Introduction

Branchial or pharyngeal cleft anomalies are the most common congenital lateral neck mass in human pediatric medicine.¹ In embryology, the branchial apparatus, defined as the complex region in the developing embryo between the head and chest, is composed of mesodermal arches separated by external clefts or grooves and internal pouches (Figure 1).^1–4 The pharyngeal pouches are a series of four to five bilateral evaginations of the pharyngeal endoderm that project laterally between the pharyngeal arches.^1–4 During development, each arch leads to a specific anatomic structure of the head and neck (Figure 1).² The external auditory tube originates from the first cleft (derived from the first arch), whereas the others invaginate to form the cervical sinus that later obliterates.^2–5 Abnormal development or regression of these structures can lead to a variety of different anomalies such as cysts (epithelial-lined structures that lack a connection to the skin or pharynx), sinuses, or fistulae.^2–5 Only a few pharyngeal cysts (also named branchial cysts) involving the head and neck have been reported in the veterinary literature (nine cases of pharyngeal cysts in the neck area in dogs and cats counted).⁵ Pharyngeal remnants in the thymus or mediastinum (anomalous regression of the pharyngeal pouches or clefts) have also been described.^4,5 Although these anomalies are usually described in the cervical region, here we describe the case of a dog diagnosed with a pharyngeal cyst in the nasopharynx, causing signs typical of nasopharyngeal obstruction.

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Case Report

A 2 yr old neutered male shih tzu was presented to the internal medicine department of the Alliance veterinary Clinic for evaluation of chronic snoring. The clinical signs were present since his adoption at the age of 3 mo and had progressively worsened. The dog developed mild heat and exercise intolerance. Two months before presentation, an acute episode of inspiratory dyspnea was noted by the owners. On presentation, the dog was bright and alert and the general physical examination was unremarkable except for mild inspiratory dyspnea and stertor. A complete blood count and a biochemistry panel were declined by the owners. Endoscopy of the upper respiratory tract was performed with a flexible endoscope.^a General anesthesia was induced with propofol^b (4 mg/kg IV and as needed) after sedation with midazolam^c (0.2 mg/kg IV) and maintained with isoflurane in oxygen after endotracheal intubation. Examination of the pharyngeal region revealed moderately hypertrophic tonsils and a moderately elongated soft palate. The nasopharyngeal region was evaluated during a J-maneuver and a well-circumscribed, smooth, round mass ∼2 cm in diameter, originating from the median plane of the soft palate and protruding into the nasopharynx, was identified (Figure 2A). The larynx did not show any abnormalities. A computed tomographic examination^d of the head was performed before and after IV injection of contrast medium (Iopamidol^e 300 mg/mL to 600 mg/kg). It revealed a 1.8 × 1 × 1 cm mass, hypoattenuating and lined by a thin peripheral rim of positive contrast enhancement, located within the soft palate and obstructing the nasopharynx (Figure 2B). These features were considered compatible with a cystic mass lesion. Fine needle aspirations of the mass were performed and a thick viscous fluid was retrieved. Cytological evaluation revealed basophilic amorphous material compatible with a mucoid or protein-rich fluid collection. No evidence of inflammation or malignancy was identified. Superficial biopsies of the mass were performed under endoscopic guidance. Histopathological analysis of the samples revealed ulcerated eosinophilic and lymphoplasmacytic pharyngitis.

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Surgical removal of the cystic lesion was performed. A dose of IV dexamethasone (dexamethasone^f 0.1 mg/kg IV) was administrated before the surgery. The dog was anesthetized with the same protocol as described above. Buprenorphine (buprenorphine^g 20 μg/kg IV q 6 hr) was administered for 24 hr after surgery.

The dog was placed in dorsal recumbency. The pharynx was packed with gauze and an incision made in the soft palate at the level of the mass. Blunt-sharp dissection was carefully performed with Metzenbaum scissors and the mass was removed “en bloc” and subjected to histopathologic analysis (Figure 2C). The soft palate was closed in three layers using poliglecaprone 25^h in a simple continuous pattern for each layer. The dog recovered from surgery and anesthesia without complications. The dog was discharged the following day with tapering doses of corticosteroids (prednisoloneⁱ starting at a dose of 0.5 mg/kg per os q 24 hr) for 7 days. Antibiotics (cephalexin^j 15 mg/kg per os q 12 hr) were administered during the first 24 hr after surgery.

Histological evaluation of the mass revealed a cyst lined by a pseudostratified columnar ciliated epithelium containing a proteinaceous material. Epithelial cells were well differentiated, polarized, and without atypia (Figure 2D). These findings were consistent with a pharyngeal cyst. Complete surgical excision was confirmed.

Follow-up 1 mo and 5 mo after the surgery revealed resolution of the clinical signs without evidence of local recurrence. No endoscopic recheck was performed at this time considering the absence of respiratory signs.

Discussion

The branchial, or pharyngeal, apparatus consists of a series of six mesodermal arches that are separated from each other, externally by ectodermally lined branchial clefts, and internally by endodermally lined pharyngeal pouches (Figure 1).^2,5–9 During development, each branchial arch gives rise to specific adult structures (Figure 1). Abnormal development or regression of these structures may result in a variety of anomalies^2,5–9: cysts, blind lesions with no external opening, sinuses characterized by a single external opening, and fistulae characterized by both internal and external openings. The lining of these cysts can vary from squamous to pseudostratified ciliated epithelium; occasionally, more than one type of epithelium is observed. In humans, most pharyngeal cleft anomalies originate from the second cleft.^1,3 These anomalies are most frequently located along the anterior border of the sternocleidomastoid muscle.^8–10 The Bailey/Proctor classification uses their anatomic location to define four types of lesions.^1,5–9 Type I lesions are superficial to the sternocleidomastoid muscle, type II and III lesions are located close to the great vessels (internal and external carotid arteries, common carotid artery, jugular vein), and type IV lesions lie against the pharyngeal wall, deep to the carotid sheath.^4,11,12 In the great majority of cases, type IV cysts are located lateral to the thyroid and parathyroid area, but they can be found in the thymus.^4,11,12 They can also rarely be found in the nasopharyngeal region—in the parapharyngeal space or the nasopharynx itself, without any connection to the palatine tonsillar fossae.^3,6,7 Although very rare, pharyngeal cyst located in the nasopharyngeal area usually secrete mucus and occur as a single, lateralized lesion.^13–15 In our case, the localization of the lesion corresponded to a type IV cyst. Furthermore, it is interesting to note that in our case, the pharyngeal cyst was in the median plane, without any macroscopic lateralization. Pharyngeal cysts are occasionally described in dogs and cats,^4,5 but cases of remnant pouches are very rare in dogs and cats. Most are derived from the second pharyngeal cleft and have been described in the head and neck area and in the thorax.^5,11,12 To our knowledge, the present case is the first report of a pharyngeal cyst located in the nasopharyngeal area in the dog.

Most pharyngeal cysts develop in the neck region and a cervical mass is often the only clinical sign.⁵ Pharyngeal cysts may also be diagnosed when clinical signs of secondary infection are identified.^1–5 When pharyngeal cysts are located within the thorax cavity, in the great majority of cases, they are serendipitously discovered.^5,8 Malignant transformation as well as the development of metastatic disease have been reported in humans.^1,3 A thymic carcinoma derived from a thymic branchial cyst has been described in a dog.¹² In humans, most pharyngeal cyst located in the nasopharyngeal area are small and asymptomatic, whereas some cause nasal obstruction, postnasal drip, occipital headache, or Eustachian tube dysfunction.^5,15,16 In our case, the dog showed clinical signs typical of a nasopharyngeal obstruction, consistent with the unusual location of the pharyngeal cyst. In humans, pharyngeal cysts can be diagnosed at any age, but most are identified in children younger than 5 yr of age.^1,3 Overall, there is no sex predilection for pharyngeal cysts, although those in the hypopharyngeal location are most commonly diagnosed in female patients.³ The scarcity of the clinical descriptions that have been reported makes it difficult to infer the epidemiology of pharyngeal cysts in dogs and cats.⁵ To date, except for one 5 mo old kitten, they have mostly been described in adults (between 2 and 13 yr old).

In humans, the imaging diagnosis is mainly based on MRI, which usually shows a low-intensity signal on T1 and a high-intensity signal on T2-weighted sequences, without peripheral contrast uptake.^7,9,15 With high-resolution MRI, the precise relationship between the lesion and the surrounding structures (i.e., great vessels) can be determined, which may be important for planning the surgical approach.¹⁵ Computed tomographic features of the pharyngeal cysts include a well-circumscribed lesion with thin walls and a central hypoattenuating area,¹ as we describe in the present case. Because of the evidence and well-defined lesion on the computed tomographic scan, no MRI was deemed necessary for this dog. In our case, the upper endoscopic examination showed a well-circumscribed, smooth mass arising from the soft palate and protruding into the nasopharyngeal space, like in reports in humans describing nasopharyngeal branchial cysts as well-delineated, smooth-surfaced, cystic-like masses attached to the lateral wall of the nasopharynx.⁸

In humans, the differential diagnosis for a nasopharyngeal mass includes nasopharyngeal carcinoma, Thornwaldt’s cyst,^6,7 Rathke’s pouch, adenoid and tonsillar mucous retention cysts, nasopharyngeal branchial cleft cyst, dermoid cyst, chordoma, herniation of the central nervous system, and sphenoid sinus mucocele.⁶ In dogs and cats, the initial differential diagnosis included neoplasia, mucus retention cyst, sialadenitis, sialocele of the major palatine salivary glands, and salivary gland infarction.^5,12,17 The definitive diagnosis of a pharyngeal cyst relies on histopathology, as performed in this case.^3,5,6 Indeed, the type of epithelium lining the cyst wall makes it possible to distinguish lesions originating from salivary glands from branchial cleft cysts. Pharyngeal cysts can be lined either by columnar ciliated (respiratory type) epithelium or by nonkeratinizing stratified squamous epithelium.^{5,8,9,12,15,17} The type of epithelium may reflect the tissue of origin (respiratory-type epithelium derives from branchial pouches and squamous epithelium derives from branchial clefts).¹⁸ It has been speculated that the squamous epithelium may progressively replace the respiratory-type epithelium spontaneously or following repeated infections.^{5,8,9,12,16–18}

As described in the literature on humans, complete surgical excision of a branchial cleft cyst is the treatment of choice, allowing definitive treatment.^{6,7,9,13–15,19}

If the cyst is infected, drainage of the pus and antibiotics are recommended as a first step. In particular, incision and drainage are good options for the type II lesions, with surgical excision reserved as a secondary procedure. The surgical excision was performed in a second attempt for this particular case.^1,18 Endoscopic marsupialization has been successfully used in a limited number of cases of pharyngeal cyst in humans.^6,9,13–15 However, in the veterinary literature, marsupialization or incision and drainage have been reported to lead to unsatisfactory healing and recurrence.⁸ Total surgical excision is highly recommended for type III and IV anomalies.¹⁹ In humans, transoral, transpalatal, transmandibular, and transcervical approaches have been described.^6,13–15 The choice of the surgical technique is based on the size and location and on the relationships to the carotid arteries (external, internal, and common) and jugular vein of the head and neck and the origin of the branchial anomaly.^15,19 The transoral approach is the least invasive technique, but great care should be taken during total excision of the nasopharyngeal branchial cleft cyst because of the risk of neurovascular injury (particularly to the facial nerve) in relation to the lateral location.^6,15 In dogs, the blood supply to the soft palate arises from the minor palatine arteries and the sensory innervation arises from the maxillary division of the trigeminal nerves.²⁰ A few complications have been reported following interference with the anatomic structures (minor palatine arteries and neurovascular supply) using the transoral approach.²⁰ In addition, in cats, the surgical time should be reduced or, alternatively, the mouth needs to be intermittently released during surgery to prevent the risk of cortical blindness associated with reduced maxillary artery blood flow.²⁰ In our case, the ventral approach of the nasopharynx allowed good exposure of the lesion and was a relatively noninvasive procedure. Blunt-sharp dissection allowed total resection without cyst rupture and an excellent long-term outcome without any signs of recurrence 5 mo after surgery.

Conclusion

Pharyngeal cysts are rarely described in dogs and are mostly located in the neck region. To our knowledge, this is the first case of pharyngeal cyst located in the nasopharyngeal region in a dog. This disease should therefore be taken into consideration during the differential diagnosis of partial upper respiratory obstruction. Definitive diagnosis is usually obtained after histopathological analysis. Total surgical excision seems to be an efficient treatment. In our case, the transoral approach allowed the safe and complete excision of the pharyngeal cyst.