Congenital cysts, sinuses, or fistulae of the branchial arch apparatus or thyroglossal duct have been reported in horses, dogs, cats, and ruminants, yet are very rare. These structures arise from persistent embryologic pharyngeal pouches, arches, or clefts, or the thyroglossal duct. Animals 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. Animals may present later in life; it is not known why a cyst may suddenly enlarge but may 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 seen on the right side, although a bilateral case has been reported. Surgical excision is curative, although complications include right laryngeal hemiplegia, seroma formation, and pneumonia. Alternatively, some horses have been treated with marsupialization and iodine sclerotherapy with good results. In dogs and cats, few complications have been noted. Embryologic origin of branchial cysts has been mostly of the third pharyngeal pouch, although anatomic location has also suggested origin of the fourth and sixth pouches in some cases.
Thyroglossal duct cysts appear similar in appearance to branchial cysts; differentiation is often made histologically by demonstration of thyroid follicles containing colloid or immunohistochemistry for 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 birth. The cyst is solitary and usually located on ventral midline, although a case of a mediastinal thyroglossal duct cyst with ectopic thyroid tissue was reported in a cat and a case of subepiglottic thyroglossal duct cyst was reported in a dog. It is hypothesized that subepiglottic cysts in horses are also of thyroglossal duct origin and may be associated with epiglottic entrapment. In Damascus goats, thyroglossal duct cysts are heritable by an unknown genetic mechanism. Aspiration of the cyst for measurement of thyroxine has not been reliably diagnostic. Surgical resection is 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. In horses, dentigerous cysts are often seen in association with the wolf teeth or canine teeth of mares; in dogs, commonly of the brachycephalic breeds, with the mandibular first premolars; and in sheep, with the mandibular incisors. They may be bilateral.
Surgical removal of the cyst(s) is required, with definitive diagnosis based on subsequent histopathologic examination. Curettage of extremely large cysts with compromise of the mandible may require bone grafts.