Guttural pouch

Guttural pouches are large, auditory-tube diverticula that contain between 300 and 600 ml of air. They are present in odd-toed mammals, some bats, hyraxes, and the American forest mouse. They are paired bilaterally just below the ears, behind the skull and connect to the nasopharynx. Guttural pouches are large, auditory-tube diverticula that contain between 300 and 600 ml of air. They are present in odd-toed mammals, some bats, hyraxes, and the American forest mouse. They are paired bilaterally just below the ears, behind the skull and connect to the nasopharynx. Due to the general inaccessibility of the pouches in horses, they can be an area of infection by fungi and bacteria, 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. The guttural pouches are located behind the cranial cavity, caudally the skull and below the wings of the atlas (C1). They are enclosed by the parotid and mandibular salivary glands, and the pterygoid muscles. The ventral portion lays on the pharynx and beginning of the esophagus, with the retropharyngeal lymph nodes located between the ventral wall and pharynx. The left and right pouches are separated by the longus capitis 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 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 and arteries that lie directly against the pouch as they pass into and out of foramina in the caudal aspect of the skull. The glossopharyngeal, vagus, accessory and hypoglossal nerves; the sympathetic trunk leaving from the cranial cervical ganglion; and the internal carotid all cause a mucosal fold indent within the medial compartment, visible when viewed endoscopically. The facial nerve 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. For many years, the functionality of the guttural pouches remained an unsolved mystery. Recent studies have shown that they play a role in cooling the blood from the internal carotid destined for the brain during hyperthermia. Even-toed artiodactyls possess a carotid rete, responsible for heat exchange, to cool arterial blood before it enters the cranial cavity. This anatomical adaptation acts to protect brain tissue from injury due to overheating. Odd-toed perrisodactyls such as horses lack a carotid rete, but since the internal carotid artery passes through the guttural pouches, it has been discovered that the air within the pouches cools the blood during exercise. Brain cooling by convection in the cerebral blood also occurs. 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. 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 be laryngeal hemiplegia (roaring) or difficulty swallowing (also if glossopharyngeal nerve is involved); and Horner’s syndrome from the involvement of sympathetic nerves. Involvement of the facial nerve is rare. As the guttural pouches are covered by respiratory epithelium 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 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 displacement may or may not be seen. Empyema is often secondary to guttural pouch tympany (distention of the pouches with air) in foals and weanlings. Arabians, in particular, are inclined to guttural pouch tympany, as many have a congenital defect 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.