Small animal - September 2024

Salivary mucocoele, also known as sialocoele, is characterised by the accumulation of saliva in the tissues surrounding the salivary glands or ducts. This results in a swelling filled with saliva that can cause discomfort, pain, and functional impairment. Effective management of salivary mucocoele requires an understanding of its aetiology, clinical presentation, diagnostic approach, and treatment options. This article provides a comprehensive overview of salivary mucocoele in dogs, with an emphasis on surgical management.

Aetiology and pathogenesis

Salivary mucocoele occurs due to the leakage of saliva from a salivary gland or duct into the surrounding tissue, leading to the formation of a saliva-filled cavity (Ritter and Stanley, 2018; Torad and Hassan, 2013). Leaked saliva causes an inflammatory response, leading to the formation of a capsule lined by inflammatory connective tissue. This encapsulated collection of saliva forms the mucocoele. Leakage of saliva can be caused by trauma (e.g., dog fight), foreign bodies, sialoliths, and tumours (Schmidt and Betts, 1978; Ryan et al, 2008; Clarke and L’Eplattenier, 2010; Benjamino et al, 2012; Philp et al, 2012; Guthrie and Hardie, 2014; Ritter and Stanley, 2018). In many cases, the exact cause of the mucocoele remains unknown (idiopathic) (Ritter et al, 2006).

Clinical presentation

In one of the largest retrospective studies of dogs with mandibular and sublingual sialocoeles, the mean age was five years. No breed or sex predisposition was determined (Ritter et al, 2006). However, in another study consisting of 14 dogs with pharyngeal mucocoeles, small breeds (50 per cent were miniature and toy poodles) and males were over-representd (Benjamino et al, 2012). A 2.2:1 ratio of males to females was also reported in another study that included 16 dogs with cervical mucocoeles (Kaiser et al, 2016). The clinical signs of salivary mucocoele depend on the location of the affected gland and the size of the mucocoele. The four major salivary glands in dogs include the mandibular, sublingual, parotid, and zygomatic. The most common offender is the sublingual salivary gland or duct but any of the major salivary glands can be involved. The four main presentations of salivary mucocoeles in dogs include cervical, sublingual, pharyngeal, and zygomatic. Clinical signs associated with sialocoeles related to the zygomatic salivary gland or duct usually include exophthalmos due to periorbital salivary accumulation, while sialocoeles related to the mandibular or sublingual glands or ducts usually manifest as cervical, pharyngeal and/or sublingual salivary accumulation (Landy et al, 2021). Sublingual sialocoeles, also called ranulas, likely result from leakage of the rostral sublingual salivary glands or duct (Ritter and Stanley, 2018).

Commonly observed signs include:

cervical mucocoele: swelling in the intermandibular or cranioventral neck region, which may be soft, fluctuant, and non-painful (Kaiser et al, 2016).

sublingual mucocoele (ranula): swelling under the tongue, which can deviate tongue position and interfere with prehending, chewing and swallowing.

pharyngeal mucocoele: swelling in the pharyngeal region. These mucocoeles occur within the wall of the pharynx and can result in respiratory distress, airway obstruction, inspiratory stridor, coughing, gagging, ptyalism/hypersalivation, and dysphagia (Benjamino et al, 2012).

zygomatic mucocoele: swelling around the eye or upper jaw region. These can result in exophthalmos, difficulty closing the eye, protrusion of the nictitating membrane, and periorbital swelling/swelling ventral to the globe (Schmidt and Betts, 1978).

A subset of dogs can have more than one sialocoele. In one study of 14 dogs with pharyngeal sialocoeles, six had a concurrent cervical mucocoele on the same side (Benjamino et al, 2012). Dogs with salivary mucocoele may also demonstrate signs of pain, reluctance to eat, drooling, and halitosis. In cases where the mucocoele becomes infected, additional signs such as pyrexia, lethargy, and purulent discharge may be present.

Diagnosis

Diagnosis of salivary mucocoele involves a combination of physical examination, cross-sectional imaging with contrast computed tomography or ultrasonography, and fine-needle aspiration for cytology. Other tests, such as periodic acid–Schiff (PAS) staining for mucin, can also be performed.

Diagnosis

Diagnosis of salivary mucocoele involves a combination of physical examination, cross-sectional imaging with contrast computed tomography or ultrasonography, and fine-needle aspiration for cytology. Other tests, such as periodic acid–Schiff (PAS) staining for mucin, can also be performed.

Physical examination

A thorough physical examination is essential for identifying the location, extent and nature of the swelling. Palpation of the mass can help differentiate it from other types of swellings, including abscesses, neoplasms, or enlarged lymph nodes. As mentioned above, the presence of a soft, non-painful, fluctuant swelling in the intermandibular or cranial cervical region would be typical of a cervical mucocoele (Figure 1). With chronicity, sialocoeles can become more firm on palpation. It would be important to palpate in this area for the mandibular salivary gland and mandibular lymph node(s) that lie immediately rostral to the salivary gland. The identification of pain on palpation of this swelling could indicate the presence of an abscess possibly secondary to a stick injury but pain could also be associated with an infected sialocoele. Oral examination should be performed in all cases to rule out the presence of a ranula (Figure 2). These ranulas are usually soft, painless, and can deviate the tongue to the contralateral side. A swelling in the region of the upper last molar may be associated with the presence of a zygomatic sialocoele (Ritter and Stanley, 2018). Upper airway examination would be required for the diagnosis of a pharyngeal sialocoele and may reveal a swelling in the region of the pharynx (Figure 3). Physical examination findings in cases with zygomatic sialocoeles may include periorbital swelling, exophthalmos, and possibly a swelling in the region of the upper last molar (Clarke and L’Eplattenier, 2010; Ritter and Stanley, 2018). There may also be inability to retropulse the globe during ophthalmologic examination (Bartoe et al, 2007)

Imaging studies

Imaging studies such as contrast computed tomography and ultrasonography are useful for diagnosis of salivary mucocoeles in dogs (Figure 4). Ultrasonography is particularly useful for visualising the fluid-filled nature of sialocoeles and assessing their relationship with surrounding structures such as local lymph nodes. It may also help identify the affected salivary gland and detect an underlying cause such as foreign bodies or sialoliths. The ultrasound appearance may depend on the chronicity of the sialocoele. Acute sialocoeles appear as round echogenic structures with central anechoic contents and a hyperechoic wall (Torad and Hasson, 2013). More longstanding sialocoeles can appear grainy or mottled, with a heterogenous appearance. Contrast computed tomographic features of salivary mucocoeles include fluid attenuating, non-contrast enhancing contents and soft tissue attenuating, contrast-enhancing walls, and possible mineralised foci within consistent with sialoliths (Oetelaar et al, 2022). Contrast sialography involves injecting contrast medium into salivary ducts and can provide information regarding the anatomy and integrity of that salivary gland and duct (Benjamino et al, 2012; Guthrie and Hardie, 2014). Sialography can be performed with radiographs or computed tomography, however, the latter is superior because superimposition artifacts with radiography can limit the diagnosis and detection of affected glands (Tan et al, 2022). CT sialography may identify intra-sialocoele contrast leakage; however, the overall sensitivity of this technique was found to be only moderate (66.7 per cent) in one study (Tan et al, 2022).

Fine-needle aspiration and cytology

Fine-needle aspiration (FNA) of the swelling can confirm the diagnosis of salivary mucocoele by revealing the presence of viscous, honey-coloured, clear or blood-tinged mucoid fluid (Ritter and Stanley, 2018) (Figure 5). Cytologic examination of the aspirate may reveal low numbers of lymphocytes, macrophages (containing abundant foamy cytoplasm), neutrophils, plasma cells, variable numbers of red blood cells, and clumps of homogenous pink- to violet-staining mucin (Figure 6) (Ritter and Stanley, 2018). Staining of the fluid with a mucin-specific stain (periodic-acid Schiff) can also confirm the fluid is saliva. FNA can also help rule out other differential diagnoses such as abscesses, aspiration of enlarged lymph nodes or neoplasms. If the diagnosis remains uncertain after imaging and FNA, an incisional biopsy may be obtained but this would be uncommon in the author’s experience.

Treatment

Sialadenectomy forms the mainstay of treatment of salivary mucocoele in dogs. Perhaps in rare cases, particularly if the mucocoele is small and asymptomatic, monitoring the swelling and providing supportive care, such as pain relief/anti-inflammatory medication, could be considered. This approach does not address the underlying cause of the mucocoele and there is a high risk of persistence or recurrence. Furthermore, incision or drainage of sialocoeles alone is not recommended as a definitive treatment because there is a high rate of recurrence (Ritter and Stanley, 2018).

Surgical management

Surgical intervention is the treatment of choice for salivary mucocoele and involves removal of the affected salivary gland and duct. The specific surgical procedure depends on the location of the mucocoele.

Cervical and sublingual sialocoeles

Mandibular and sublingual sialadenectomy: surgical removal of the mandibular and sublingual salivary glands is the treatment of choice for cervical and sublingual sialocoeles. Both lateral and ventral approaches are described, with the author preferring the ventral approach (Cinti et al, 2021). In one study, while overall rates of intraoperative and postoperative complications did not differ, the recurrence rate was higher with the lateral approach and wound-related complications were more likely with the ventral approach (Cinti et al, 2021). In another study, no difference in the rate of intraoperative complications, recurrence, or postoperative complications was identified among dogs that underwent a lateral or ventral approach (Swieton et al, 2022). It is vitally important in cases with intermandibular or cranioventral cervical swelling that the correct sided gland is removed. Affected animals should be placed in dorsal recumbency and the swelling should deviate to the affected side. CT should also identify the affected gland(s). The mandibular salivary gland is located cranial to the bifurcation of the maxillary and linguofacial veins and has a pale greyish appearance. The mandibular and sublingual glands, along with their associated ducts, are carefully dissected and removed (Figure 7). Both glands are removed together because they are intimately associated with one another. The sublingual salivary gland consists of monostomatic and polystomatic components. The most caudal part of the monostomatic component abuts the mandibular salivary gland and shares a capsule with it. The monostomatic component continues rostrally as loose clusters of glandular tissue alongside the mandibular salivary duct (Ritter and Stanley, 2018). Clusters of salivary tissue along this path rostral to the lingual nerve (Figure 7) is considered the polystomatic component of the sublingual salivary gland. Both the mandibular and sublingual salivary ducts exit at the sublingual caruncle. With dissection of these ducts in a rostral dissection, the digastricus muscle will be observed along the ventral border of the caudal mandible, crossing and obstructing the path of dissection. The muscle is dissected and elevated and large, curved Kelly or Crile haemostatic forceps passed from medial to lateral, deep to this muscle. The duct is grasped with the forceps and the mandibular and monostomatic part of the sublingual salivary glands excised. The remaining duct is pulled from lateral to medial deep to the digastric muscle and dissection continued rostral to the lingual nerve (Marsh and Adin, 2013) (Figure 7). The actual sialocoele itself can be punctured, its contents suctioned, and its outer connective tissue wall left in place, or it can be removed as part of the sialadenectomy.

Marsupialisation: for sublingual mucocoeles (ranulas), marsupialisation involves creating a permanent opening in the floor of the mouth to allow continuous drainage of saliva. This procedure can be performed in conjunction with sialadenectomy or as a standalone treatment for small ranulas. For sublingual sialocoeles, the author typically performs mandibular-sublingual sialadenectomy with dissection as rostral as oral mucosa (rostral to the lingual nerve) combined with either lancing and drainage of the ranula or marsupialisation.

Investigators in a recent study evaluated the response rate and clinical course of 11 dogs with cervical sialocoele treated with radiation therapy (Poirier et al, 2018). Six dogs (54 per cent) achieved a complete response, whereas five dogs (45 per cent) achieved a partial response. Three dogs had progression of their sialocoele two, three, and nine months after radiation therapy.

Pharyngeal mucocoele: pharyngeal mucocoeles require prompt surgical intervention due to the risk of airway obstruction. The surgical approach involves excision of the mandibular and sublingual salivary glands +/- aspiration of drainage of the sialocoele and excision of the mucocoele or marsupialisation of the mucocoele. Marsupialisation of the sialocoele alone without sialadenectomy has also been described (Benjamino et al, 2012).

Zygomatic mucocoele: zygomatic mucocoeles are treated by removing the zygomatic salivary gland. This involves a more complex surgical approach due to the gland’s deep location near the eye. Sialadenectomy can be performed with or without temporary take down of a portion of the zygomatic arch (modified lateral orbitotomy) or via an intraoral approach (Viitanen et al, 2023; Dörner et al, 2021). In a recent cadaveric study, an approach ventral to the zygomatic arch allowed complete removal of the zygomatic gland in all dogs, with good surgical overview and reducing tissue trauma (Dörner et al, 2021). The intraoral approach involves making an incision in the oral mucosa and muscular tissue caudal to the upper last molar, and was recently shown to be faster than the lateral orbitotomy approach, but technically not any easier (Viitanen et al, 2023).

Parotid mucocoele: descriptions of parotid mucocoeles are sparse in the literature (Guthrie and Hardie, 2014). Surgical excision of the parotid salivary gland is technically challenging. One report described a young mixed breed dog that developed right-sided facial swelling after a dog fight (Guthrie and Hardie, 2014). Surgical excision of the right parotid salivary gland and duct was performed along with drainage of the mucocoele.

Prognosis

The-prognosis for dogs with salivary mucocoele is generally good with appropriate surgical intervention. Most dogs recover well and the recurrence rate is low when the affected salivary gland and duct are completely removed. However, potential complications such as postoperative infection, seroma, and recurrence or intraoperative damage to surrounding neurovascular structures can occur, emphasising the importance of careful surgical technique and diligent postoperative care.

Conclusion

Salivary mucocoele is a relatively common condition in dogs and can have a number of presentations. Understanding the aetiology, clinical presentation, and diagnostic approach is crucial for timely and accurate diagnosis. Surgical intervention remains the treatment of choice, with sialadenectomy being the most commonly performed procedure. Proper postoperative care and monitoring are essential for ensuring successful recovery and minimising complications. With appropriate management, the prognosis for dogs with salivary mucocoele is generally good.