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The outer surfaces of both eardrums are exposed to outside air, so the pressure on these outer surfaces is the same as atmospheric pressure. However, the inner surfaces of the eardrums are not exposed to outside air, so the pressure on them is unaffected by changes in atmospheric pressure. When atmospheric pressure decreases, as it does when you (or your horse) go up into the mountains, the pressure on the outer surface of the eardrum is lower than the pressure on the inner surface. Without the eustachian tube, this pressure difference would cause the eardrum to bulge outward and, if the difference in pressure became great enough, to rupture. When the atmospheric pressure increases, as it does when you come down from the mountains, the eardrum would bulge inward. How do the eustachian tubes function to equalize the pressure on both sides of the eardrums? Every time you (or your horse) swallow, the pharyngeal (lower) ends of the eustachian tubes open. If the pressure in the eustachian tube (which is the same as the pressure on the inner surface of the eardrum) is higher than atmospheric pressure, air passes from the eustachian tube into the pharynx with each swallow until the pressures are equal. If the inside pressure is lower than atmospheric pressure, air passes from the pharynx into the eustachian tube with each swallow.
The pressure-regulating function of the eustachian tube is important in keeping the eardrum from rupturing, but it has a downside. Every time an animal swallows, the eustachian tube opens, allowing any bacteria in the nose or mouth a free passage to the middle ear. If the pressure in the eustachian tube is higher than that in the pharynx, bacteria tend to be forced out. If the pressure in the eustachian tube is lower than that in the pharynx (as happens with a rapid decrease in altitude, for example), bacteria tend to be sucked in. All other factors being equal, the pressure in the eustachian tubes is normally slightly lower than atmospheric pressure. As a result when atmospheric pressure is steady, air and bacteria are sucked into the eustachian tubes every time an animal swallows. Excessive mucus (which may be sucked in from the pharynx when an individual with a cold swallows) can trap bacteria inside one of the eustachian tubes. In the human, such bacteria commonly invade the middle ear. In horses, however, the bacteria encounter the guttural pouch before they can reach the middle ear. As we have said, the guttural pouches are outpouchings of the eustachian tubes of horses. They are quite large, each having a volume of approximately 10 ounces (more than 100 times the volume of the remainder of the eustachian tubes). Their function, if any, is unknown, but air entering the horse's eustachian tubes when he swallows must pass through the guttural pouches. A suggested, but unproved, function of the air passing into the guttural pouches is to cool the horse's head. We can also speculate that the guttural pouches may serve as a buffer to cause the horse's eardrums to be less affected than those of other animals by changes in atmospheric pressure. Pouch Diseases The guttural pouches may protect the horse from ear infections by trapping bacteria that enter his eustachian tubes before they reach the middle ears. However, for the same reason the guttural pouches are vulnerable to disease. They are exposed to any bacteria that enter the horse's nose or mouth, and they detain such bacteria, giving them time to invade the walls of their prison. Guttural pouch disease can have serious consequences, because numerous large and important arteries and nerves pass through the guttural pouches. These arteries and nerves have little defense against infection. As illustrated by the horses with guttural pouch tympany, guttural pouch empyema, and guttural pouch mycosis, diseases of the guttural pouch can have a wide variety of symptoms. In the remainder of this article we'll discuss the symptoms caused by each of these three diseases and consider what can be done to treat them. Guttural Pouch Tympany Some horses have a defect in the pharyngeal opening of one of their eustachian tubes (rarely both) so that it acts as a one-way valve. Air can enter, but it can't get back out. In these individuals, the affected guttural pouch becomes very distended. If you tap on these air-distended pouches, they resonate like a drum. Hence the name: guttural pouch tympany. Affected horses are generally born with the defect that leads to guttural pouch tympany. As a result the condition most commonly becomes apparent when a foal is only a few days or weeks old. Signs: In guttural pouch tympany there is an obvious swelling in the throatlatch. Even though only one guttural pouch is generally affected, the swelling usually becomes so extensive that it involves the entire throatlatch. In some cases the foal looks like an amorous prairie chicken out to attract a harem. It often is impossible to tell which pouch is affected by just looking at the foal. In some cases the swelling causes no apparent inconvenience to the foal. However, if the swelling is severe, the foal might have trouble breathing and swallowing because everything in his throat is pushed out of position and/or obstructed by the enlarged guttural pouch. Milk may run back out of his nostrils when he suckles. In particularly severe cases, a foal's swallowing difficulties may lead to milk being sucked down his trachea (windpipe) and into his lungs, causing pneumonia. Diagnosis: Guttural pouch tympany is not difficult to diagnose since foals with this condition almost always have the characteristic swelling in the throatlatch. However, a veterinarian may have to use X-rays and/or perform an endoscopic examination to confirm the diagnosis and determine which guttural pouch is involved. To determine which side is involved, the veterinarian may need to pass a catheter through the pharyngeal opening of each eustachian tube to deflate the guttural pouches, one at a time. Treatment: Surgery is required to treat guttural pouch tympany. If only one side is involved, the surgery is relatively simple, and the prognosis is good. If both sides are involved, the surgery is much more complicated, and the prognosis is less favorable. In the past the surgery required to allow the air to escape from both pouches often led to the formation of scar tissue that made the tympany a permanent condition. Although excessive scar tissue is less likely to occur with modern laser surgery, it is still a worry. Guttural Pouch Empyema Guttural pouch empyema is defined as pus in a guttural pouch. The persistent nasal discharge in the 3-year-old filly was due to this condition. Empyema generally occurs after an infection in a horse's pharynx, most commonly equine strangles. As we have pointed out, bacteria in the pharynx have ready access to the guttural pouches. Every time a horse swallows the pharyngeal ends of the eustachian tubes open and negative pressure pulls in any bacteria in the vicinity of the opening. In equine strangles abscesses commonly form in the pharynx and, when these break, they drain large quantities of creamy, streptococcus-laden fluid (pus) into the eustachian tubes. Ordinarily the horse's defenses are efficient in removing bacteria entering the eustachian tubes. But, if the horse's system is occupied in fighting off equine strangles, or if a very large number of bacteria enter one of the guttural pouches, the eustachian tube's defenses may be overwhelmed. When bacteria gain a foothold inside a guttural pouch, the horse's defenses are mobilized against them, and a major battle ensues. The creamy fluid that forms during this conflict consists of living and dead bacteria and white blood cells as well as debris from the damaged guttural pouch. Since the pharyngeal opening is above the majority of the space in a guttural pouch, the fluid is not easily drained away. However, when the pharyngeal end of the eustachian tube opens when the horse swallows, fluid near the top of the infected pouch may escape and pass out the nostril on the affected side. The infection in the guttural pouch may last for weeks to months. As time passes the fluid remaining in the bottom of the pouch becomes thicker until it consists largely of solid bodies with a cottage cheese-like consistency. These bodies, which are called chondroids, will not drain through the eustachian tube. Signs: The cardinal sign of guttural pouch empyema is a persistent creamy nasal discharge, usually only from one nostril. The discharge increases when the horse lowers his head, because more of the contents of the guttural pouch are above the pharyngeal opening when the horse's head is down. If the horse swallows with his head down, as he does when grazing, a large volume of pus may escape from the guttural pouch as the eustachian tube opens. In some cases there is noticeable swelling over the affected guttural pouch; however, a large amount of exudate must accumulate before such swelling is discernable. Unless the swelling is sufficient to interfere with breathing, or the nerves in the guttural pouch are damaged, the horse with guttural pouch empyema generally shows little sign of overall ill health. In rare cases of empyema, fluid draining from the eustachian tube is sucked back into the lungs, causing aspiration pneumonia, a condition that is quite difficult to treat. Diagnosis: Guttural pouch empyema should be suspected in any horse with a persistent white to yellow discharge from one nostril, especially if the discharge is more evident when the horse's head is lowered. A veterinarian can confirm the diagnosis, determine the amount of exudate present, and identify any chondroids (which may necessitate a different approach to treatment) by means of an endoscopic examination of the guttural pouches. He may need to take X-rays to determine the amount the exudate and whether chondroids have developed. Treatment: Many cases of guttural pouch empyema resolve without treatment, the fluid gradually draining out as the grazing horse swallows. In fact, a high percentage of cases of strangles probably result in a transitory guttural pouch empyema that clears up on its own. However, if there is a large volume of thick fluid in the guttural pouch or if chondroids have developed, the horse will have to be treated. Any horse in which a nasal discharge persists for more than a week or two is a candidate for treatment. Antibiotics may be required to kill the bacteria, but antibiotics alone are not sufficient to cure persistent empyema. To clear the fluid from the guttural pouch, it is necessary to flush large volumes of sterile fluids through the pouch via the eustachian tube or to surgically drain the pouch. Surgical drainage is generally required when there are very large volumes of thick pus in the guttural pouch. Surgery is almost always necessary to remove chondroids. Following flushing or drainage, it is helpful to place the horse's feed on the floor of the stall so that any remaining fluid can be expelled when he swallows. A horse has a very good chance of recovering from guttural pouch empyema if his treatment is begun before any complications, such as nerve damage, have had time to develop. Guttural Pouch Mycosis Guttural pouch mycosis, a fungal infection of the wall of the pouch, is by far the most serious of the diseases affecting the guttural pouches. As with empyema, generally only one pouch is affected. Fungi are common in the horse's environment and frequently enter the guttural pouch. However, in most cases they don't invade the wall of the pouch. If the fungi do establish themselves, they usually do so directly over one of the major arteries that traverse the wall of the pouch. Some authorities believe that an abnormality in an artery is essential to the development of guttural pouch mycosis. This belief is supported by the fact that surgically obstructing the flow of blood through an involved artery frequently cures the condition without any other treatment. At any rate, once the fungi become established, they invade the artery and cause repeated episodes of bleeding. At first the bleeding generally amounts to little more than a trickle, and the blood passes out the nostril on the affected side. However, if the condition is not treated, the flow of blood becomes so extensive that blood flows from both nostrils, even though it is coming from just one of the guttural pouches. A high percentage of horses with guttural pouch mycosis will eventually bleed to death if they are not treated. The 13-year-old gelding described at the beginning of this article bled to death from guttural pouch mycosis. In addition to damaging the arteries, the fungi may attack some of the vital nerves that traverse the guttural pouches. Depending on the nerves that are damaged, the horse with guttural pouch mycosis may become a roarer, develop a dorsally displaced soft palate, have a paralyzed and drooping ear and/or lip on the affected side or, most importantly, have difficulty in swallowing. The last is an especially severe complication. Normal swallowing is unlikely to return once the nerves are disrupted, and the affected horse generally wastes away because he is unable to eat normally. Signs: The cardinal sign of guttural pouch mycosis is repeated nosebleeds, occurring when the horse is resting in his stall or on pasture. Nosebleeds associated with exercise are generally due to conditions other than guttural pouch mycosis. Repeated nosebleeds are cause for genuine alarm and warrant an immediate examination by a veterinarian to determine their cause. In some cases of guttural pouch mycosis, damage to the nerves in the guttural pouch precedes damage to the arteries. Sometimes horses with this disease, being unable to swallow because of nerve damage, die of starvation without ever having a nosebleed. Diagnosis: The best (often the only) way to diagnose guttural pouch mycosis is by endoscopic examination of the guttural pouches. If the horse is bleeding heavily, the veterinarian may have to wait until the bleeding stops and stabilize the horse by giving intravenous fluids (or even a blood transfusion) before he can complete the examination. The diagnosis is more challenging in horses who have difficulty in swallowing in the absence of bleeding, because guttural pouch disease may not be suspected. However, endoscopic examination of the guttural pouches of such horses will detect the fungal lesions. Treatment: Various antifungal drugs have been used to treat guttural pouch mycosis, but drug treatment alone is seldom successful. The only treatment that has given consistent results is surgical ligation of the affected arteries. Such surgery is complicated because, due to the complex blood supply to the brain, blood can enter the affected arteries from two directions. Thus it is necessary to block the flow of blood to the affected artery from both sides of the fungal lesion. The surgery must not be delayed, once the diagnosis has been confirmed, because the horse may bleed to death at any time. In most cases the fungal lesions will disappear with no treatment other than surgery, but antifungal drugs are sometimes placed in the guttural pouch following surgery. The prognosis in guttural pouch mycosis is always guarded, even after surgical correction, because the blood flow to the arteries may be re-established and the horse may begin to bleed again several months to years after the surgery. A horse who has difficulty swallowing due to nerve damage associated with guttural pouch disease has a poor chance of recovering. Photo Gallery: **Caution this gallery contains very graphic photos of equine patients with Gutteral Pouch disease.** Click here. Dwight G. Bennett, a regular contributor to the Western Horseman magazine, is professor emeritus of equine medicine at Colorado State University. Although retired, he remains active in consulting and writing.
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