Hidden at the base of the horse’s skull is a pair of air-filled guttural pouches. These are pockets, or “out-pouchings”, of the auditory (Eustachian) tubes, which connect the nasopharynx (the area between the nasal cavity and the larynx) to the middle ear.
Humans do not have these structures — they are present only in the horse, the rhino, the tapir, a small rabbit-like creature called the hyrax, certain bats and a species of South American mouse.
The role of the guttural pouches is not known for sure. The most recent and promising theory is that the air in the pouches cools the blood supply to the brain. Preventing overheating of this sensitive organ is particularly important during fast exercise, when the temperature of circulating blood can increase by up to 2°C.
In an average 500kg horse, each of the guttural pouches measures about 450cm3 in volume (roughly the size of a fist). They are bordered by many vital anatomical structures, including the base of the first vertebra, the pharynx, various muscles and the parotid and mandibular salivary glands. This makes both medical imaging and surgical access challenging.
The left and right pouches are completely separate, and are divided by a thin membrane called the midline septum at the top, and the rectus capitus and longus capitus muscles at the bottom. Each pouch is then partially divided by the stylohyoid bone into an outer (lateral) and inner (medial) compartment.
There are numerous crucial structures running within the walls of the guttural pouches. These include large arteries and veins, plus cranial nerves that are vital for a variety of neurological functions including swallowing, pharyngeal tone and sympathetic nerve supply to the head. There are also large lymph nodes in the floor of the pouches, which can harbour infection — for instance, in cases of strangles.
Diseases affecting the guttural pouch can damage some or all of these structures. This is what makes guttural pouch disorders so serious and often life-threatening. Because of their location deep within the horse’s head, by far the easiest way to view what is going on inside the pouches is by using a flexible endoscope. A camera is passed up the nose and guided through the narrow Eustachian tubes, the entrances of which are marked by a flap of cartilage.
It can sometimes be difficult to pass the scope into the pouches, particularly if the horse is small or if the pouches are narrowed by inflammation. X-ray examination of the pouches can at times be useful.
Fungal infection
The most feared disease of these structures is guttural pouch mycosis, which carries an overall death rate of 50% in untreated horses.
A fungal plaque develops on the back wall of the guttural pouches. This fungus has a predilection to grow right on top of the internal or external carotid arteries, or both. Erosion of the wall of these major arteries causes bleeding, and blood is seen at one or both nostrils.
Often, a small, seemingly innocuous nosebleed will precede a major, life-threatening haemorrhage. It is for this reason that all nosebleeds (other than those in horses known to have sustained facial trauma, or racehorses after fast exercise), should be investigated with relative urgency.
The fungus can also erode the cranial nerves. This results in various neurological abnormalities, including laryngeal paralysis (roaring), and Horner’s syndrome, which causes changes to the eyelid position and the pupil of the eye, along with abnormal facial sweating. A further abnormality is difficulty swallowing, which may be so severe that the horse develops aspiration pneumonia.
The fungal plaque itself is malodorous. Many affected horses also have a history of an unpleasant-smelling purulent (pus-like) discharge from both nostrils. Treatment involves surgery to block the affected artery, thereby preventing a fatal bleed. This can be achieved by simple ligation (tying) of the vessel, or the insertion of a catheter into it to block off a longer segment of the artery.
More recently, insertion of metal coils into the artery to block off the bleeding section has been described. This has the advantage of being performed in a minimally invasive way via a small incision in the neck.
Topical administration of anti-fungal solution to the plaque within the guttural pouches on a regular basis is also recommended, once ligation of the artery has taken place.
Pus in the pouches
Guttural pouch empyema is a condition associated with infection with Streptococcus equi var. equi, a bacterium known as strangles. Abscesses may form in lymph nodes in the floor of the pouches, which then burst and leak pus into the pouch interior. Pus may be seen draining from the Eustachian tubes into the nasopharynx, and the horse may have enlarged lymph nodes.
Pus, either in liquid form or as solid concretions called chondroids, may also occur on the floor of the pouches. Laboratory testing of fluid pooling within the pouches should confirm the diagnosis of strangles. Chronic carriers may harbour the bacterium within the guttural pouches, with or without obvious pooling of fluid or chondroid formation.
Horses afflicted with viral upper respiratory tract infections, or bacterial infections other than strangles, can also occasionally develop inflammation and pus within the guttural pouches. Daily lavage (flushing) of the pouches may be useful to encourage drainage of purulent material, while chondroids must be physically removed before infection can be resolved. Lavage is often followed by direct installation of penicillin solution.
Various surgical approaches can be used to remove chondroids from the pouches if they are too large or too numerous to be retrieved with trans-endoscopic instruments. Other disorders are relatively rare but include guttural pouch tympany, which is an accumulation of air within the pouches, and neoplasia — the development of tumours.
Ref Horse & Hound; 27 October 2016
