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CASE REPORT: Branchial Cleft Retention Cyst in a Dog.

History

A two year old, female, neutered Cocker Spaniel presented with a life-long history of stertor.  Signs had recently worsened, and the referring veterinary surgeon had examined the upper respiratory tract under general anaesthesia (GA).  This had revealed a left-sided pharyngeal mass, extending to the midline to involve the soft palate.  Fine needle aspiration had yielded a greenish, viscous fluid containing granules of apparent dystrophic calcification.  Removal of fluid had improved the clinical signs.

Investigations

fig. 1 - transverse scan of pharyngeal region showing large, fluid filled structure to left of oropharynx

General physical examination was unremarkable apart from upper respiratory noise.  Routine haematology and biochemistry revealed no abnormalities.  Examination of the oropharynx under GA had confirmed the earlier findings.  MRI scans of the

head (figs. 1 & 2) revealed a large, fluid filled mass, closely associated with the dorsal aspect of the left mandibular salivary gland.

Treatment

The pharyngeal region was explored surgically via a ventral approach.  The mandibular salivary gland appeared enlarged, but otherwise grossly normal.  It was removed and submitted for analysis; this revealed mild inflammatory changes, but was otherwise unremarkable.  Dorsal to the gland, a distinct, encapsulated cyst was identified, with no obvious anatomical connection to the salivary gland.  This was dissected out en bloc, taking care to avoid traumatising the vital structures in the area (internal carotid, vagosympathetic trunk, etc) and submitted for histopathology and bacteriology.   A passive drain was secured into the dead-space and the surgical wound was closed routinely.  The drain was removed three days after surgery.  The patient made an unremarkable recovery, and the stertor ceased.

Results

Histological analysis revealed a cyst-like structure, multiloculated and lined in most areas by a thin single layer of cuboidal epithelium. There was a thick capsule of mature fibrostromal connective tissue samples surrounding the cyst lumen. Within this connective tissue and infiltrating into the cyst lining cells was a moderately dense population of plasma cells and lymphocytes. A focal area of mature cartilage was also noted and adjacent to this was a small focus of degenerating bone, consistent with a congenital cyst.

Discussion

parasagittal T2w MRI scan of the head showing cyst dorsal to mandibular salivary gland

Although abutting the salivary gland, the presence of a true cyst lining, associated capsule and degenerate bone, together with the absence of any direct anatomical connection to the salivary gland supports the diagnosis of a congenital cyst rather than sialocoele.  The latter typically have a pseudocapsule and track along soft tissue planes as well as exhibiting a marked granulomatous response to the presence of saliva.

True congenital pharyngeal cysts are rare in the small animal population.1 Indeed there is very little published information on the condition in the veterinary literature.  The majority of pharyngeal cysts are branchial cleft retention cysts where there is an embryological failure of epidermal tissue (often of apocrine origin) to involute.  Other congenital cysts found in the head/ neck region arise from remnants of the thyroglossal tract and Rathke’s pouch, but these have a different anatomical location to the case above.

Classification of human branchial clefts is well established;2 this case most closely corresponds to a first cleft, type II presentation, given its close proximity to the mandibular salivary gland.  This is the commonest type of branchial cyst found in people, and has a similar presentation too (stertorous juveniles).  Interestingly in small animals, branchial cysts are more often found within the mediastinum – ‘thymic branchial cysts’ – and usually present as dyspnoea.3 Branchial cysts derived from the first cleft—as is the case here are extremely rare.

Treatment of all types of branchial cysts is surgical excision, although aspiration of the fluid contents will result in a temporary relief of signs.  Untreated cysts usually result in progression of signs and can become secondarily infected.  Common long-term complications post-surgery would include fistula formation from inadequate excision.

References

1.  Clements DN et al. (2006) Diagnosis and surgical treatment of a nasopharyngeal cyst in a dog. JSAP 2006 47 674-7

2.   Dallan I et al (2008) Parapharyngeal cyst: considerations on embryology, clinical evaluation, and surgical management. JCraniofac Surg. 6, 1487-90

3.  Liu S, Patnaik AK, Burk RL (1983) Thymic branchial cysts in the dog and cat. JAVMA 10, 1095-8