While the function of the guttural pouch in athletic performance is unclear, disease of the area can be deadly
(www.thoroughbredtimes.com).- A HORSE'S guttural pouch is the Bermuda Triangle of horse anatomy. Though the area can be well defined and located, it remains somewhat a mystery as to its purpose. Diseases that occur in this region can be quite severe, and, like the Bermuda Triangle, small problems can have deadly consequences.
Guttural pouch diseases do not occur frequently. They represented less than 0.5% of all cases presented to the Western College of Veterinary Medicine in California over a two-year period. But the potential adverse effects of these conditions on the individual horse warrant attention and discussion.
What is a guttural pouch?
A guttural pouch is an air-filled out-pouching of the auditory or Eustachian tube. While all mammals have auditory tubes, not all have these pouches. Horses, mules, and donkeys have the largest, with one pouch lying on each side of the back of the throat.
Eustachian tubes extend from the inner surface of each eardrum to the sides of the pharynx. The pharynx is the large opening in the back of the mouth/throat where the nasal passages and the oral cavity join before separating into the trachea (airway) and the esophagus (feeding tube). Eustachian tubes connect to this large pharyngeal cavity as slit-like openings on either side of the back of the throat.
Auditory tubes serve an important function by regulating pressure on the eardrums. (Some children have had surgery to implant drainage tubes in the Eustachian tubes to reduce earaches.) The outer ear of all animals is exposed to atmospheric pressure while the inner ear and the ends of the auditory tubes are exposed to the pressure within the throat, which can be very different.
As a horse or human ascends a high mountain, the atmospheric pressure becomes lower. If the pressure is severe enough, the eardrum begins to bulge outward, and, if not for the auditory tubes, it probably would rupture. The auditory tubes, however, allow the body to equalize this pressure.
Each time an individual swallows, the pharyngeal or lower end of the tube opens. If the outer ear pressure is higher, some air is taken into the tube until the pressure on both sides of the eardrum is equalized. The reverse occurs if the outside pressure is less than the inner pressure.
Because the horse spent much of its evolutionary development on flat grassland, pressure regulation does not appear to be a factor in explaining the extreme size of the equine guttural pouch. More recent research points to its ability to significantly cool the horse's brain by moving air in and out of the guttural pouch as probably its key function.
The exercising horse generates a tremendous amount of heat because of the massive amount of muscular work required for high-speed motion. This heat must be removed from the body or internal temperatures will rise to debilitating levels. Many large arteries and veins that supply the head and brain are closely associated with the inner lining of the guttural pouches.
Researchers now believe that heat is removed from the exercising horse's brain via transfer from the blood to the air in the guttural pouch and then out through the pharynx. This mechanism also provides a clue as to why diseases occur in this anatomical area and why those diseases can be deadly.
Potential for disaster
With each swallow, air enters or leaves the guttural pouch of the horse. This means that any bacteria, fungi, or other infectious agents inhaled or ingested by the horse have ready access to both pouches. These potentially disease-causing particles can enter the pouches and usually become trapped in the mucus that lines these structures.
Most of the time, the horse's immune system successfully destroys these agents, but bacteria or fungi occasionally survive and continue to grow by invading the lining of the pouches.
The guttural pouch is a two-chambered space separated by the stylohyoid bone, and it can hold roughly 20 ounces of fluid or air. The medial or innermost chamber has several important blood vessels and nerves located on its surface. These nerves are responsible for various crucial body functions, such as breathing, swallowing, and chewing.
The lateral or outermost chamber is associated with more nerves and contains the internal carotid artery and the maxillary-facial vein. Proximity of the guttural pouches and the nerves, arteries, and veins associated with them also exposes these critical structures to infection and damage. The design of the guttural pouch is a beneficial marvel for the horse, but it also is a potential disaster waiting to happen.
There are three main diseases of the guttural pouch. The first is guttural pouch tympany.
Horses with this problem are born with a defect that causes the pharyngeal opening of the Eustachian tube to act like a one-way valve. Air can get in, but it cannot get out. This condition usually occurs in only one pouch, but it can affect both.
Because it is a problem present from birth, most cases of tympany are noted within the first few days of a foal's life. Several recent studies have looked at the genetic component of guttural pouch tympany, and links with the disease have been made to some Arabian and Hanoverian bloodlines.
Horses with tympany show massive swelling of the neck and throatlatch. If this swollen area is tapped with a finger, it resonates like a drum, which gives the condition its name. Affected horses might exhibit mild discomfort or be unable to breathe and swallow correctly if the swelling is severe enough.
Diagnosis is made based on the observed swelling and the foal's age, then the condition usually is confirmed by radiographs and endoscopic examination. Surgical correction is the treatment of choice, and one-sided tympany usually is easily corrected with a favorable prognosis. Those rare foals with both pouches affected require a much more complicated surgery and have a less favorable outcome, though recent advances employing laser surgery could soon improve their chances.
Guttural pouch empyema, or the presence of pus in the pouches caused by bacterial growth, is more common than tympany. Empyema usually occurs following an infection of the pharynx. Often significant infection can be present before external swelling of the pouch is noted.
The most common organism involved is Streptococcus equi, the causative agent of strangles. Horses with strangles often have abscesses in the pharynx that rupture and drain, which allows Streptococcus bacteria to gain entry into the guttural pouch. This bacterial strain can live in the guttural pouch for weeks to months causing a large amount of mucus that contains white blood cells from the immune system, bacteria, and necrotic tissue from the guttural pouch. A thick, pasty material develops that is not easily drained away.
The top part of the pouch might contain more fluid, but the bottom of the pouch holds a thicker material of almost cottage-cheese consistency with occasional solid masses of debris called chondroids.
An affected horse will show a persistent, creamy discharge, usually from one nostril. When the horse lowers its head to graze, the discharge usually increases as gravity allows more pus to exit the pouch into the pharynx and flow out the nasal passage.
Initially, these horses are rarely sick, but chronic infection will result in progressive weight loss, debilitation, and potential damage to the nerves and vessels within the pouch.
If the nerves become involved, horses can exhibit difficulty eating and swallowing. They also can have difficulty breathing. Research has shown that one potential cause of laryngeal paralysis, or roaring, is infection of the guttural pouch. Dorsal displacement of the soft palate, which also affects breathing, can be a complication. If the facial nerves are affected, some horses also might develop a drooped lip or ear.
Some cases of empyema resolve themselves without treatment as the pus eventually drains from the pouch. The majority of horses, however, will require aggressive flushing of the pouch and antibiotic therapy. Specialized catheters are placed in the affected pouch, and large volumes of fluids are repeatedly flushed in and out with significant pressure.
If the condition is chronic and severe enough, surgery sometimes is needed to drain the pouch. This is especially true if large chondroids have formed. Though the surgical approach is technically difficult, horses that receive treatment before damage occurs to nerves and blood vessels show good response and might recover fully.
Danger to carotid artery
Guttural pouch mycosis is the most serious of the three pouch diseases and is caused by the presence of fungi in the pouch. Aspergillus, Candida, Penicillium, and Mucor are the fungi most commonly found; they easily are encountered in hay, forage, and other parts of the horse's natural environment.
Fungal infections in the guttural pouch usually begin over one of the main arteries that traverse the pouch walls. Interestingly, research has shown that a defect in the artery wall is necessary for the fungal infection to start and that restricting blood flow through the artery might cure the condition without other treatment. If the mycosis is not resolved, however, the fungal infection slowly will erode the walls of the blood vessel, and the horse will begin to bleed.
The first sign of guttural pouch mycosis might be intermittent bleeding from one nostril, or epistaxis. If this mild bleeding is allowed to continue untreated, the blood vessel eventually will rupture, and the horse will bleed to death.
Fungal damage also can occur to the nerves within the walls of the pouches. Horses with this condition, even if they are treated and do not succumb to bleeding, could remain unable to eat and swallow. They often lose tremendous amounts of weight and eventually die.
Treatment involves a complicated surgery to ligate or tie off the blood vessels traveling through the affected pouch. Balloon catheters and surgical lasers now are being used in these procedures, but the prognosis for these horses remains guarded.
Antifungal medication is administered along with surgery for optimal chances of return to function in these cases. If the diagnosis can be made early, there is a better chance that surgery can be done and that no permanent damage will occur to crucial blood vessels and nerves.
Pay close attention
Few other areas in the horse's body are as important to its ability to function as an athlete but can lead to its destruction. Add the fact that the guttural pouches cannot be seen and are difficult to comprehend even with a description.
Yet, knowing about their existence, their function, and the signs of possible disease might allow you seek early treatment for your horse. Problems in this area do not happen often, but when they do, quick response could save a horse's life.