Guttural pouches are large, auditory-tube diverticula that contain between 300 and 600 ml of air. They are present in odd-toed mammals, some

bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most ...

s, hyraxes, and the American forest mouse. They are paired bilaterally just below the ears, behind the skull and connect to the

nasopharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struct ...

. Due to the general inaccessibility of the pouches in horses, they can be an area of infection by

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

and

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

, and these infections can be extremely severe and hard to treat. The condition ''guttural pouch tympany'' affects several breeds, including the Arabian horse. The condition predisposes young horses to infection, often including severe swelling and often requires surgery to correct. The guttural pouch is also the site of infection in equine strangles.

Structure

The guttural pouches are located behind the cranial cavity, caudally the skull and below the wings of the

atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

(C1). They are enclosed by the

parotid The parotid gland is a major salivary gland in many animals. In humans, the two parotid glands are present on either side of the mouth and in front of both ears. They are the largest of the salivary glands. Each parotid is wrapped around the ma ...

and mandibular salivary glands, and the

pterygoid Pterygoid, from the Greek for 'winglike', may refer to: * Pterygoid bone, a bone of the palate of many vertebrates * Pterygoid processes of the sphenoid bone ** Lateral pterygoid plate ** Medial pterygoid plate * Lateral pterygoid muscle * Medi ...

muscles. The ventral portion lays on the pharynx and beginning of the esophagus, with the

retropharyngeal lymph nodes The retropharyngeal lymph nodes, from one to three in number, lie in the buccopharyngeal fascia, behind the upper part of the pharynx and in front of the arch of the atlas, being separated, however, from the latter by the Longus capitis. Their ' ...

located between the ventral wall and pharynx. The left and right pouches are separated by the

longus capitis The longus capitis muscle (Latin for ''long muscle of the head'', alternatively rectus capitis anticus major), is broad and thick above, narrow below, and arises by four tendinous slips, from the anterior tubercles of the transverse processes of th ...

and rectus capitis ventralis muscles dorsomedially. Below these muscles, the two pouches fuse to form a median septum. The guttural pouches connect the middle ear to the pharynx. The opening into the pharynx is called the nasopharyngeal ostium, which is composed of the pharyngeal wall laterally and a

fibrocartilaginous Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of ...

fold medially. This opening leads to a short soft tissue passageway into the respective guttural pouch. The openings are located rostrally to enable drainage of mucous when the head is lowered and prevent fluid build-up. The
plica salpingopharyngea
'' a mucosal fold at the caudal portion of the Eustachian tube, forms an uninterrupted channel between the medial lamina of the Eustachian tube and the lateral wall of the pharynx. The plica salpingopharyngea can sometimes act as a one-way valve trapping air in the pouch, causing ''guttural pouch tympany''''.'' Each pouch is moulded around the stylohyoid bone which divides the medial and lateral compartments. The medial compartment is much larger, and protrudes more caudally and ventrally. The epithelium is pseudostratified and ciliated containing mucous-secreting goblet cells; lymph nodules are also present. The compartments of each guttural pouch contain many important structures including several

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

and arteries that lie directly against the pouch as they pass into and out of

foramina In anatomy and osteology, a foramen (;Entry "foramen"
in

in the caudal aspect of the skull. The

glossopharyngeal The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. B ...

vagus The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right v ...

, accessory and

hypoglossal The hypoglossal nerve, also known as the twelfth cranial nerve, cranial nerve XII, or simply CN XII, is a cranial nerve that innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus, which is innervated by ...

nerves; the

sympathetic trunk The sympathetic trunks (sympathetic chain, gangliated cord) are a paired bundle of nerve fibers that run from the base of the skull to the coccyx. They are a major component of the sympathetic nervous system. Structure The sympathetic trunk lies j ...

leaving from the cranial cervical ganglion; and the

internal carotid The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these ...

all cause a mucosal fold indent within the medial compartment, visible when viewed

endoscopically An endoscopy is a procedure used in medicine to look inside the body. The endoscopy procedure uses an endoscope to examine the interior of a hollow organ or cavity of the body. Unlike many other medical imaging techniques, endoscopes are insert ...

. The

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste ...

is in contact with the dorsal part of the pouch. The external carotid artery passes ventral to the medial compartment before crossing to the lateral wall of the lateral compartment. The pouch also covers the temporohyoid joint.

Function

The function of the guttural pouches has been shown with experimental data to participate in the rapid cooling of arterial blood destined for the brain and surrounding structures. In other words, the horse's guttural pouches are 'brain-cooling devices', cooling blood within the internal carotid arteries during

hyperthermia Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...

that occurs during heavy exercise. However, this proven function has been called into question by another study that neither examined the guttural pouches nor the internal carotid arteries; and others have argued that a cooling function would require an unattainable high rate of inspiratory air flow partly diverted into the guttural pouches. The issue of necessary guttural pouch air flow rates, to provide rapid cooling of the internal carotid arteries, has been solved by further supporting evidence from microvascular studies of the guttural pouch mucosa. Many of the guttural pouch mucosal superficial arterioles and capillaries extend outwards, forming two types of vascular plexuses surrounding the internal carotid arteries: one with capillary bundles parallel to the internal carotid arteries in the outer layer of the tunica adventitia (outer peri-arterial plexus) and the other with vein-artery-vein triads within the inner layer of the tunica adventitia (inner peri-arterial plexus). These guttural pouch microvascular plexuses, engulfing the internal carotid arteries, are typical of countercurrent heat exchangers recognised in other animal species, supporting the data that guttural pouches participate in selective brain cooling, even at lower air flow rates. This completes the triad of internal carotid artery cooling that protects the horse's brain from

. The triad including the function of guttural pouches achieving arterial cooling, via both by utilizing inspiratory air cooling as well as microvascular countercurrent heat exchangers surrounding internal carotid arteries, and finally afterwards where the internal carotid arteries project upwards passing through the intracranial cavernous venous sinuses accepting cooled venous blood from the nasal sinuses.

Pathology

If the drainage tract becomes blocked for any reason, the mucous secretions can accumulate and cause the pouch to distend, producing a visible and palpable protrusion behind the

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

. The exudate may become contaminated with pathogens. The bacteria Streptococcus equi, the causative agent of strangles, is commonly involved. Mycotic infections can also occur. Some visible symptoms of guttural pouch disease include abnormal head and neck carriage, nasal discharge, painful swelling and occasional abnormal functioning of the structures associated with the pouch. Secondary problems may include inflammation of the middle ear due to migration of the infection along the auditory tube; nasal bleeding caused by damage to the internal carotid artery; with vagus nerve involvement there may b
laryngeal hemiplegia
(roaring) or difficulty swallowing (also if glossopharyngeal nerve is involved); and

Horner's syndrome Horner's syndrome, also known as oculosympathetic paresis, is a combination of symptoms that arises when a group of nerves known as the sympathetic trunk is damaged. The signs and symptoms occur on the same side (ipsilateral) as it is a lesion ...

from the involvement of sympathetic nerves. Involvement of the facial nerve is rare. As the guttural pouches are covered by

respiratory epithelium Respiratory epithelium, or airway epithelium, is a type of ciliated columnar epithelium found lining most of the respiratory tract as respiratory mucosa, where it serves to moisten and protect the airways. It is not present in the vocal cords o ...

and mucosa, they have the potential to be affected by all respiratory pathogens. Most infections are self-limiting, requiring no or little medical intervention. Upon endoscopy, affected guttural pouches often house mucopurulent fluid that is in the process of draining.

Guttural Pouch Empyema

Guttural pouch empyema is characterized by the accretion of purulent, bacteria infested exudate in the pouch. The bacteria is primarily ''Streptococcus equi'', the infectious agent of strangles. Clinically apparent symptoms include painful swelling of the parotid area and recurrent infected nasal discharge, and in severe circumstances, difficulty breathing and abnormal head carriage may be observable. Fever, anorexia, difficulty swallowing and

soft palate The soft palate (also known as the velum, palatal velum, or muscular palate) is, in mammals, the soft tissue constituting the back of the roof of the mouth. The soft palate is part of the palate of the mouth; the other part is the hard palate. ...

displacement may or may not be seen. Empyema is often secondary to guttural pouch tympany (distention of the pouches with air) in foals and

weanling A weanling is an animal that has just been weaned. The term is usually used to refer to a type of young horse, a foal that has been weaned, usually between six months and a year. Once it is a year old, the horse is referred to as a yearling. The ...

Arabians The Arabs (singular: Arab; singular ar, عَرَبِيٌّ, DIN 31635: , , plural ar, عَرَب, DIN 31635: , Arabic pronunciation: ), also known as the Arab people, are an ethnic group mainly inhabiting the Arab world in Western Asia, No ...

, in particular, are inclined to guttural pouch tympany, as many have a

congenital defect A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can r ...

in the pharyngeal orifices of the pouches. The infection can also be due to the rupture of the nearby retropharyngeal lymph nodes, usually caused by an abscess. Diagnosis is established through and endoscopic examination. Radiographic examination of the area will show an opaque fluid line in the pouch and if a retropharyngeal lymph node is involved, it may reveal a mass. In mild, acute cases of empyema, a saline or polyionic solution lavage is often performed via an endoscope or

catheter In medicine, a catheter (/ˈkæθətər/) is a thin tubing (material), tube made from medical grade materials serving a broad range of functions. Catheters are medical devices that can be inserted in the body to treat diseases or perform a surgi ...

repeatedly until the exudate drains. Antimicrobial therapy without a lavage seldom remedies the infection. In more complex cases, where concretions have formed, surgical intervention may be necessary to ensure appropriate drainage and removal of the hardened material. The area can be accessed surgically through the Viborg's triangle^,.

Guttural Pouch Tympany

Guttural pouch tympany is an uncommon ailment in which excessive amounts of air become trapped in the pouch, resulting in abnormal expansion. Tympany is usually unilateral, but in some cases can affect both pouches. It is seen most often in young foals and is more common in females than in males. Tympany results in non-painful, soft swelling beneath the ear and behind the jaw. Additional symptoms may include roaring, difficulty breathing, and difficulty swallowing and/or

aspiration pneumonia Aspiration pneumonia is a type of lung infection that is due to a relatively large amount of material from the stomach or mouth entering the lungs. Signs and symptoms often include fever and cough of relatively rapid onset. Complications may inc ...

. Diagnosis is achieved through radiography and endoscopic evaluation. The specific cause of guttural pouch tympany is not known, but it is suspected that it is more common when there are defects of the plica salpingopharyngeus, and/or the pharyngeal orifice where they act as a one-way valve that does not allow air to escape. Because of the risk of secondary infection, it is crucial that tympany be treated as soon as possible. Treatment protocols may include, but are not limited to, surgical intervention and in cases where surgery is not an option, insertion of a transnasal Foley balloon catheter in an attempt to remodel the pharyngeal orifice.

Guttural Pouch Mycosis

Guttural pouch mycosis (GPM) is a fungal disease that is rare but potentially life-threatening. GPM is of unknown

pathogenesis Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...

currently and no predisposing factors have been identified. Fungal plaque is usually located in the medial guttural pouch, near the internal carotid artery. Clinical signs include unilateral or bilateral

epistaxis A nosebleed, also known as epistaxis, is bleeding from the nose. Blood can flow down into the stomach, and cause nausea and vomiting. In more severe cases, blood may come out of both nostrils. Rarely, bleeding may be so significant that low bl ...

due to erosion of the artery walls, nasal discharge and cranial nerve dysfunction. GPM is a dangerous condition as spontaneous fatal

hemorrhaging Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, vag ...

can occur, usually within a few days to weeks after the first bout of epistaxis^,. The most common fungus associated with GPM is ''

Aspergillosis Aspergillosis is a fungal infection of usually the lungs, caused by the genus ''Aspergillus'', a common mould that is breathed in frequently from the air around, but does not usually affect most people. It generally occurs in people with lung di ...

''. Diagnosis is made based on the history of the animal, presenting clinical signs and endoscopic exploration. Pharmaceutical treatment is not suggested without coinciding surgery. Treatment typically consists of topical as well as systemic antifungal medication, paired with surgery to occlude or embolise affected arteries. Early intervention is necessary to ensure the best chance of survival. Horses that experience

dysphagia Dysphagia is difficulty in swallowing. Although classified under "symptoms and signs" in ICD-10, in some contexts it is classified as a condition in its own right. It may be a sensation that suggests difficulty in the passage of solids or liq ...

or other forms of nerve dysfunction as a result of GPM have a poorer prognosis that those who have not exhibited those symptoms.

References

{{Reflist, 30em Equidae Hyraxes Animal anatomy Horse anatomy Mammal diseases Horse diseases