Surgical Management of a Subepiglottic Thyroglossal Duct Cyst in a Dog

Introduction

The thyroglossal duct is created as the embryonic thyroid gland migrates from the base of the tongue to its normal position in the neck. In humans, the duct normally involutes by the fifth week of gestation. If the duct does not obliterate prior to development of the mesodermal anlage of the hyoid bone, the duct remains patent and at risk for cyst formation.¹ Thyroglossal duct cysts (TDCs) are common in human pediatric patients, but are only rarely reported in animals. Recently, there have been reports of TDCs in cats, horses, and one calf.^2–5 Two reports of TDCs in dogs have been published.^6,7 The purpose of this report is to describe the diagnosis and surgical management of a subepiglottic TDC in a dog.

Case Report

A 2 yr old castrated male mixed-breed dog weighing 31.2 kg presented for dysphagia, coughing, and halitosis. The dog had originally been examined by another veterinarian 6 mo prior to presentation at the authors’ hospital for Orthopedic Foundation for Animals radiographs of the hips. Upon sedation, the veterinarian noted a fluctuant, subepiglottic mass. Based on its location, the mass was suspected to be a salivary mucocele and was lanced intraorally. Cytology was not performed at that time. The dog subsequently presented to his primary veterinarian 1 wk after the hip radiographs for dysphagia, coughing after eating and drinking, and exercise intolerance. Bilateral mandibular and sublingual sialoadenectomy was performed by another veterinarian, but the dysphagia and the subepiglottic mass persisted.

Upon presentation to the authors’ hospital, the dog was bright, alert, and responsive and had a rectal temperature of 38.6°C. A paroxysmal cough and mild inspiratory stertor were noted. An oral examination revealed a 4 cm × 4 cm turgid mass rostral and adjacent to the epiglottis at the base of the tongue (Figure 1). Initial diagnostic work-up included a complete blood count, serum biochemistry profile, thoracic radiographs, cervical (laryngeal) radiographs, and an esophagram performed under fluoroscopy. Results of the complete blood count, serum biochemistry profile, and thoracic radiographs were unremarkable. Cervical radiographs revealed a soft-tissue opacity in the caudal oropharynx, extending to the cranial laryngopharynx, associated with the base of the tongue (Figure 2). There were also metal opacities and heterogenous tissue markings in the region of the laryngopharynx, consistent with hemostatic clips placed during the sialoadenectomy. The esophagram confirmed the presence of a lobular pharyngeal mass associated with the base of the tongue. During swallowing the mass displaced caudally and partially entered the cricopharyngeal sphincter and cranial esophageal lumen (Figure 3). During the liquid and paste phases of the esophagram, there was a subjectively mild delay in bolus formation, but no abnormalities of swallowing or esophageal motility were noted during the food phase.

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Contrast computed tomography (CT) of the head and neck revealed a cystic hypoattenuating mass that was in contact with the epihyoid bone and contiguous with the base of the tongue (Figures 4A, B). The mass was not contrast-enhancing and when directly injected with iodinated contrast, no direct connections with salivary structures were identified (Figures 4C, D). The medial retropharyngeal lymph nodes were bilaterally enlarged, but the other cervical structures were unremarkable. Tissue was obtained by incisional biopsy and submitted for histopathology. Cytology of aspirated fluid revealed a serosanguinous, noninflammatory fluid consistent with a cyst, and bacterial culture had no growth over 48 hr. Histopathology from the biopsy sections revealed the following: areas of granulation tissue and mixed cuboidal and squamous epithelium; degenerate and nondegenerate neutrophils, macrophages, lymphocytes and plasma cells; aggregates of erythrocytes; and scattered extracellular Gram-positive cocci. These findings were all consistent with an abscess and hematoma.

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Surgical excision was performed 1 wk later. The dog was premedicated with intramuscular acepromazine^a (0.05 mg/kg) and intramuscular methadone^b (0.3 mg/kg). General anesthesia was induced with ketamine^c (5 mg/kg IV) and diazepam^d (0.25 mg/kg IV). The dog was maintained on isoflurane^e in 100% oxygen. An esophagostomy tube was placed prior to mass excision because the postoperative plan was to withhold oral intake of food and water until healing was complete. The dog was placed in dorsal recumbency, and an 8 cm midline incision was made through the skin and subcutaneous tissues beginning 1 cm caudal to the thyroid cartilage and extending cranially. The myohyoideus and hyoglossus muscles were retracted laterally, and a combination of sharp and blunt dissection was used to isolate the mass. Delineation of the mass was facilitated by intraoral palpation during the dissection. A marginal excision was completed with a circumferential incision into the oropharyngeal mucosa surrounding the mass. The residual sublingual salivary glands were also removed at this time because they were in the surgical field and readily resectable. There was no grossly apparent connection between the salivary glands and the mass. The oropharyngeal mucosa was closed in a simple interrupted pattern with 3-0 polydioxanone^f, the hyoglossus and myohyoideus muscles were apposed in a simple continuous pattern with 3-0 polydioxanone, the subcutaneous tissue was closed in a simple continuous pattern with 3-0 polyglecaprone^g, and the skin was closed in a Ford interlocking pattern with 3-0 nylon^h. The mass and the salivary tissue were submitted for histopathology. Postoperative management consisted of IV fluid support with lactated Ringer’s solutionⁱ (2 mL/kg/hr), methadone (0.1 mg/kg IV q 6 hr), cefazolin^j (22 mg/kg IV q 8 hr), and a cold pack on the incision q 6 hr. The dog was discharged 1 day postsurgically and prescribed tramadol^k (3 mg/kg per os q 8 hr for 5 days) and amoxicillin trihydrate/clavulanate potassium^l (12 mg/kg per os q 12 hr for 14 days). All food and water was administered through the esophagostomy tube by the owner for 4 days. Appropriate healing was noted during a sedated oral examination performed 5 days after surgery. Oral intake of water and soft foods was subsequently initiated (Figure 5), but the esophagostomy tube was left in place until 10 days after surgery.

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Histopathology of the mass revealed a stratified squamous epithelium with focal areas of ciliated epithelial cells lining the majority of the cyst. There was granulation tissue and a focus of neutrophils that was deemed to be the previous biopsy site. Based on the histopathologic characteristics, anatomic location, and age of the dog, a diagnosis of TDC was made. Fourteen months postsurgically, there was no evidence of recurrence.

Discussion

The thyroglossal duct is created as the embryonic thyroid migrates from the base of the tongue to its final position in the neck. In human pediatric patients, TDCs are the among the most common causes of ventral cervical swelling, accounting for 75% of midline cervical swellings in children and 7% in adults.^1,8,9 Thyroglossal duct remnants only occur in 7% of the population, and the majority of remnants are asymptomatic.⁹ A TDC can be located nearly anywhere in the neck, but presents most commonly as a painless swelling between the tongue and the thyroid gland on the ventral midline of the neck.⁹ In humans, 66% of TDCs are located adjacent to the hyoid bone, but they can also be between the tongue and hyoid, between the hyoid and pyramidal lobe, within the thyroid gland itself, or even located within the tongue. The cyst often moves with swallowing.^9,10 Because of the close association with the oral cavity, approximately 33% of TDCs in humans present with an infection, and an infected TDC is the most common presentation in adulthood.⁹ In animals, TDCs have reportedly occurred as ventral cervical swellings, but a TDC has also been observed in the caudal mediastinum in one cat.^2,4,6,7 The subepiglottic location of the TDC reported in this case has not been previously described in dogs. The clinical signs observed with TDCs are the consequence of a space-occupying mass in the cervical region and include dysphagia, dyspnea, coughing, dysphonia, and nonpainful swelling.^2–7 In humans, the cyst can contain ectopic thyroid tissue that may be the only functional thyroid tissue in the patient.⁹ Clinical signs and thyroid stimulating hormone levels consistent with hypothyroidism may also be present in these cases, as the ectopic thyroid tissue usually fails to produce adequate thyroxine (T₄).⁹

Although an incisional biopsy was performed in this dog, it is not recommended to biopsy a suspected TDC in humans because there is an increased recurrence rate following biopsy.⁹ Moreover, ectopic thyroid tissue that would definitively identify a TDC is only reported in 62% of excisional biopsies in humans.¹¹ Indeed, the biopsy in this case did not yield a definitive diagnosis, as the lesion still could not be differentiated between an abscess, mucocele, or cyst after histopathology. However, the biopsy was helpful in ruling out oral malignancies, which are common causes of oropharyngeal masses in dogs.¹² In this case and in another case of one cat, the lesions were biopsied prior to the definitive surgery, and no recurrence of disease occurred.⁴ Due to the apparent rareness of TDCs in animals, the authors recommend performing a preoperative biopsy of the lesion to rule out a malignant process.

In one case of a cat with a TDC, the authors reported that the cyst T₄ concentration can be used to help diagnose the condition.² However, in that patient, the cyst fluid had a T₄ concentration similar to the serum T₄ concentration.² In the case described in this report, neither the cyst nor serum T₄ concentration was measured. Cyst T₄ concentration, however, was not useful in the case report of the cat with a TDC and is not part of the normal diagnostic paradigm in humans.^2,9,10 In contrast, serum thyroid stimulating hormone may raise suspicion for a papillary adenocarcinoma within the TDC in human patients, but the diagnostic value of serum thyroid stimulating hormone levels is not known in dogs. As TDCs are an embryologic abnormality of the thyroid system, thorough evaluation of the native thyroid gland may also prove useful in dogs suspected to have a TDC.

In this case, diagnostic imaging, especially CT, was beneficial because it revealed the cystic nature of the lesion. As shown in Figure 4, after contrast was injected into the lesion, the cyst lacked any intimate association or communication with any of the surrounding structures. This finding was further confirmed under fluoroscopy where the mass was seen to move freely in and out of the proximal esophagus (Figure 3). These findings suggested that the lesion was neither a mucocele nor a solid tumor. Additionally, the imaging results were consistent with what is reported in the human literature in that the mass in this patient was seen moving with swallowing on the esophagram, and the CT revealed a well-defined, low-attenuating, and minimally contrast-enhancing cystic structure.¹³

Differential diagnoses in this case included a pharyngeal mucocele, salivary mucocele, abscess, neoplasia, and a cyst. A definitive diagnosis was not achieved in this case because histopathology was not absolutely conclusive. However, although the cyst lacked thyroid tissue, the stratified squamous epithelium with focal areas of ciliated epithelium lining the cyst was consistent with that described for TDCs.¹¹ In human TDCs, histopathology can reveal a variety of findings, including evidence of infection/abscessation, cystic spaces that either have or lack a lining, ciliated columnar epithelium, squamous epithelium, or occasionally thyroid follicles.¹⁴ Because solid thyroid tissue is only seen on histopathology in 62% of cases, lack of thyroid tissue does not preclude a diagnosis of TDC. Moreover, this patient had two previous surgeries that may have disrupted the architecture of the mass. The cystic nature of the lesion and its location on the ventral midline are consistent with the presentation of TDCs.

The standard treatment of TDCs in humans is surgical excision via the Sistrunk procedure, in which the entire duct is dissected and excised from the foramen cecum in the tongue caudally to the thyroid gland. Part of the hyoid bone is also often removed because it is intimately associated with the cyst-duct complex.⁹ If the entire cyst-duct tract is excised, the recurrence rate is between 2.6% and 5%.⁹ Failure to remove the entire tract can result in a 38–70% recurrence rate.⁹ Although the mass was only marginally excised in the dog described in this report, the authors anticipate the likelihood of recurrence to be low. In this case, a tract from the base of the tongue to the thyroid gland was not identified. The lack of an identifiable duct from the tongue to the thyroid is consistent with other reports in the veterinary literature.^2,4 Fourteen months postoperatively, despite the marginal excision in this case, the cyst had not recurred. There have been no reports of recurrence after marginal surgical excision of TDCs in other animals.^2,4,6,7

The authors elected to excise the mass through a ventral midline approach to the oropharynx in this case. Due to the caudal location of the mass within the pharynx, the authors suspected that surgical manipulation and dissection may have been difficult via an oral approach. Intraoral resection of the mass would have been especially challenging, compared with a similar resection in humans, as the more prominent protruding mandibles and comparatively large tongue precludes good exposure to the pharynx in dogs. Ventral exposure of the mass was relatively simple because of the minimal amount of soft tissue between the skin and the base of the tongue. As the mass was on midline, exposure was achieved with bilateral retraction of the lingual musculature. The oral cavity was included in the surgical field to allow direct palpation and ventral pressure on the mass, which facilitated delineation of abnormal tissue during the dissection. Given the recurrent nature of the mass after lancing, marsupialization was not considered. The mass may have been amenable to laser resection via endoscopy; however, the specialized equipment required for this treatment option was not available at the authors’ institution.

Because the larynx is pulled caudally during swallowing, the authors suspected there would be a significant amount of tension placed on the mucosal closure adjacent to the larynx each time the animal swallowed. By placing an esophagostomy tube before surgery and providing all of the dog’s food and water via the tube during the early phase of wound healing, the number of times the dog swallowed was reduced, thus minimizing the risk for dehiscence.

Conclusion

TDC should be considered a likely differential diagnosis for a subepiglottic cystic mass in dogs. Biopsy is recommended to rule out a malignancy; however, histopathologic confirmation of TDC is not always achieved. Diagnostic imaging appears most helpful for confirming the location and cystic nature of TDCs. Marginal surgical excision of TDCs in dogs appears to be curative.