Zygomatic Salivary Gland Diseases in the Dog: Three Cases Diagnosed by MRI
This article describes three original cases of zygomatic gland disease in the dog diagnosed by low-field MRI and treated by a modified lateral orbitotomy with zygomatic osteotomy. Presenting complaints included exophthalmia, protrusion of the third eyelid, and periorbital swelling without any history of trauma. Low-field MRI allowed for adequate diagnosis of zygomatic gland disease in all cases and provided detailed information about both the specific tissue characteristics of each lesion and extension into surrounding structures. MRI findings were also helpful for surgical planning and dictated the choice of a modified lateral orbitotomy without removal of the orbital ligament. Histopathologic diagnosis for each of the three dogs was a mucocele, a malignant mixed salivary tumor, and sialadenitis.
Introduction
A wide variety of imaging techniques has been used in the diagnosis of orbital disease, including radiography, ultrasound, computed tomography (CT), and MRI. MRI enables topographic evaluation of the orbit to allow clinicians to characterize, localize, and determine the extent of orbital disease.1 Salivary gland disease is uncommon in dogs and cats, with a reported overall incidence of < 0.3%. Of the four salivary glands, the zygomatic gland is least frequently involved in disease.2 Recognized diseases of the canine salivary glands mainly include mucocele, sialadenitis, sialadenosis, sialolithiasis, and neoplasia. In one retrospective study of 245 histopathologic salivary gland samples, neoplasia was found in 30%, sialadenitis in 26%, and mucocele in 9% of the cases.2
To the authors’ knowledge, less than 20 cases of zygomatic gland disease have previously been reported in the veterinary literature. The zygomatic gland, also known as the orbital gland, is located ventral to the zygomatic arch.3 Because of its location in the orbit, enlargement of the gland causes clinical signs similar to retrobulbar disease, including exophthalmia, protrusion of the nictitating membrane, signs of pain on either opening of the mouth or on palpation of the orbit, as well as conjunctival congestion or chemosis.4 This report describes three cases of zygomatic gland disease diagnosed with low-field MRI and treated with modified lateral orbitotomy.
Case Report
Case 1
A 3 mo old male basset hound was referred for the investigation of an acute left exophthalmia, protrusion of the third eyelid, and periocular tissue swelling. No history of trauma was reported. Physical examination revealed pain on opening of the mouth and a soft-tissue swelling of the alveolar mucosa dorsal to the left maxillary third molar tooth. Physical examination was otherwise unremarkable. Complete blood count showed a mild leukocytosis. Fine-needle aspiration of the oral swelling was performed and yielded a small amount of mucoserosanguineous material. Cytology was compatible with neutrophilic inflammation, and the bacterial culture was negative. The dog received oral meloxicama (0.05 mg/kg q 24 hr for 2 wk) and amoxicillin trihydrate/clavulanate potassiumb (12.5 mg/kg q 12 hr for 2 wk) with no objective clinical improvement. MRIc was subsequently performed and revealed an enlarged zygomatic gland that appeared hypointense on T1-weighted images and hyperintense on T2-weighted images compared with the adjacent muscle. After IV contrastd administration (0.01 mmol/kg), peripheral ring enhancement of the mass was noted. An additional finding was the presence of a retrobulbar cystic structure caudal to the left maxillary third molar (Figure 1).
Citation: Journal of the American Animal Hospital Association 49, 5; 10.5326/JAAHA-MS-5882
A modified lateral orbitotomy surgical approach was performed. The skin incision was made over the entire length of the zygomatic arch. The aponeurosis of the temporalis and masseter muscles were incised and reflected dorsally and ventrally, respectively. Osteotomy sites were planned, and two holes were drilled on each side of the osteotomy site. The zygomatic arch was reflected dorsally without transection of the orbital ligament. The zygomatic gland and the cystic lesion were removed, and the zygomatic arch was replaced and stabilized with 18 gauge stainless steel wire sutures through the previously drilled holes. The masseter and temporalis muscles were reapposed with 3-0 glycomer 631e simple interrupted sutures, and the subcutaneous tissues were apposed with a 3-0 glycomer 631 simple continuous suture. The skin was apposed with 3-0 nylonf simple continuous sutures. Postoperatively, the dog was treated with cephalexing (12.5 mg/kg per os [PO] q 12 hr) and carprofenh (4.4 mg/kg PO q 24 hr) for 6 days.
The histopathology report was consistent with a salivary mucocele associated with sialadenitis. The architecture of the salivary tissue was preserved, and an inflammatory infiltrate characterized by the presence of numerous macrophages was noted. The salivary duct was dilated, surrounded by a loose connective tissue and diffusely infiltrated by mucinous material. Two wk after the surgical procedure, ocular signs resolved with the exception of the development of a mild enophthalmia. Telephone consultation with the owner 2 yr later confirmed that the dog was healthy and free of clinical signs.
Case 2
A 10 yr old male Staffordshire bull terrier was referred for evaluation of a 2 wk history of right periorbital swelling. Ophthalmologic examination of the right eye showed chemosis, protrusion of the third eyelid, and a serous ocular discharge. No history of trauma was reported. Physical examination was otherwise unremarkable. On clinical examination, the dog exhibited no pain on opening the mouth, and no intraoral swelling was seen. A complete blood count revealed a mild leukocytosis (20 × 109/L; reference range, 6–13 × 109/L). Aspiration of the periorbital swelling yielded a small amount of serosanguineous material. The cytology report described the presence of erythrocytes, leukocytes, and histiocytes. Microbial culture was positive for Staphylococcus intermedius colonization. MRI was performed and revealed a 2 cm (in diameter) cavitated mass related to the rostrolateral part of the enlarged right zygomatic gland, causing dorsal displacement of the right eye. The mass showed mixed signal intensity on T1-weighted and T2-weighted images, with rim enhancement on T1-weighted images after contrast administration (0.1 mmol/kg). The zygomatic gland showed increased signal intensity in this rostral portion (Figure 2).
Citation: Journal of the American Animal Hospital Association 49, 5; 10.5326/JAAHA-MS-5882
A modified lateral orbitotomy was performed, and a marginal resection of the mass was realized. A Penrose drain was placed at the surgical site, exiting ventral to the zygomatic arch, which was removed 2 days postsurgically. The dog was discharged with carprofen (4.4 mg/kg PO q 24 hr) and amoxicillin trihydrate/clavulanate potassium (12.5 mg/kg PO q 12 hr) for 5 days.
Histopathologic analysis described salivary glandular tissue, diffusely infiltrated by poorly differentiated neoplastic cells, compatible with a malignant mixed salivary tumor without clean margins. Thoracic radiographs and abdominal ultrasound were within normal limits. Radiotherapy was recommended but was declined by the owner. The dog was euthanized 3 mo later due to local recurrence.
Case 3
A 14 yr old spayed female Dalmatian was referred for the investigation of a 1 wk history of a painful swelling ventral to the left orbit. The left eye was exophthalmic with a serous ocular discharge, and a protrusion of the third eyelid was present. No history of trauma was reported. On physical examination, the dog showed pain on opening of the mouth. Hematology and serum biochemical profiles were unremarkable. Cytologic evaluation of fine-needle aspirates of the swelling revealed the presence of numerous neutrophils, and the microbial culture was negative. MRI showed a left zygomatic gland enlargement with decreased signal intensity on T1-weighted images and concomitant increased signal intensity on T2-weighted and gradient-echo short tau inversion recovery images. Following contrast administration (0.1 mmol/kg), marked homogeneous contrast enhancement of the zygomatic gland was noted. Multiple areas of increased signal on T2-weighted and gradient-echo short tau inversion recovery images within the temporal muscle, the medial pterygoid muscle, and the subcutaneous tissues lateral to the temporomandibular joint, with moderate contrast enhancement, were present as well.
Surgical therapy was elected, and the zygomatic gland was removed by a modified lateral orbitotomy. Postoperatively, prednisolonei (0.5 mg/kg PO q 12 hr) for 10 days and cephalexin (12.5 mg/kg PO q 12 hr) were administered for 6 days. The histopathologic report described a salivary gland diffusely infiltrated with neutrophils and a small number of macrophages, lymphocytes, and plasma cells. Those findings were consistent with chronic suppurative sialadenitis. Telephone consultation with the owner 6 mo later confirmed that clinical signs did not recur.
Discussion
This report describes zygomatic gland diseases in three dogs diagnosed by low-field MRI: a mucocele, a neoplasm, and a sialadenitis. Several imaging modalities have historically been used to differentiate zygomatic gland disease from other retrobulbar lesions. Those include radiography, ultrasound, CT, and MRI. In the three cases described in this report, MRI was performed using a 0.2 Tesla system, and those images were successfully used to evaluate zygomatic disease in all three cases. Compared with high-field MRI, the main disadvantage of low-field MRI is the reduced signal/noise ratio, generally associated with longer scan times and decreased resolution. Field strength is not the sole determining factor for image quality, and the diagnostic performance seems to be similar for both systems.5,6 In the mucocele, fluid was clearly differentiated from solid tissue based on its MRI characteristics of hypointensity on T1-weighted images and marked hyperintensity on T2-weighted images. MRI characteristics of mucoceles described in the literature include heterogeneity on both T1- and T2-weighted images, hypointensity on T1-weighted, and hyperintensity on T2-weighted images.7 MRI characteristics of neoplasia included an obvious mass with medium intensity on T1-weighted images and variable hyperintensity on T2-weighted images. Following contrast administration, the neoplasm showed a modest diffuse enhancement, and extension of the tumor into surrounding structures was easily seen. Despite the less precise evaluation of cortical bone reported with MRI, osteolysis may also be clearly seen.7 In the case with sialadenitis, inflamed tissue appeared hypointense on T1-weighted images, hyperintense on T2-weighted images, and showed contrast enhancement of adjacent muscles after contrast injection.
Compared with radiography and ultrasonography, MRI and CT provide more accurate imaging of the orbit due to their cross-sectional nature, their ability to image tissues beyond bony margins (nasal and cranial cavities), and their superior spatial and contrast resolution.7 In comparison with CT, MRI offers direct multiplanar imaging capability and superior soft-tissue resolution and contrast.8–10 Disadvantages include longer data acquisition time, increased tissue slice thickness, and less precise detection of cortical bone and soft tissue mineralization.11 MRI imaging for intra- and extraocular lesions provides more precise information about location, extent of disease, and tissue characteristics relative to other imaging techniques, thus allowing optimal selection of the surgical approach.7,11 In the present series, definitive diagnosis was subsequently obtained by histopathologic examination.
A mucocele may be defined as a cavity in either the subcutaneous or submucosal tissues created by, and containing, mucoid saliva.12 Mucoceles reportedly occur secondary to trauma, foreign bodies, sialoliths, neoplasia, salivary gland infarction, necrosis, and obstruction of the duct secondary to inflammation and fibrosis.2,12,13 Salivary mucoceles most commonly affect the sublingual glands, and in a retrospective study of 60 cases, only one mucocele was attributed to the zygomatic gland.12 The location of the mucocele will often help determine what salivary gland it is arising from. In the patient presenting with a salivary mucocele described herein, the clinical signs were periorbital swelling, exophthalmia, chemosis, protrusion of the nictitating membrane, swelling in the mouth next to the maxillary molar teeth, and pain on opening the mouth. The clinical presentation was similar in others cases previously described.4,14
Salivary gland tumors are uncommon in dogs and cats. In a retrospective study, histopathologic diagnosis of neoplasia was made in 30% of cases of salivary gland masses. In that study, carcinoma and adenocarcinoma were the most common histopathologic types of tumor.2 They are locally invasive and can rapidly metastasize, but early diagnosis significantly improves the survival time in dogs.15 In the case presented here, a marginal resection was made without clean margins and radiotherapy was offered, but declined by the owner. The dog had no diagnosed distant metastases at the time of the surgery but had local recurrence 3 mo later. Previously published reports of salivary gland neoplasia in dogs describe carcinoma as the most common tumor type.2,16 The case presented here describes a malignant mixed tumor, with both adenocarcinomatous and sarcomatous components. This mixed histology is rare in domestic animals with salivary gland neoplasia but is commonly reported in humans.17–19 Retrobulbar tumors originating from the zygomatic salivary gland may be difficult to distinguish from those arising from orbital structures based on clinical examination.20 Specific tumor localization using MRI may help to determine the tissue of origin.
Sialadenitis is uncommon in dogs and may occur secondary to a hematogenous infection, ascending infections from oral trauma (foreign bodies), or represent an immune-mediated process.21 In one study of 245 cases of salivary gland diseases, a diagnosis of sialadenitis was obtained in 45 cases; however, the zygomatic gland could not be identified histologically as the primary site of involvement in any of those cases.2 Sialadenitis may be primary or secondary and was considered secondary in the case described herein because of the presence of inflammation in the tissues adjacent to the gland. In the study by Splanger et al. (1991), there was consistent association between sialadenitis and the presence of a salivary mucocele.2 That case described an unusual presentation of sialadenitis as it was not associated with a mucocele.
Many surgical approaches to the orbit have been described, including transconjunctival, transpalpebral, dorsal, lateral, and a modified lateral approach with osteotomy of the zygomatic arch. A modified lateral approach with zygomatic arch osteotomy was elected in the three cases described here. Zygomatic arch osteotomy was performed without removal of the orbital ligament. The temporalis and masseter muscles were incised from the zygomatic arch, and the zygomatic arch was reflected dorsally to provide access to the dorsolateral orbit and retrobulbar area. The zygomatic arch was replaced. This technique provided optimal exposure of the orbit, including the dorsolateral orbit from its rostral border to the caudal bony orbital wall, while maintaining the integrity of orbital structures and a visual globe.22,23
Conclusion
Even though zygomatic gland disease is rare in dogs, it should be considered in any dog presenting with periorbital swelling, exophthalmia, or protrusion of the nictitating membrane. Low-field MRI appeared to be a valuable diagnostic tool, providing information about characteristics of the lesion and invasiveness into surrounding tissues, and was particularly helpful in planning the surgical approach in each case.
Transverse T1-weighted MRI of a zygomatic mucocele. The left zygomatic gland is enlarged and appears hypointense to adjacent structures.
Transverse T1-weighted MRI of a salivary gland tumor after IV contrast administration. There is a mass located rostrolaterally to the right globe. The mass shows rim enhancement, and the globe appears to be displaced caudodorsally.
Contributor Notes
G. Payen’s updated credentials since article acceptance are DMV, ECVO.
