Surgical Excision of the Parotid Salivary Gland for Treatment of a Traumatic Mucocele in a Dog

Introduction

Salivary mucoceles are the most common disease of the salivary glands in dogs, defined as an accumulation of saliva outside of the salivary system arising from a disruption in either the duct or the gland.¹ Salivary mucoceles are cystic in appearance; however, they are not true cysts because they do not possess an epithelial lining. The salivary glands in the dog include the sublingual, mandibular, zygomatic, and parotid, and all have the potential to develop a mucocele. Salivary mucoceles may occur in several locations, including the cervical region, sublingually, in the pharyngeal region, and on the side of the face. Mucoceles of zygomatic origin usually result in exophthalmos. Mucoceles typically are nonpainful and do not cause clinical signs unless they become either inflamed or infected; however, mucoceles in the pharyngeal region can cause potentially life-threatening respiratory distress. Potential causes for salivary mucoceles include sialoliths, neoplasia, and trauma, but for the majority of dogs, a specific cause cannot be identified. The sublingual salivary gland (and duct) is the most commonly affected gland, whereas reports of parotid mucoceles are rare.^1–11 This report describes the successful diagnosis, treatment and outcome of a parotid salivary mucocele of traumatic origin in a dog.

Case Report

A 3 yr old spayed female mixed-breed German shepherd dog was presented to the University of Wisconsin Veterinary Medical Teaching Hospital for evaluation of a salivary mucocele on the right side of the face. Approximately 1 mo prior to presentation, the dog was in a fight with another dog and was injured on the right side of the face, resulting in an approximately 1 cm laceration in the skin ventral to the zygomatic arch. A few days later, a fluctuant subcutaneous swelling developed at the site of the trauma. The referring veterinarian drained the swelling and diagnosed a mucocele based on gross appearance of the fluid. The facial swelling recurred after a few days, and the dog was referred to the University of Wisconsin Veterinary Medical Teaching Hospital.

On physical examination at the time of referral, a moderately firm, nonpainful swelling measuring 4 cm in diameter was present on the right side of the face just ventral to the zygomatic arch. A 1 cm scar, consistent with the previous trauma, was present at the ventral aspect of the swelling.

A fine-needle aspirate of the swelling was performed, and approximately 3 mL of slightly viscous, straw-colored fluid were removed. Cytologic examination of the fluid revealed a mostly acellular proteinacious background with rare macrophages and red blood cells. Based on the physical examination and cytologic examination of the fluid, a presumptive diagnosis of parotid salivary gland mucocele was made.

To confirm the gland of origin of the mucocele, survey radiographs of the skull and contrast sialography were performed with the dog under general anesthesia. In preparation for anesthesia, blood was drawn for a serum biochemical analysis, packed cell volume, and total protein. Serum chloride concentration was slightly elevated at 120 mmol/L (reference range, 109–119 mmol/L), and all other values were within the reference ranges. The dog was premedicated with 1 mg of acepromazine^a and 4 mg (0.17 mg/kg) of hydromorphone^b intramuscularly. Carprofen^c was administered at a dose of 4.4 mg/kg subcutaneously prior to induction of anesthesia, and perioperative antibiosis was achieved with cefazolin^d at 22 mg/kg q 90 min for the duration of surgery. Anesthesia was induced with 110 mg (4.6 mg/kg) of propofol^e and maintained with inhaled isoflurane^f and 100% O₂ via endotracheal intubation.⁶ A balanced crystalloid solution was administered perioperatively IV at a rate of 10 mL/kg/hr.

Survey skull radiographs revealed the presence of a soft-tissue opacity ventrolateral to the right zygomatic arch. The right parotid salivary gland papilla, located next to tooth 108 (using the modified Triadan system, available at http://www.rvc.ac.uk/review/Dentistry/Basics/triadan/dog.html), was cannulated with a 24 gauge catheter, and sialography of the right parotid gland was performed by injecting 3 mL of a contrast agent^g (300 mg/mL) into the catheter. Parotid sialography highlighted the distal portion of the duct as a thin line of contrast extending from the level of tooth 108 caudally to the level of tooth 110, then the contrast coursed ventrally at a sharp angle, with pooling of contrast ventral and axial to the soft-tissue swelling. No contrast was identified in the caudal portion of either the duct or gland (Figure 1A). To rule out involvement of the right mandibular gland, the right mandibular salivary gland papilla, located medial to tooth 407, was cannulated with a 24 gauge catheter, and sialography of the right mandibular gland was performed by injecting 3 mL contrast agent into the catheter. Mandibular sialography highlighted the duct as a thin line of contrast extending from the level of tooth 407 to the caudal aspect of the ramus of the right mandible before coursing ventrally to an ovoid structure with multiple thin lines of contrast, consistent with a normal right mandibular salivary gland (Figure 1B).

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Based on the results of the sialography, a right parotid mucocele was diagnosed and parotid gland resection and drainage of the mucocele was performed. The dog was placed in left lateral recumbency and the right side of the face was clipped and aseptically prepared. After clipping the hair on the face, the scar on the ventral aspect of the mucocele was more evident (Figure 2A). A 10 cm incision was made, extending from the ventral aspect of the vertical ear canal to the caudal extent of the ramus of the mandible coursing over the mucocele. The platysma and parotidoauricularis muscles were incised over the parotid salivary gland and reflected ventrally. Using a combination of blunt and sharp dissection, the parotid gland was dissected from the surrounding tissue, taking care to avoid injury to the superficial temporal artery and the facial nerve and its branches (Figure 2B). The glandular tissue was intimately associated with the surrounding musculature, making dissection laborious. With traction on the parotid gland, the caudal portion of the duct (approximately 2 cm) was traced rostrally toward the caudal extent of the mucocele, ligated with 3-0 glycomer 631^h and transected. The mucocele was drained and flushed with sterile saline, and a Penrose drain was placed within the subcutaneous space exiting ventral to the incision. The incision was closed routinely in three layers, taking care to avoid the vessels and nerves exposed after removal of the gland. Gauze sponges were applied to the exit site of the Penrose drain, and a stockinette was placed over the head to hold the gauze in place.

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Recovery from anesthesia was uneventful, and postoperative analgesia (0.05 mg/kg hydromorphone intramuscularly) was administered q 4–6 hr pro re nata for 24 hr. Postoperative assessment of facial nerve function revealed normal palpebral reflex and no other signs of facial nerve deficits. Based on minimal fluid production from the drain site, the Penrose drain was removed on day 1 postoperatively. No complications occurred, and the dog was discharged 48 hr postsurgically.

Histopathology of the excised parotid gland revealed mild lymphoplasmacytic adenitis with duct ectasia. A section of the tissue surrounding the mucocele was also examined revealing fibrillar mucinous exudate with mild granulomatous response and early granulation tissue formation. No evidence of either neoplasia or sialolithiasis was evident.

A follow-up phone call was made 4 mo after surgery. The owner reported no recurrence of the mucocele or other complications.

Discussion

To the authors’ knowledge, this is the first report of a traumatic parotid salivary gland mucocele in a dog. Previously reported parotid mucoceles in dogs have been caused by either sialolithiasis or iatrogenic damage.^6–10 Given the superficial location of the parotid salivary gland and duct, it is surprising that there are not more reports of traumatic parotid gland mucoceles in the literature.¹¹

Based on the history of previous trauma to the face and the general location of the mucocele, the parotid gland was considered the most likely source; however, the zygomatic and mandibular glands could not be definitively ruled out. Therefore, sialography was performed to confirm the gland of origin. Cannulation of the parotid and mandibular papillae is very challenging and may not be feasible in all cases depending on the size of the dog and the individual anatomy. The interpretation of the sialograms was fairly straightforward in this case; however, because of the difficulty in performing sialography, a lack of a normal sialogram may not always indicate pathology.

Computed tomography (CT) has also been used to identify the parotid salivary gland as the source of a mucocele; however, in that case, the etiology of the mucocele was a sialolith, which resulted in dilation of the duct, making the duct more identifiable with CT.⁹ For the case reported here, it is unlikely that CT would have been useful for diagnosing the parotid gland as the gland of origin because of the very small diameter of the duct and lack of contrast within the caudal portion of the duct and gland. CT sialography, however, would have been a feasible diagnostic modality, and that technique has been described previously.¹²

Factors other than trauma, iatrogenic damage, or sialolithiasis may contribute to mucocele formation. Attempts to produce salivary mucoceles experimentally by either disruption or ligation of the duct have been largely unsuccessful, which indicates that a predisposition for mucocele formation may exist.¹³ Indeed, breed predispositions have been reported in poodles, German shepherd dogs, greyhounds, Australian silky terriers, and dachshunds, suggesting a genetic predisposition.³ Mucoceles have also been reported to occur in littermates, further supporting the hypothesis for a genetic predisposition.¹⁴ It is worth noting that the dog in this report is a mixed-breed German shepherd dog, which may have predisposed the dog to mucocele formation following the traumatic injury to the face.

The current treatment of choice for a salivary mucocele of mandibular and/or sublingual gland in origin is excision of the gland(s) and drainage of the mucocele.^2,3 However, excision of the salivary gland may not be necessary in all cases. Ligation of salivary ducts generally results in atrophy of the respective glands, without adverse effects.^13,15,16 One study describes an experiment designed to create salivary mucoceles in dogs, by either ligating or longitudinally incising the ducts.¹³ In dogs that had the ducts ligated, the salivary glands atrophied within days, and no mucoceles were created in any of the dogs, regardless of treatment. Another study describes attempts to experimentally create mucoceles in cats by ligating the mandibular, sublingual, and parotid ducts.¹⁵ For all three glands, atrophy occurred, with results most consistent in the parotid salivary gland. Mucoceles formed only in glands where the duct had not been completely ligated, possibly from either lack of sufficient backpressure in the gland or potentially from lack of denervation that is accomplished with complete duct ligation. In rats, ligation of the parotid duct led to rapid atrophy of the glandular tissues and cessation of saliva production.¹⁶ Although ligation of the sublingual and mandibular ducts has not been described as a treatment of mucoceles in dogs, there are reports of successful treatment of parotid mucoceles by ligation of the duct rather than sialoadenectomy.^7,17,18 In two separate studies, Harvey described parotid duct ligation in one cat and one dog for parotid salivary duct rupture, and in one dog for treatment of bilateral parotid gland enlargement. All three animals recovered uneventfully and presumably went on to develop atrophy of the parotid salivary glands.^7,17 Kuhns et al. (1979) report ligation of the parotid duct in a dog following complications from a parotid duct transposition performed for treatment of keratoconjunctivitis. Ligation of the duct successfully led to atrophy of the gland and cessation of saliva production.¹⁸

Based on those reports, it follows that ligation of the duct of the offending salivary gland proximal to the disruption may be a viable treatment option for mucoceles arising from any gland, with the benefit being a less invasive surgical approach. However, in dogs, most mucoceles occur in either the submandibular or ventral cervical region and arise from either the mandibular or sublingual glands. In those cases, the mucocele typically surrounds both glands, making it impossible to determine what gland or duct is the source of leakage. Therefore, the most efficient treatment option is to remove both glands and as much of the duct(s) as possible.

For the case reported herein, it is possible that the small segment of the parotid duct caudal to the mucocele could have been identified and ligated as a simpler form of treatment given the technical challenge of excising the parotid gland, which required meticulous and time-consuming dissection between the gland and surrounding tissue. Duct ligation would have also avoided the risk of iatrogenic injury to the facial nerve, which is intimately associated with the gland. However, based on the recent report of successful resolution of a parotid mucocele due to sialolithiasis after parotid gland resection, the decision was made to resect the gland as a more definitive treatment.⁹

Conclusion

Although rare, parotid gland mucoceles should be considered as a differential for any swelling around the zygomatic region of the face. A key reason for the success of this case was the correct identification of the gland of origin. The authors, therefore, recommend performing further imaging, such as sialography or CT, in any case where the location of the mucocele is either unusual or may be associated with several glands. In the case reported herein, excision of the parotid gland was curative and resulted in an excellent outcome for the dog, indicating that excision of the parotid salivary gland is a viable treatment option for parotid mucocele of traumatic origin. However, further investigation of treatment options is indicated to compare the success rates of ligation of the duct proximal to the disruption versus excision of the affected gland(s), as ligation of the duct may be associated with less morbidity.