Guttural pouches are a paired extension of the eustachian tubes that can be found in perissodactyls (ungulate mammals). Several functions have been proposed for guttural pouches, including: pressure equilibration across the tympanic membrane; brain cooling; and acting as a resonating chamber for vocalisation. They are separated into two compartments – medial and lateral – by the stylohyoid bone. The medial compartment is three times bigger than the lateral compartment.
Most cranial nerves (vagus, glossopharyngeal, hypoglossal, spinal accessory, cranial laryngeal nerves, pharyngeal branch of the vagus and facial nerve) are located within the guttural pouch itself or closely connected with its walls, which relates to some of the clinical signs encountered with guttural pouch mycosis.
The vestibulocochlear nerve (CN VIII) does not enter the guttural pouch directly but may be involved in guttural pouch diseases that affect the middle ear, such as temporohyoid osteoarthropathy. Finally, the internal carotid artery lies within the caudal wall of the medial compartment, while the external carotid artery and maxillary artery are located within the lateral compartment of the guttural pouch.
Aspergillus fumigatus is the most common isolate identified in cases of guttural pouch mycosis. The fungal mycelia together with various bacteria, necrotic tissues and cell debris form what is called the diphtheric membrane, which is the typical lesion found in horses affected with guttural pouch mycosis (Figure 1).
Clinical signs are variable and closely related to the structures involved in the mycosis. Epistaxis (sometimes unilateral but more commonly bilateral) is probably one of the most common clinical signs seen in horses and varies from repeated episodes of mild epistaxis to severe haemorrhage, which can be fatal.
Other common signs encountered in cases of guttural pouch mycosis include dysphagia, secondary aspiration pneumonia, pharyngeal paresis/laryngeal hemiplegia and Horner’s syndrome. Less common clinical signs may be seen in some cases and include mucopurulent nasal discharge, corneal ulcers, head shyness, blindness, tongue paralysis and mycotic encephalitis.
Diagnosing the mycosis
Diagnosis is made on the basis of history and clinical signs and is confirmed using diagnostic imaging. Endoscopy is the most commonly used diagnostic tool for guttural pouch mycosis as it permits direct visualisation of the mycosis but also allows for complete evaluation of the affected structures (including the larynx region and the presence of dysphagia/laryngeal hemiplegia), which will guide treatment choices. Other diagnostic tools that have been used to diagnose guttural pouch mycosis include radiographs and computed tomography.
Treatment options
Both medical and surgical treatments have been described. It has been reported that up to 50 percent of horses presenting with signs of haemorrhage die from this complication, so preventing haemorrhage should be the first aim of treatment.
Medical treatment
Medical treatment includes daily lavage through the endoscope or through a Chambers/Foley catheter introduced within the affected guttural pouch under endoscopic guidance, detachment of the diphtheric membrane using biopsy forceps or cytology brushes under endoscopic guidance and anti-fungal therapy (systemic and/or topical). Itraconazole (5mg/kg PO or used as a topical infusion of 30ml of the 10mg/ml solution) and enilconazole (60ml of a 33mg/ml solution) are most commonly used. Supportive treatment should also be initiated depending on the clinical signs seen (such as fluid therapy, blood transfusion and nasogastric tubing, as indicated).
Medical treatment can be slow to resolve the mycosis and generally provides inconsistent results. In some cases, a spontaneous regression has been described over time and so the efficacy of treatment has been questioned when only assessing the regression of the mycotic plaque.
Medical treatment does not prevent fatal haemorrhage secondary to the erosion of one of the carotid arteries and may even dislodge blood clots that are occluding the defect; case selection should be made carefully. In addition, the length of treatment needed to completely resolve the lesion should be taken into consideration when discussing prognosis (especially for horses showing neurological signs) and finances with the owners (Dobesova et al., 2012).
Surgical treatment
Various surgical treatments have been proposed over the years and aim at occluding the affected vessel to prevent fatal haemorrhage (Freeman, 2015). It has also been suggested that it may hasten recovery, but this remains controversial. Recently, balloon catheterisation and coil embolisation seem to be the treatment of choice in most cases.
Both techniques rely on the exact knowledge of the structure affected (internal versus external carotid artery and location of the defect), as this will dictate the surgical approach. Occlusion using a balloon catheter (Fogarty venous thrombectomy catheter or Foley catheter) inserted through an arteriotomy in the appropriate vessel provides immediate occlusion of the affected vessel and prevents retrograde blood flow (Figure 2).
It is a simple, effective and inexpensive method to occlude the affected artery. It does, however, require general anaesthesia and removal is necessary two to three weeks after the initial procedure. One of the main disadvantages of this technique is the inability to directly visualise the vessels to be catheterised; aberrant branches may be occluded, which could lead to complications such as blindness. Infection of the surgical site and breakage of the catheter are also reported complications.
Occlusion of the affected vessels using coil embolisation has been described both as a procedure under general anaesthesia and also standing under sedation (Benredouane and Lepage, 2012). It relies on the use of stainless steel coils that selectively occlude arterial segments. The procedure is performed using the injection of contrast material under fluoroscopic guidance, allowing direct visualisation of the vessel occluded and limiting complications associated with occlusion of aberrant branches. Though this technique is less invasive than the balloon catheterisation technique, it is also associated with higher cost, concerns with radiation shielding, limited availability and coil migration.
Both techniques have a reported success rate of approximately 80 percent. Ligation of the ipsilateral common carotid artery has also been reported in the past. The use of this technique may be controversial today as some studies have shown that it could potentially increase the blood flow within the internal carotid artery; but it may be beneficial for haemorrhages originating from the external carotid artery. One study showed a 21 percent recurrence of epistaxis and 17 percent fatality using a combination of ligation of the ipsilateral common carotid artery and topical treatment (Cousty et al., 2016).
Conclusion
Guttural pouch mycosis is a rare but potentially fatal disease affecting horses. Although the diagnosis is often straightforward based on history, clinical signs and endoscopy, its treatment can be challenging. Risks and benefits should be explained to the owner and financial implications should also be taken into consideration when making a decision. Owners of horses that present with neurological signs (dysphagia and laryngeal hemiplegia) should be warned that these signs are likely to persist or have incomplete resolution, which may also have an effect on treatment selection.
