Parotid Duct Foreign Body in a Dog Diagnosed with CT

Introduction

Parotid duct foreign bodies in dogs are a rarely reported clinical scenario.^1,2 To the authors’ knowledge, computed tomography (CT) has been used in cases of parotid duct sialoliths but has not been reported for parotid duct foreign bodies.³ This case describes the use of CT in diagnosing a parotid duct foreign body and aiding surgical removal.

Case Report

A 12 mo old castrated male German shorthaired pointer was referred with a 3 mo history of a recurrent left-sided facial swelling. The dog was initially presented to the referring veterinary surgeon with a sudden onset swelling of the left side of the face. Initial antibiotic management with 20 mg/kg of a clavulanate-potentiated amoxicillin^a per os (PO) for 26 days and 2 mg/kg carprofen^b PO for 5 days produced a minimal reduction in the size of the swelling. Ultrasonography showed a fluid-filled space but no obvious foreign body. A bacteriology swab produced no significant growth. The left parotid duct was catheterized and flushed with saline^c revealing purulent material. Clindamycin^d (11 mg/kg PO q 12 hr for 5 wk) resolved the swelling. Five days after cessation of antibiotics, the swelling recurred. A second catheterization 1 wk prior to referral was only possible up to 5 cm from the opening of the parotid duct due to stenosis.

Physical examination at the time of referral was unremarkable other than a firm, fixed nonpainful swelling on the left ventrolateral cheek. CT of the skull was performed with a third generation CT scanner^e (1 mm contiguous slices at 60 kVp, 120 mAs) under sedation with 8 μg/kg IV medetomidine^f and 0.2 mg/kg butorphanol^g. This study showed a tubular structure measuring 0.7 cm diameter and 2 cm length with a mean density of 30 Hounsfield units (HU). Soft tissue and/or fluid density was present at the ventrolateral aspect of the left masseter muscle, at the level of the angle of the mandible, which was consistent with a focally dilated parotid salivary duct (Figure 1A). After IV injection of contrast medium^h (600 mgI/kg), the wall of the focally dilated left parotid duct appeared thickened with marked uptake of contrast (Figure 1B). The walls of the dilated parotid duct precontrast had a density of 40 HU, and the walls of the dilated parotid duct postcontrast had a density that ranged from 60 HU to 100 HU that was more marked on its periphery. An area that was tubular in shape had a lower density (20 HU) than the surrounding thickened duct wall, and no uptake of contrast was present in the center of the dilated duct (Figure 2). The significance of that finding was not clear at the time of the study. There was no evidence of contrast uptake by the wall of the short section of the duct proximal to the marked dilatation.

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Next, 1 mL of iohexol (300 mI/mL) diluted in 1 mL of saline solution (2 mL total volume) was then injected into the parotid duct through a cannula placed through its respective papilla, and CT of the skull was repeated. This revealed partial filling of the duct with contrast medium from its opening to its main branches into the parotid gland (Figure 3A). Findings included focal accumulation of contrast medium at the level of the previously detected dilatation. The short section of duct proximal to main dilatation was also mildly dilated (2 mm in diameter) as shown in Figure 3B.⁴ The parotid gland did not show uptake of contrast, but was normal in size and shape when compared with the contralateral side. In addition to a foreign body lodged within the parotid duct, differential diagnoses at this point included either inflammation or infection of a segment of the parotid duct that was incompletely responsive to antibiotics and neoplasia of the duct. In view of the diagnostic imaging findings and lack of complete resolution with medical treatment, surgical exploration was recommended.

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Surgery was performed the following day under general anesthesia, following premedication with 0.02 mg/kg acepromazineⁱ and 0.3 mg/kg methadone^j intramuscularly. Anesthesia was induced with 4 mg/kg IV propofol^k and maintained with isoflurane^l and O₂^m. Crystalloid fluid therapyⁿ at 10 mL/kg/hr was maintained throughout surgery, perioperative antibiotic coverage was provided with 22 mg/kg cefuroxime^o, and 0.2 mg/kg meloxicam^p was given IV perioperatively.

A 5 cm long skin incision was made over the parotid duct swelling parallel with the duct. The platysma muscle was sectioned, and the dorsal buccal branch of the facial nerve visualized and preserved. The parotid duct was cannulated from the oral cavity to allow clear identification of the duct. The cannula was palpated as it passed to a region of stenosis but would advance no further. The parotid duct was ligated with 3 metric polydioxanone^q distal to the stenosis and sectioned. The parotid duct was dissected proximally in the dilated region of the duct as visualized on CT where it was closely adherent to the underlying masseter muscle. The sheath of the masseter muscle was incised in an ellipse around the dilated parotid duct. During this dissection, the dilated parotid duct ruptured, producing purulent material of which a swab taken for bacteriology. After suctioning the exposed purulent material from the operating field, a grass seed was visualized protruding from the duct (Figure 4). The dilated duct was thick-walled and was lined with chronic granulation tissue. The parotid duct was dissected proximal to this abnormally dilated portion of the duct into the area where the duct had a normal gross appearance. The parotid duct was double ligated proximal to the dilation with 3 metric polydioxanone. The surgical site was flushed with sterile saline, closed with 2 metric polyglecaprone^r using a simple continuous pattern in the masseter sheath over two closed suction drains^s. The platysma muscle was closed with 2 metric polyglecaprone followed by 3 metric nylon^t cruciate mattress skin sutures.

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The dog was hospitalized for 3 days following surgery and received 0.02 mg/kg buprenorphine^u intramuscularly q 6–8 hr for the first 24 hr. Analgesia was then continued with meloxicam PO q 24 hr for 5 days. Antibiotic coverage was provided with 500 mg cephalexin^v PO q 12 hr for 14 days and 300 mg metronidazole^w PO q 12 hr for 14 days. One drain was removed 38 hr postoperatively after producing 2.4 mL in that period. The second drain was removed 72 hr postoperatively after producing 7.3 mL in total.

Histopathology of the submitted parotid duct confirmed extensive chronic inflammatory changes consistent with an ascending infectious process of the parotid duct, likely secondary to the presence of the foreign body. In some areas, the duct appeared to have ruptured, forming a sinus cavity with extensive surrounding granulation tissue. Microbiology of the purulent material revealed scanty growth of Pasteurella multocida, which was sensitive to the antibiotics administered.

Physical examination was performed 12 mo later by the referring veterinary surgeon. No recurrence of the facial swelling was noted.

Discussion

Nonenteric foreign bodies are a common cause of recurrent soft-tissue swellings and are often intermittently responsive to antibiotic medication. Foreign bodies associated with the parotid duct are rarely reported, with only two other cases reported in the literature.^1,2 The history, normal appearance of the surrounding tissue on CT, and surgical findings indicated that the grass seed originated from the oral cavity, entered the left parotid duct via the oral opening of the duct, and migrated caudally within the duct. It is likely the stenosis formed as a result of the damage to the duct due to the presence of the grass seed. A similar etiology was suggested previously, and percutaneous entrance followed by migration into the duct has also been reported.^1,2

In dogs, radiography, ultrasonography, and sinography are all recommended initial imaging modalities for localizing superficial foreign bodies. CT has been recognized as a good imaging modality for the detection of nonenteric foreign bodies in dogs.^5–8 Ultrasound and a radiographic contrast sialogram have been used to diagnose dilation and obstruction of the duct.² Ultrasonography showed a curvilinear echogenic interface with clean far-field acoustic shadowing, suspected to be a foreign body. The radiographic sialogram showed marked dilatation of the caudal third of the parotid duct with filling terminating abruptly close to the expected emergence of the duct from the gland. The contrast failed to reach the duct. Use of a radiographic sialogram has been reported, showing localized dilation of the parotid duct.¹ To the authors’ knowledge this is the first reported use of CT sialography to diagnose a parotid duct foreign body. In this case, although the foreign body itself could not be clearly visualized, the images demonstrated that the pathology was grossly confined to the dilated portion of the duct.

The combination of contrast administered IV and via sialography, allowed the presence of either a sinus tract or rupture of the duct wall with extension of changes into the surrounding soft tissues to be ruled out. The combination of those techniques also showed no abnormalities in the left parotid gland and only mild dilatation of the proximal part of the duct with no contrast enhancement of the wall. This was in agreement with the authors’ findings during surgical exploration and added weight to the decision to confine the surgery to resection and ligation of the duct alone, leaving the normal parotid salivary gland in situ. In addition, as parotid sialoadenectomy involves dissection extending to the region of the horizontal ear canal, facial nerve paresis or paralysis is reported as a common complication.⁹ Ligation of the parotid duct is an accepted alternative to sialoadenectomy and is thought to lead to atrophy of the salivary gland.¹⁰ Experimental studies in rodents have documented that duct ligation of salivary glands produces glandular atrophy without mucocele formation.¹¹ The surgery performed in the dog described in this report is less extensive than previously documented for removal of a grass seed in the parotid duct where the parotid duct and parotid, mandibular, and sublingual glands were removed.² This is likely a reflection that the etiology of the grass seed in that case was percutaneous entry, and migration into the gland system led to more extensive pathology.

Ideally, the dog in this report would not have received pre- and perioperative antibiotics to allow an accurate bacterial culture to be obtained. In this case, the dog had received several weeks of antibiotics prior to referral, continuing to admission and intraoperatively. Despite this, a positive culture was achieved, albeit scanty in nature.

In the human literature, grass seed parotid duct foreign bodies have been reported.^12,13 In one of those cases, interestingly, CT was performed but showed no specific changes. A diagnosis was made with contrast sialography and ultrasound.¹³ In another case, suppurative parotitis caused by a hair foreign body in the parotid duct, removed via endobuccal examination, resulted in resolution of clinical signs without excision of the salivary gland.¹⁴

Conclusion

CT revealed the presence of an abnormal, focally dilated portion of the left parotid duct with a thickened wall, but no gross pathology of the proximal part of the duct and salivary gland. Crucially, the CT revealed the surrounding soft tissues to be normal. This information helped in the decision to perform a minimal surgical resection of the affected portion of the parotid salivary duct (with its grass seed foreign body) with ligation of the proximal duct. The authors were confident that all of the affected tissue was removed. No complications were reported 12 mo postoperatively.