The guttural pouch is a unique structure found in horses and only a few other species. It is an extension of the Eustachian tube, which is an air-filled canal that connects the throat to the middle ear. There are two guttural pouches (one on each side) that are located just below the ear in the throatlatch region (Figure 1). The function for these specialized air-filled sacs remains unknown but possible uses include pressure equalization across the ear drum, warming of inhaled air, a resonating chamber for vocalization, and to aid in the cooling of blood that flows to the brain during exercise.
Each guttural pouch is divided into two compartments by the stylohyoid bone (Figure 2). There are a number of important nerves that run along the walls of the guttural pouch and control the muscles involved in swallowing, upper airway function, and facial expression. Additionally, several important blood vessels, namely the internal carotid artery, external carotid artery, and maxillary artery, all pass along the walls of the guttural pouch in order to provide blood supply to the brain and head.
Guttural pouch mycosis is a fungal infection of one or both guttural pouches. Fungal plaques form within the guttural pouches, most commonly along the walls of the major blood vessels (internal carotid, external carotid, and maxillary arteries) (Figure 3). Overtime, the fungus can erode through the wall of vessels, resulting in hemorrhage that can be life threatening. If left untreated, a fatal bleed often ensues. The cause of guttural pouch mycosis is unknown, but the fungus Aspergillus is the most common type of fungus identified. There is no age, sex, breed, or geographical predisposition.
The most common clinical sign that horse owners will see is a moderate to severe nose bleed. This is due to the fungus eroding through the wall of one of the arteries located within the guttural pouch. The blood is usually bright red in appearance. Several smaller nose bleeds may precede a fatal episode, but any episode of blood coming from one or both nostrils should be considered an emergency and your primary care veterinarian contacted immediately for further evaluation.
The second most common clinical sign is dysphagia or difficultly eating and swallowing. This occurs if the fungus damages one or more of the nerves that control tongue movement and swallowing. Dysphagic horses may struggle to prehend food, be seen quidding or dropping feed, cough and have feed containing nasal discharge while eating, and pack feed in their cheeks because they are unable to properly swallow.
Other clinical signs less commonly observed with guttural pouch mycosis include the development of Horner’s syndrome (drooping eyelid, constricted pupil, sunken eye, and patchy sweating on the neck observed on only one side), white nasal discharge, abnormal head posture, and pain in the throatlatch region. Additionally, abnormal respiratory noise can occur due to damage to the nerves that innervate the muscles of the throat.
Endoscopy of the guttural pouch is the goal standard to diagnose guttural pouch mycosis. This involves passing an endoscope, which is a small flexible camera, up the nose and into the guttural pouches. Blood can often be seen coming from one or both guttural pouch openings if the horse is examined shortly after an episode of a nose bleed (Figure 4). Within the guttural pouches, fungal plaques, which appear as white, tan, and black membranes overlying one or more blood vessels, can be seen (Figure 3). The appearance of fungal plaques within the guttural pouch confirms the diagnosis of guttural pouch mycosis. The size of the fungal plaques can vary, but the size of the lesions bears no relationship to the severity of the disease.
The aftercare for horses following surgical treatment for guttural pouch mycosis varies greatly depending on what structures were affected by the fungus. In horses in which bleeding is the only pre-operative sign, aftercare is usually minimal. The fungal lesions usually regress over 30–180 days after the affected artery has been occluded, and additional antifungal treatments are not necessary. If a horse does not die from a significant nose bleed and surgery is performed as soon as possible, the horse usually has a good prognosis.
If signs of nerve dysfunction are present before surgery, then these horses may require additional aftercare or surgical procedures. Horses having difficultly eating may require intravenous nutrition or to be fed via a tube that is passed into their stomach. Horses with upper airway dysfunction may require additional surgery to treat the airway problem. Recovery of nerve function can be slow and may take up to 6–18 months, with some horses never fully recovering.