiotics and/or flushing of the sinus. The sinus is entered by drilling a small hole through the bone. After the hole is made, a piece of tubing can be inserted into the hole and a variety of flush solutions can be pumped into the sinus; the flush solution will drain out the naso-maxillary opening and come out the nostril.
Other problems that can cause a nasal discharge related to a sinus problem include the growth of a tumor inside one of the sinuses with or without secondary infection. In addition to radiographs, an endoscope can be inserted through a drill hole to aid in the evaluation of the sinus's interior; samples of abnormal material can be acquired for culture or microscopic evaluation via the endoscope if necessary.
Another problem that can occur within the sinuses, but is relatively rare, is a fungal infection. The specific predisposing factors to sinus fungal infections are unknown, but most of the fungal sinus infections I have observed were associated with chronic antibiotic administration.
The Guttural Pouch
Guttural pouch empyema (pronounced em-py-ema) is a bacterial infection of the guttural pouch. Affected horses might be free of clinical signs, with the exception of a thick white to yellow nasal discharge; some are systemically ill. The discharge is most obvious when the horse is eating off of the ground and swallowing. As in us, the opening of the eustachian tube (guttural pouch in the horse) opens up when horses swallow, and with their head down low, the stuff runs right out.
Guttural pouch empyema is most often associated with a chronic Streptococcus (strep) infection, which includes one species of Streptococcus that causes strangles (Strep. equi). It is important to rule out strangles and the presence of any swollen lymph nodes. If the infection is chronic, there might be the development of concretions of pus called "chondroids." The presence of chondroids complicates treatment as they must be removed before the infection will clear up. The treatment of guttural pouch empyema typically involves flushing the affected pouch with any one of a variety of flush solutions and the administration of systemic antibiotics.
Guttural pouch tympany is a disease of foals. The classic clinical appearance of a foal with guttural pouch tympany is that of a chipmunk with its cheeks full. The bulging in these foals is right behind the jaw and might be quite tight; the swelling can be tight enough to cause a partial obstruction of the upper airway and respiratory distress. If a young horse is standing with his head and neck extended in respiratory distress with a large swelling behind his jaw, veterinary assistance should be obtained immediately.
Guttural pouch tympany is caused by an overfilling of either one or both guttural pouches with air. The inciting cause is a malformation of the opening of the guttural pouch that acts as a one-way flap valve such that air goes into the pouch, but not back out of the pouch. The correction of guttural pouch tympany involves surgically connecting the affected pouch to the normal pouch (if one of the pouches is normal). If both pouches are affected, the correction involves attempting to reconstruct the guttural pouch openings surgically. These procedures can be performed with the aid of an endoscope and might involve the use of laser surgery.
Guttural pouch mycosis is a fungal infection of the guttural pouch. The guttural pouch is open to the environment and as a result is not sterile. It is unknown what exactly is responsible for the development of guttural pouch mycosis. Fungal infections generally are associated with fungal species that are primary pathogens (organisms that have the ability to cause disease in healthy beings) or immune-suppressed individual horses or those receiving long-term and chronic antibiotic therapy.
The cellular immune system is very important in fighting fungal infections, and as a result, any deficiency in that system will predispose to fungal infection. With respect to chronic antibiotic therapy, many areas of the body are not sterile and have a normal population of bacteria, fungal, and yeast organisms. There is some aspect of this normal population that is self-controlling so that the bacteria suppress the fungi and so forth. So, if part of the normal bacterial population is suppressed by chronic antibiotic therapy, the fungal part of the population can overgrow and become a problem.
As mentioned before, the guttural pouch is full of vital structures (the nerves that control swallowing and the internal carotid artery). The typical site of fungal infection within the guttural pouch, unfortunately, is right on top of the internal carotid artery--the fungal infection often involves the internal carotid artery. For this very fact, any evidence of bleeding from either one or both nostrils should be treated as an emergency and promptly evaluated.
The opening of the guttural pouch is before the nasal septum, so rapid bleeding can come from both nostrils. But, because the guttural pouch opening is on the outer wall of the nasopharynx, if the bleeding or discharge is only a trickle, it might just run down the side of the opening and only be evident in the nostril on the same side as the affected pouch.
The onset of guttural pouch mycosis can be extremely insidious with, at times, very little warning of a catastrophic hemorrhage. There often are several episodes of minor bleeding prior to a more serious bleeding episode, and there can be a potential history of non-hemorrhagic nasal discharge for variable periods of time prior to the discharge containing blood.
It is important to note that horses suffering from guttural pouch mycosis can suddenly hemorrhage to death from a bleeding internal carotid artery. Again, I must stress that evidence of bleeding from the nostrils should be treated as an emergency and promptly evaluated.
It is possible that the bleeding might be associated with other sources of hemorrhage, but if the cause is the guttural pouch, time is of the essence with respect to treatment.
The diagnosis of guttural pouch mycosis is made easily with the use of an endoscope--the general cost of basic endoscopic equipment approaches $15,000. Although technically difficult, the endoscope can be passed up the nostril and into the guttural pouch in a standing, sedated horse. The entire interior of the guttural pouch can be evaluated.
The classic appearance of guttural pouch mycosis is a green fluffy (not unlike the appearance of mold on the bread in your kitchen) fungal growth in the upper corner of the pouch--typically right on top of the internal carotid artery.
Due to the risk of spontaneous fatal hemorrhage, medical management of guttural pouch mycosis is attempted only if financial constraints prohibit surgical intervention. There are several different surgical approaches for the treatment of guttural pouch mycosis--all of which by some methodology involve the occlusion of the arteries in the guttural pouch area. These procedures are technically complicated and include a simple ligation (to tie off) of the internal carotid artery or the passage of balloon tip catheters up the various arteries.
With the balloon catheter technique, when the catheter is in position, the balloon is inflated and left in place until the blood clots and the arteries are permanently occluded. The balloon techniques are the most effective as they can occlude all potential routes of blood flow.
Once the blood supply coursing through the fungal infection is eliminated, the infection generally resolves. Sometimes the guttural pouch is flushed with various anti-fungal agents to aid in the elimination of the fungal infection.
Guttural pouch mycosis is a terrible disease that potentially can have a fatal outcome. Remember, any horse with evidence of bleeding from the nostril should be promptly evaluated.
The Laryngeal Area
Making a diagnosis of laryngeal abnormalities can be extremely difficult. This is because many of the abnormalities might only occur at high speed after the horse has been exercising for some period of time.
The development of the high-speed treadmill has led to great advancements in both the diagnostic capabilities and evaluation of treatments with respect to laryngeal dysfunction. The high-speed treadmill allows the horse to reach near-racing speeds while allowing for the insertion of the endoscope during exercise. This direct visual evaluation of the laryngeal area while exercising is of great diagnostic use.
Dorsal Displacement Of The Soft Palate
Dorsal displacement of the soft palate is a disease process well known to performance horse owners. The abnormality might be intermittent or persistent and might only occur at racing speed (and then only after the horse has been exercising for a period of time). All of these factors can make an accurate diagnosis more difficult or require treadmill evaluation.
If the displacement is persistent, there could be nerve or muscle dysfunction contributing to the persistence of the problem. The majority of horses suffering from displacement do so intermittently; intermittent dorsal displacement of the soft palate is a commonly reported cause of upper airway obstruction in the racehorse. Of 151 horses which were presented to Cornell University with complaints of respiratory problems or poor performance, 29 (19%) were diagnosed as having dorsal displacement of the soft palate when exercised on the high-speed treadmill.
Dorsal displacement of the soft palate has been a recognized affliction of horses since 1949, but the specific cause of this problem remains unknown. Factors that contribute to displacement of the soft palate range from an attempt to breathe through the mouth--causing the palate to be pushed upwards--to a lack of general muscle tone in the palate area associated with unfitness.
Remember that the soft palate separates the oral cavity from the nasal cavity. If the soft palate displaces, it flips up over the top of the epiglottis and partially obstructs the airway; normal placement of the soft palate is essential for unobstructed breathing. The classic history of a horse which displaces the soft palate includes exercise intolerance, generally poor performance, and/or the making of a sudden respiratory noise and simultaneous loss of exercise tolerance--the horses are said to "choke-up" or "swallow the tongue." Remember, the horse cannot breathe through his mouth, so until he swallows a few times and replaces the soft palate, he cannot breathe.
The diagnosis is made based on history and endoscopic examination of the laryngeal area with the horse at rest and during exercise. Many horses can displace their soft palate at rest normally (and even more so if sedated for the examination), a fact that makes the observation at rest less meaningful. Treadmill evaluation can greatly aid in this diagnosis. It has been observed that up to 30% of horses suffering from intermittent dorsal displacement of the soft palate have some other throat problem observed during high-speed treadmill examination.
Treatment of a displacing soft palate can involve something as simple as using a "tongue-tie" or a "figure-eight" noseband. In addition, if the horse is observed to be generally unfit, an attempt should be made to raise his level of fitness. Also, if during the examination there is any evidence of inflammation in the throat, an attempt should be made to reduce the inflammation. This can be accomplished by using systemic anti-inflammatory drugs and/or the topical application (in the back of the throat) of a variety of anti-inflammatory throat sprays.
The tongue tie is an attempt to "pull" the larynx forward and make it harder for the soft palate to displace. Should the tongue tie, noseband, or anti-inflammatory therapy fail as a treatment, there are several surgical options. The three main surgical options involve the trimming of part of the soft palate and/or the resection (surgical cutting) of part of the "strap" muscles (the long thin muscles on the underside of the neck), or changing the shape of the soft palate by injecting it with certain materials.
Both of the palate surgeries can be performed utilizing the endoscope and can involve the use of the laser. The strap muscle surgery usually is performed with the horse under sedation and given local anesthetic. In recent reports, all three of the surgical options indicate a success rate of approximately 60%.
The epiglottis is the triangle-shaped piece of tissue that sits pointing forward at the entrance to the larynx. The epiglottis flips up and covers the opening to the wind-pipe during swallowing and protects against food material's entering the respiratory system. There are several problems that can occur involving the epiglottis. There is a generalized inflammation of the epiglottis that is called epiglottitis ("itis" meaning inflammation), the cause of which is unknown. It is hypothesized that other laryngeal abnormalities can predispose to the inflammation, as well as irritation by allergens, foreign bodies, poor quality roughage (hay), other environmental irritants such as dirt/dust, and/or the stresses involved with performance training. It has been observed that approximately 90% of horses with inflammation of the epiglottis are racehorses.
The clinical signs associated with epiglottic inflammation include exercise intolerance, making respiratory noise, coughing, and, occasionally, trouble swallowing or breathing. The diagnosis of epiglottitis is made using the endoscope and observing the hallmark signs of inflammation (redness, swelling, ulceration) of the epiglottis. The treatment involves rest for a minimum of two weeks, with endoscopic rechecks prior to resumption of exercise. The use of systemic and topically applied (sprayed up the nose with a catheter) anti-inflammatories is necessary. Inflammation of the epiglottis is a serious problem; it has been reported that up to 50% of racehorses affected with epiglottic inflammation developed performance-limiting complications.
Another problem occurring with the epiglottis is called epiglottic entrapment. The epiglottis has a firm cartilage inner make-up, so the structure is rather stiff. Epiglottic entrapment occurs when some of the soft tissue surrounding the epiglottis envelops it and stretches across it like a balloon. The clinical signs associated with epiglottic entrapment include exercise intolerance, making respiratory noise, coughing, and, occasionally, trouble swallowing or breathing. The diagnosis is made by use of the endoscope and observing the classic appearance of the epiglottis entrapped by the soft tissue. Epiglottic entrapment has been observed as a complication in 5% of horses having inflammation of the epiglottis. Treatment of epiglottic entrapment involves cutting away the soft tissue that has entrapped the epiglottis; this procedure often is performed with the horse under sedation and by using a laser via the endoscope.
Next to the vocal cord area of the larynx are structures called the arytenoids (pronounced ah-ret-e-noids). The arytenoids open and close in a vertical manner and are at the window to the windpipe. During inspiration, the arytenoids open wide and allow for maximal air flow through the laryngeal area into the trachea. The arytenoids are moved by a series of small muscles surrounding them under control of the laryngeal nerves. The right laryngeal nerve has a relatively straight shot from the brain to the larynx, but, due to some fluke of nature, the left laryngeal nerve travels from the brain all the way down the neck, flips around one of the major blood vessels near the heart, and travels all the way back up the neck before finally connecting to the muscle responsible for opening the left arytenoid.
A disease process has been described where the left recurrent laryngeal nerve degenerates, causing the left arytenoid to open improperly, if at all. The specific cause of this degeneration is unknown. The paralysis of the left arytenoid is usually not complete, with only 10% of horses having this abnormality showing complete paralysis. However, 70% of horses examined have some degree of asynchronous movement of the arytenoids when viewed via the endoscope. The draft breeds appear to be at greater risk for the development of this disease.
The clinical signs of laryngeal hemiplegia (meaning paralyzed on one side) include the making of a respiratory noise on inspiration and/or variable degrees of exercise intolerance. Not all horses experience exercise intolerance; that depends on both severity of the lesion and the horse's individual performance sport. The noise is a "roaring" type of sound; hence these horses often are called roarers. For high-stress endurance horses, the paralysis can affect overall athletic performance. For the less strenuous performers, such as low hunters, the noise produced is considered an "unsoundness" and might result in decreased placement in the ribbons.
The diagnosis of left laryngeal hemiplegia is made based on history, clinical signs, resting endoscopic examination, and potentially, high-speed treadmill endoscopic examination. The degree of paralysis is graded on a scale of 1 to 4 with respect to degrees of severity.
If the exercise intolerance is confirmed to be resulting from the airway paralysis, or the respiratory noise is unacceptable, surgical intervention is the currently accepted treatment. Surgical correction involves placing a permanent suture into the affected arytenoid and fixing it into the open position. In addition to the arytenoid suture, various pieces of tissue within the larynx are trimmed away depending on the individual case. The surgery is technically difficult and requires a great deal of skill.
For adult horses, the surgery is not a guaranteed fix--the success rate is reported to be 40-60% in adult racehorses. The outcome is reported to be slightly better for sport horses or young racehorses, with an 80-90% success rate.
Another treatment option currently being evaluated is the nerve-muscle pedicle graft. With the nerve-muscle pedicle graft, a small piece of muscle and its accompanying nerve are "transplanted" onto the muscle responsible for opening the arytenoid. This procedure shows promise, with successful cases having normal laryngeal function restored within one year of the surgery.
Other Inflammatory Conditions
In addition to the epiglottis, the other parts of the throat's anatomy can become inflamed for a variety of reasons. As with the epiglottis, other laryngeal abnormalities can predispose the horse to inflammation, as well as irritation, by allergens, foreign bodies, poor quality roughage (hay), other inhaled environmental irritants, and/or the stresses involved with performance training. The entire area might be inflamed, or just parts of the soft palate, surrounding tissue of the throat, or parts of the larynx itself (the flappers) could be affected.
The clinical signs associated with throat inflammation include exercise intolerance, making respiratory noise, coughing, and, occasionally, trouble swallowing or breathing; coughing might be one of the more prominent signs along with generalized throat inflammation. The diagnosis is made using the endoscope and observing the hallmark signs of inflammation (redness, swelling, ulceration). The treatment involves rest for variable periods of time in addition to the use of systemic and topically applied (sprayed up the nose with a catheter) anti-inflammatories.
There are many lymph nodes within the tissues of the head. A series of these lymph nodes surround both guttural pouches and the laryngeal area. Due to the location of these lymph nodes, there is great risk for them to affect the respiratory system. One of the more common causes of severe lymph node swelling and abscess formation in the head area is strangles. (For more information on strangles, a bacterial infection caused by Streptococcus equi, see The Horse of May 1997.)
As the lymph nodes swell and become large abscesses, they often take the path of least resistance, which sometimes is inward. It is ideal if the abscess enlarges toward the skin and ruptures/drains to the outside world, but sometimes it goes the other way. In severe cases of strangles, the lymph node swelling can become so great that there is enough compression of the laryngeal/upper trachea that airway obstruction occurs. Acute respiratory distress, to the point of requiring a tracheotomy (emergency surgery opening the lower part of the trachea for breathing), can be a complication of strangles. I also have seen several cases where a strangles lymph node ruptured into the guttural pouch and caused a thick white nasal discharge. I also have seen several cases where the lymph node ruptured into the trachea, spilling pus directly into the windpipe and causing extreme respiratory distress. If your horse has any evidence of lymph node swelling in the head area, or difficulty in swallowing/breathing, you should seek veterinary consultation and carefully monitor the horse.
Next month in Part 2 of Common Respiratory Problems, we will cover the lower airway, including reactive airway disease, viral pneumonia, bacterial pneumonia, shipping fever, pleuritis, and COPD.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: Rehabbing the Injured Horse