Grande surprise in the guttural pouch

The guttural pouches (GP) in horses are diverticulae of the left or right tuba auditiva (TA), each diverticulum able to hold between 300 and 600mL of air. As either TA connects the nasopharynx with the middle ear of the ipsilateral side, a key function of the TA is to regulate pressure differences between the outer and middle ear. A lack of middle ear decompression leads to discomfort and hearing compromise. Inflammation is a main cause for loss of decompression. Middle ear decompression is achieved by allowing valves at the nasopharyngeal end of the TA to open with each swallowing. While TA are present in all mammals, only equids, odd-toed mammals, rarely bats and few rodent species possess a TA that enlarges into a GP or diverticulum. The function of this anatomical curiosity is still up for debate. In the horse, current opinion focuses on cooling of arterial blood prior to entering the brain via the carotid arteries. The GP is lined by a thin layer of mostly ciliated cuboidal epithelial or goblet cells similar to most parts of the upper respiratory tract. Also, spuriously distributed in both GPs are primary lymphoid follicles and M cells for antigen scavenging and processing. The actual GP cavity is divided by the tongue apparatus stylohyoid bone into 2 compartments, a lateral and medial division.

Endoscopic entry into the GP is through a cartilage-supported valve at the proximal TA. Advancing the endoscope into the GP always leads into the medial compartment, which opens ventrally to the examiner. For orientation, the stylohyoid bone (SB) is always lateral to the endoscope. The fold medial and dorsal to the SB is the actual TA continuing into the middle ear. The caudal end of the SB forms tongue skeleton part of the temporo-hyoid joint articulating with the temporal bone of the skull. Counterclock-wise in the left GP medial compartment there are ample views of cranial nerve branches including the cranial cervical ganglion, branches of the internal carotid artery, as well as of the occipital vein. The retropharyngeal lymph node typically protrudes into the GP from ventral during lymphadenopathy. Further medial is the longus capitis muscle, which is situated close to the inter GP septum separating left from right medial compartment. The lateral compartment is smaller, and contains facial nerve (VII), external carotid artery and maxillary vein.

Thus, inspecting the GPs can provide unique views of arterial blood vessels, nerves, a joint capsule and more, and should always be pursued when cranial nerve damage is evident (especially VII, IX, X, XII); in case of a head tilt, or when horses present with hemorrhagic nasal discharge.

Conditions affecting areas or function of parts or the entire GP can be divided into conditions caused by dysplasia, trauma, degeneration, neoplasia or inflammation. A malformation of the valve mechanism at the entrance to the nasopharynx can cause air trapping and tympany. Trauma can lead to skull fractures, basosphenoid, petrosal bone or stylohyoid bone fractures. Hemorrhage into the GP can be evident, and there also might be swelling. GP mycosis more commonly can be localized to the large arteries crossing through the pouch. The growth of Aspergillus fumigatus weakens the arterial wall and can cause exsanguinating hemorrhage. Rupture of the longus capitis muscle often leads to fatal hemorrhage, occurring typically retropharyngeally and outside of the GP, however, compressing the GP.

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