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Guttural pouch mycosis in horses

Author : Safia Barakzai

Categories : Vets

Date : July 12, 2010

Safia Barakzai discusses the anatomy of the guttural pouch in equines, signs of fungal plaque, and the treatment and prognosis of mycotic infection

ASPERGILLUS fumigatus is an opportunistic respiratory pathogen responsible for mycotic infection of the guttural pouch.

The fungus has a predilection site in the dorsal aspect of the medial compartment of the pouch, which is usually at, or just below, the sigmoid flexure of the (Figure 1).

The fungus often erodes the major blood vessels that lie within the pouch, and severe arterial epistaxis, not associated with exercise, is a common presenting sign in equines.

Guttural pouch anatomy overview

The guttural pouches (Figure 2) are large mucosa-lined outpouchings of the auditory (eustachian) tubes that connect the nasopharynx to the middle ear. In a 500kg horse they are around 350cc in volume. Each pouch is incompletely divided into lateral and medial compartments by the stylohyoid bone.

The medial compartment contains the internal carotid artery (ICA), which runs from ventral to dorsal in the caudal wall of the medial compartment, and forms a sigmoid flexure just before reaching the dorsal limit of the pouch. IX (glossopharyngeal), X (vagus), XI (accessory) and XII (hypoglossal) emerge from the jugular foramen and hypoglossal canal. They course ventrally through the medial compartment of the guttural pouch, enveloped in a thin fold of

1 / 5 mucosa.

These nerves and the sympathetic trunk are intimately associated with the ICA for much of their course within the pouch. The pharyngeal branch of the (X) runs along the floor of the medial compartment before joining the pharyngeal plexus. The ventral straight muscles of the head (rectus and longus capitis muscles) form the caudomedial border of the pouches. The lateral compartment of the guttural pouch contains the , which enters the pouch ventrally and courses dorsally. The maxillary vein runs lateral and deep to the maxillary artery.

Clinical signs of guttural pouch mycosis

The most frequently observed clinical sign of guttural pouch mycosis (GPM) is epistaxis at rest, which may be severe enough to be fatal. Commonly, an affected horse will have one or two small to moderate-volume bleeds before a large-volume epistaxis occurs. Therefore, it is important to investigate all cases of acute epistaxis occurring in horses at rest with some urgency, unless there is a history of recent head trauma. Although the mycotic infection in GPM is usually unilateral, epistaxis may be unilateral or bilateral, depending on the volume of blood draining into the nasopharynx or if mycotic infection has caused erosion of the median septum of the guttural pouch.

The second most frequent clinical sign exhibited is pharyngeal , which manifests as nasal regurgitation of food – with or without water. can develop in more severe or protracted cases. Pharyngeal paralysis occurs due to damage to cranial nerves IX and X, which lie in close association with the ICA underneath the fungal plaque. Damage to the (XII), may cause tongue weakness or hemiparesis.

Other GPM clinical signs can be similar to those in many other diseases affecting the pouches and include mucopurulent to purulent unilateral or bilateral nasal discharge, swelling of the ipsilateral submandibular lymph nodes and parotid region, hyperextension of the head and neck (Figure 3) and dyspnoea. Neurological signs – such as laryngeal hemiplegia (recurrent laryngeal branch of X), Horner’s syndrome (cranial sympathetic nerve) and, less commonly, persistent dorsal displacement of the (pharyngeal branch of X) and (VII) paralysis – can occur when other cranial nerves that run within the guttural pouch are damaged.

Occasionally, fungal erosion of the stylohyoid bone may result in stylohyoid bone osteopathy and/ or pathological fracture. Affected horses have acute onset of severe dysphagia and quidding.

Endoscopic examination

Endoscopic examination of the guttural pouch is easier to perform with a small-diameter (such as 8mm) endoscope, because the auditory tube is a narrow structure. In addition to the endoscope, some sort of blunt probe is required to elevate the fibrocartilaginous ostium of the guttural pouch within the nasopharynx and guide the endoscope into the auditory tube.

2 / 5 With the endoscope introduced via the ventral meatus into the nasopharynx, the guttural pouch ostia and adjacent nasopharyngeal mucosa should be examined for the presence of blood or mucopurulent/purulent discharges (Figure 4).

If a recent haemorrhage has occurred, and a stream of blood or a blood clot is seen emanating from the ostium, great care should be taken when advancing the endoscope into the pouch in case the clot is disrupted and a fatal haemorrhage ensues. The history of profuse epistaxis not associated with exercise, plus the endoscopic finding of blood emanating from a guttural pouch ostium, provides enough evidence to warrant referral to a surgical facility without performing endoscopy of the pouch itself.

The nasopharynx and larynx should be examined endoscopically for evidence of concurrent neuropathies that may affect the long-term prognosis of the case. Figure 5 shows a case of rightsided GPM that has secondary right-sided laryngeal neuropathy and pharyngeal paralysis.

Once at a surgical facility, the pouches should be examined endoscopically, pre-operatively to confirm that GPM is indeed the cause of epistaxis, to verify which pouch is affected (erosion of the median septum may result in blood emanating from the contralateral ostium in the nasopharynx) and to ascertain which artery within the pouch is affected. If the affected pouch is still filled with blood, determination of the affected artery may not be possible. However, the ICA is the site of haemorrhage in the majority of cases.

Endoscopy of the guttural pouch reveals characteristic grey, black or white fungal and fibrinous plaques (Figures 1 and 6). Occasionally, the plaque can be very extensive, and may erode the median septum (Figure 6) and affect the contralateral pouch, or even affect the atlanto-occipital joint. However, the size of the lesion is not necessarily a prognostic indicator, as even small lesions can cause dysphagia that necessitates euthanasia and, conversely, some very extensive lesions can be treated successfully.

Treatment and prognosis of GPM

If a horse has experienced signifi– cant blood loss, it should initially be stabilised with intravenous fluids. Non-surgical treatment is slow and there is a considerable risk of a fatal haemorrhage occurring if arterial occlusion is not performed, even in cases that present initially with dysphagia and not epistaxis. The author’s treatment of choice is balloon catheterisation for occlusion of the internal carotid artery and endoscopically guided postoperative lavage of the fungal plaque with topical antifungal solution.

Although resolution of GPM has been reported with arterial ligation alone, no clear reason can be seen as to why this should occur (the ICA and ECA do not supply the mucosa of the guttural pouch), hence a “belt-and-braces” approach to such a potentially devastating disease appears logical. The response to any treatment method should be interpreted with caution, because

3 / 5 spontaneous regression of the lesion over a variable course of time is typical (Freeman and Hardy, 2006).

Regardless of the choice of treatment, NSAIDs should be administered to reduce neuritis of the cranial nerves.

• Non-surgical treatment

Endoscopically guided lavage of the fungal plaque and associated diphtheritic membrane (Figure 7) can, in the author’s experience, help to speed resolution of GPM by providing topical treatment of the mycosis, and also by physically breaking down and eliminating the fungal plaque.

Great care should be taken if arterial occlusion has not been performed prior to topical lavage because of the risk of physically disturbing the plaque sufficiently to cause haemorrhage from the vessels lying underneath. Lavage fluids used for this purpose include dilute povidine iodine, natamycin and enilconazole. Systemic antifungals have been used in horses, but treatment may be required for several months and is usually cost-prohibitive.

• Surgical treatment

Surgical occlusion of the affected artery must be performed to prevent an episode of fatal epistaxis. This can be achieved by simple ligation of the artery on the cardiac side of the lesion (ICA only), use of a balloon to occlude the affected segment(s) of artery (ICA or ECA), or use of more complex surgical methods, such as transarterial coil embolisation or detachable balloon catheter systems (ICA or ECA).

• Prognosis

If horses present with epistaxis, a 50 per cent mortality rate is reported if left untreated (Cook, 1968). However, the mortality rate is considerably higher for horses that present with dysphagia (Greet, 1987). With arterial occlusion procedures, the prognosis for horses presenting with epistaxis and no neurological complications is excellent.

Horses with dysphagia can recover, but recovery times can be very prolonged (six to 18 months) and may be incomplete (Freeman and Hardy, 2006).

Recovery from other cranial neuropathies, such as Horner’s syndrome, recurrent laryngeal neuropathy and facial nerve paralysis have also been reported, although, again, the resolution period can be quite prolonged.

References

4 / 5 Cook W R (1968). The clinical features of guttural pouch mycosis in the horse, The Veterinary Record, 83: 336. Greet T R C (1987). Outcome of treatment in 35 cases of guttural pouch mycosis, Equine Vet J, 19: 483. Freeman D E and Hardy J (2006). Guttural pouch. In: Auer J A and Stick J A (eds), Equine Surgery, Elsevier: 591-608.

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