34th Annual Scientific Meeting proceedings

Anatomical considerations
The zygomatic salivary gland (ZSG) is globular to pyramidal in shape and only present in carnivores. This gland is situated medial to the zygomatic bone between the eyeball and the pterygoid muscle. The ducts open into the vestibule near the last maxillary molar tooth and caudal to the parotid duct (Evans and de Lahunta, 2010).

Background
A salivary mucocele develops from leakage of saliva from the gland itself or associated ducts into surrounding tissue, with subsequent local reaction to the accumulating material. The cystic structure encapsulating the saliva is lined by granulation or fibrous tissue (Bartoe and others, 2007). Of all the salivary glands, the ZSG is the least frequently involved in salivary gland disease (Schmidt and Betts, 1978; Gray and others, 2020).

Clinical signs associated with a mucocele of the ZSG vary from mild to severe including swelling of the cheek or below the eye, divergent strabismus, trismus, retrobulbar/orbital swelling, exophthalmos, hypersalivation and pain opening the mouth between others (Mason and others, 2001; Slatter and others, 2003; Cannon and others, 2011; Maggs and others, 2013). For this reason, diagnosis can be challenging. FNAB and fluid analysis, radiographic sialography, ultrasound, CT scan with or without sialography and MRI, have been reported to aid diagnosis (Mason and others, 2001; Tadjalli and others, 2004; Cannon and others, 2011; Kneissl and others, 2011; Boland and others, 2013; Durand and others 2016; Tan and others, 2022; Son and others 2023;)

Surgical options
Various surgical approaches to the orbit have been described. The most commonly and well described is the lateral approach with ostectomy of the zygomatic arch (Gilber and others, 1994; Ritter and Stanley, 2018; Bartoe and others, 2007; Dörner and others, 2021). This approach offers the best exposure to the ZSG but it is also considered the most traumatic and time consuming. Variations of the technique are described including transection versus preservation of the orbital ligament and the exact location and extent of the ostectomy.

A partial V-shaped zygomatic arch ostectomy has been described as a less invasive approach (Ibrahim and others, 2020). The procedure was described in 6 healthy dogs where a partial V-shaped segment of the zygomatic arch was resected. The technique was considered satisfactory for the exploration of ZSG and the salivary gland was successfully excised in all dogs. No severe complications or deaths were recorded. Although promising, further studies are required to assess the exposure and successful removal of the ZSG in dogs with naturally occurring pathology.

A comparison of the lateral approach, a ventral approach and a dorsal approach was published in 2021 (Dörner and others, 2021). In this study, 20 cadavers had the lateral approach performed on the left side (with ostectomy of the zygomatic arch), and a ventral (incision at the ventral border of the zygomatic arch) and a dorsal approach (incision at the dorsal border of the zygomatic arch) were performed on the right side in 10 cadavers respectively. Both the dorsal and ventral approach did not encompass ostectomy of the zygomatic arch. The surgical view and tissue trauma were graded for each approach. The lateral approach was found to have the best surgical exposure, allowed complete removal of the zygomatic gland in all dogs but resulted in more tissue trauma. The ventral approach offered a reasonable surgical exposure, and the dorsal component of the gland, although partially covered by the zygomatic arch, could be mobilised and subsequently dissected. Tissue trauma was graded as moderate and all ZSG were resected in their entirety. The dorsal approach resulted in the most restricted surgical view and gland dissection had to be carried out blindly beneath the zygomatic arch. Tissue trauma was rated as minimal, but complete ZSG excision was not possible in 9/10 cases. The results of this study suggest that the ventral approach is less traumatic and can result in successful complete resection of the ZSG. However, this is a cadaveric study, performed only on mesocephalic breeds and with normal “thawed” salivary glands. Further clinical studies are required to validate these results.

Other less invasive approaches include the ventral transpalpebral anterior orbitotomy (McDonald and others, 2016 and the transconjunctival approach (Cho, 2008; Ramsey, 1997).  The ventral transpalpebral anterior orbitotomy approach has been described to effectively remove two diseased ZSG with resolution of the clinical signs and its similar in surgical technique to the dorsal approach described by Dörner and others, 2021. This raises the question if incomplete removal of the ZSG (with the dorsal approach) in the later study was due to the friable nature of thawed tissue or if complete resolution of the clinical signs in the former study occurred despite possible incomplete removal of the ZSG.

An Intraoral approach (Viitanen and others, 2023) was reported in 10 cadavers (with a lateral approach in the contralateral side) and 3 clinical cases. In this approach the gland was dissected through a small oblique incision into the oral mucosa and muscular tissue overlying the rostral part of the pterygopalatine fossa, oriented caudomedial to rostrolateral direction and ending immediately caudal to the zygomatic major papilla. The intraoral approach reduced the surgical time, precluded preoperative clipping and required less tissue dissection in comparison to the lateral approach. Adequate exposure to the ZSG, complete extirpation of eight normal glands and near-complete removal of the remaining two ZSG (in cadavers), as well as excellent outcome in the 3 clinical cases with no intraoperative or short-term complications was achieved. Further clinical studies are required to further validate and understand these results.

Conclusion
Salivary mucoceles of the zygomatic salivary gland are rare and can result in a plethora of clinical signs. Advanced imaging is useful to determine the extent of soft tissue changes as well as distortion of the local anatomy. This may play a crucial role in surgical planning as well as choosing the preferred surgical approach. A variation of techniques has been described in the literature most with the main intent to decrease surgical time and tissue trauma associated with the standard or modified lateral approach with ostectomy of the zygomatic arch. Most of these techniques have shown promising results in a small number of clinical cases or successful gland removal in a cadaveric trial but further studies are warranted to validate these results. Ultimately each patient must be evaluated individually, and a decision made based on the extent of soft tissue changes, breed conformation, comorbidities, and local anatomy.

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