Congenital cysts, sinuses, or fistulas of the branchial arch apparatus, or thyroglossal duct, have been reported in horses, dogs, cats, and ruminants, yet they are very rare. These structures arise from persistent embryologic pharyngeal pouches, arches, or clefts, or the thyroglossal duct. Patients typically present with nonpainful, fluid-filled masses in the cervical region. Clinical signs are typically due to the space-occupying mass and include dyspnea, respiratory stridor, intermittent esophageal obstruction, and coughing. Patients may present later in life; it is not known why a cyst may suddenly enlarge, but such enlargement could be associated with respiratory infection. Diagnostic imaging includes radiography, ultrasonography, video endoscopy, and contrast CT to determine whether there is communication with the pharynx.
Branchial cysts (also called lateral cervical cysts) in horses are typically observed on the right side, although one bilateral case has been reported. Surgical excision is curative, but possible complications include right laryngeal hemiplegia, seroma formation, and pneumonia. Furthermore, the proximity of vital structures in the neck of the horse, including the recurrent laryngeal nerve, vagosympathetic trunk, jugular vein, and carotid arteries, increases the risks associated with cyst excision. Alternatively, some horses have been treated with marsupialization and iodine sclerotherapy with good results. In dogs and cats, few complications have been noted. Branchial cysts originate mainly in the second or third pharyngeal pouch of the embryo, but some originate in the fourth or sixth pouch.
Thyroglossal duct cysts appear similar to branchial cysts; often the two types of cysts are only differentiated by histologic demonstration of thyroid follicles containing colloid or by immunohistochemistry for the presence of thyroglobulin. These cysts arise from the thyroglossal duct, which in the embryo is present from the base of the tongue to the eventual location of the thyroid, and which is normally absent by the time of birth. The cyst is solitary and usually located on the ventral midline of the cranial cervical region, but exceptions of location can occur. Subepiglottic cysts in horses are also thought to originate in the thyroglossal duct, and they may be associated with epiglottic entrapment. In Damascus goats, thyroglossal duct cysts are heritable by an unknown genetic mechanism. Aspiration of the cyst to measure thyroxine has not been reliably diagnostic. A few cases of neoplastic transformation of thyroglossal cysts to thyroglossal duct cyst carcinoma have been reported in geriatric dogs. Even in the case of neoplastic cysts, surgical resection is usually curative.
In horses, congenital cystic lesions of the esophagus are typically of two types: intramural inclusion cysts and esophageal duplication cysts. Both have been described in yearling horses that presented for recurrent choke, dysphagia, and aspiration pneumonia. Both occur in the cervical esophagus and result in compression of the esophageal lumen and its function. Diagnosis is aided by imaging, including video endoscopy, contrast radiography, and ultrasonography. Described treatments include surgical resection and marsupialization with sclerotherapy. Complications after excision have included left laryngeal hemiplegia and esophageal fistula formation. Histologically, esophageal duplication cysts include a layer of muscle, whereas inclusion cysts contain only keratinized squamous epithelium.
Heterotopic polyodontia, or teeth outside the dental arcade, includes both dentigerous cysts, which have been described in most domestic animal species, and the ear teeth, or temporal teratoma, of horses. Dentigerous cysts contain all or part of at least one tooth (including the crown). The cysts are lined by epithelium and often cause facial swelling or draining tracts, if fistulated. Dentigerous cysts often occur in association with the wolf teeth or canine teeth of mares in horses; with the mandibular first premolars in dogs, commonly of the brachycephalic breeds; and with the mandibular incisors in sheep. They may be bilateral.
Surgical removal of dentigerous cysts is required, and definitive diagnosis is based on subsequent histopathologic examination. Diagnostic imaging, particularly CT, is useful to determine the extent that tissues surrounding the cyst are involved, and to aid in surgical planning. Depending on the location of the cyst, complications of surgical excision include temporomandibular joint sepsis, cranial nerve trauma, and development of fistulous tracts. Curettage of extremely large cysts with compromise of the mandible may require bone grafts.