History
A 5-year-old sexually intact male Shih Tzu was referred to the University of Tennessee College of Veterinary Medicine with a 2-week history of exophthalmia and edema around the left eye and the left side of the face. A large, soft, fluctuant swelling surrounded the left eye and extended to the upper lip. Chemosis and slight dorsotemporal deviation of the left globe were seen. The direct pupillary light response of the left eye was negative, and intraocular pressure was high (31 mm Hg). There was a fleshy, pink mass behind the leftmost caudal maxillary molar. Fine-needle aspiration of the mass yielded a yellow-tinged viscous liquid. Cytologic examination revealed moderate neutrophilic inflammation with the presence of mucus. Computed tomography (CT) of the head was performed; pre- and postcontrast images were obtained (Figure 1).
Pre- (A and C) and postcontrast (B and D) transverse computed tomographic images (slice thickness, 3 mm) of the head at the level of the orbit and the zygomatic arch, respectively, in a 5-year-old male Shih Tzu evaluated because of a 2-week history of exophthalmia and edema around the left eye and the left side of face.
Citation: Journal of the American Veterinary Medical Association 237, 12; 10.2460/javma.237.12.1375
Pre- (A and C) and postcontrast (B and D) transverse computed tomographic images (slice thickness, 3 mm) of the head at the level of the orbit and the zygomatic arch, respectively, in a 5-year-old male Shih Tzu evaluated because of a 2-week history of exophthalmia and edema around the left eye and the left side of face.
Citation: Journal of the American Veterinary Medical Association 237, 12; 10.2460/javma.237.12.1375
Pre- (A and C) and postcontrast (B and D) transverse computed tomographic images (slice thickness, 3 mm) of the head at the level of the orbit and the zygomatic arch, respectively, in a 5-year-old male Shih Tzu evaluated because of a 2-week history of exophthalmia and edema around the left eye and the left side of face.
Citation: Journal of the American Veterinary Medical Association 237, 12; 10.2460/javma.237.12.1375
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Diagnostic Imaging Findings and Interpretation
A large hypoattenuating well-circumscribed mass extends from the ventral aspect of the left orbit deviating the left globe dorsally. The caudal aspect of the mass extends to the level of the left temporalis muscle and to the ventromedial aspect of the zygomatic arch. Septa are faintly visualized within the mass with slightly higher attenuation than the mass that forms the bulk of the lesion. The density of the mass ranges from approximately 45 to 65 Hounsfield units. The margins of the expansile mass and the septate nature of the lesion are more clearly identified in the postcontrast image (Figure 2). Otherwise, there is no evidence of increased density (contrast enhancement) within the mass. An expansile, septate, well-defined, fluid-filled mass, without evidence of invasion of normal structures, suggests a cystic rather than infiltrative process. The combination of the clinical and imaging findings strongly suggests the formation of a zygomatic salivary gland mucocele. Other, much less likely inflammatory or neoplastic lesions cannot be excluded on the basis of clinical and imaging findings alone.
Same computed tomographic images as in Figure 1. A large hypoattenuating well-circumscribed mass extends from the ventral aspect of the left orbit deviating the left globe (asterisk) dorsally (A and B; arrows). The caudal aspect of the mass extends to the level of the left temporalis muscle and to the ventromedial aspect of the zygomatic arch (C and D; arrows). Septa are faintly visualized within the mass with slightly higher attenuation than the mass that forms the bulk of the lesion. The density of the mass ranges from approximately 45 to 65 Hounsfield units. The margins of the expansile mass and the septate nature of the lesion are more clearly identified following the IV administration of contrast medium. Otherwise, there is no evidence of increased density (contrast enhancement) within the mass (B and D).
Citation: Journal of the American Veterinary Medical Association 237, 12; 10.2460/javma.237.12.1375
Same computed tomographic images as in Figure 1. A large hypoattenuating well-circumscribed mass extends from the ventral aspect of the left orbit deviating the left globe (asterisk) dorsally (A and B; arrows). The caudal aspect of the mass extends to the level of the left temporalis muscle and to the ventromedial aspect of the zygomatic arch (C and D; arrows). Septa are faintly visualized within the mass with slightly higher attenuation than the mass that forms the bulk of the lesion. The density of the mass ranges from approximately 45 to 65 Hounsfield units. The margins of the expansile mass and the septate nature of the lesion are more clearly identified following the IV administration of contrast medium. Otherwise, there is no evidence of increased density (contrast enhancement) within the mass (B and D).
Citation: Journal of the American Veterinary Medical Association 237, 12; 10.2460/javma.237.12.1375
Same computed tomographic images as in Figure 1. A large hypoattenuating well-circumscribed mass extends from the ventral aspect of the left orbit deviating the left globe (asterisk) dorsally (A and B; arrows). The caudal aspect of the mass extends to the level of the left temporalis muscle and to the ventromedial aspect of the zygomatic arch (C and D; arrows). Septa are faintly visualized within the mass with slightly higher attenuation than the mass that forms the bulk of the lesion. The density of the mass ranges from approximately 45 to 65 Hounsfield units. The margins of the expansile mass and the septate nature of the lesion are more clearly identified following the IV administration of contrast medium. Otherwise, there is no evidence of increased density (contrast enhancement) within the mass (B and D).
Citation: Journal of the American Veterinary Medical Association 237, 12; 10.2460/javma.237.12.1375
Comments
A modified lateral orbitotomy1,2 was performed, and the presumptive mucocele and zygomatic salivary gland were excised. Recovery from anesthesia was uneventful; the patient was discharged 1 day following surgery to continue treatment at home with a carbonic anhydrase inhibitor and β-adrenergic receptor blocking agent ophthalmic solution, triple antibiotic ophthalmic ointment, and pain medication (tramadol and carprofen). Histologic evaluation of tissue obtained during surgery confirmed the presumptive diagnosis of a salivary mucocele.
The zygomatic salivary gland is 1 of 4 major pairs of glands in the canine salivary system. Mucoceles, while rare in dogs, are one of the most frequently diagnosed salivary gland disorders.2 They occur when saliva leaks from the zygomatic salivary gland into the surrounding tissues causing inflammation and tissue fibrosis.2,3 The cystic structure is normally lined by granulation or fibrous tissue.2 Dogs with zygomatic mucoceles may have clinical signs of orbital swelling, exophthalmos, protrusion of the third eyelid, protrusion of the oral mucous membrane behind the last upper molar, and protrusion of a mass beneath the conjunctiva.3–5 Previous trauma is often reported with zygomatic mucoceles4,5; however, there was no history of trauma in the dog of this report. In agreement with previous reports4,6 of zygomatic mucoceles, this patient had a swelling in the mouth next to the molars. Results of cytologic evaluation of the fine-needle aspirates were consistent with a mucocele but can also be seen with a mucus-producing salivary adenoma or myxosarcoma.5
Ultrasonography, CT, and magnetic resonance imaging are considered the best imaging modalities for evaluating orbital disease.2 Ultrasonography is beneficial in compromised patients because it does not require anesthesia, and it is the least expensive imaging modality.6 Both CT and magnetic resonance imaging are superior to ultrasonography in evaluating extent of the lesion and assessing adjacent soft tissues and bony structures.2 Computed tomography was performed in the dog of this report and allowed assessment of the full extent of the mass prior to surgery.
Other mucus-producing masses include neoplasias such as salivary adenomas and myxosarcomas, which can be difficult to distinguish cytologically from a mucocele because they yield a similar tenacious, straw-colored fluid.5 If osteolysis is visible on CT images, a more aggressive process such as myxosarcoma should be suspected.
1 Gilger BC, Whitley RD, McLaughlin SA. Modified lateral orbitotomy for removal of orbital neoplasms in two dogs. Vet Surg 1994; 23: 53–58.
2 Bartoe JT, Brightman AH, Davidson HJ. Modified lateral orbitotomy for vision-sparing excision of a zygomatic mucocele in a dog. Vet Ophthalmol 2007; 10: 127–131.
3 Maggs DJ, Miller PE, Ofri R. Orbit. In: Slatter's fundamentals of veterinary ophthalmology.4th ed. St Louis: Saunders, 2008; 352–373.
4 Martin CL, Kaswan RL, Doran CC. Cystic lesions of the periorbital region. Compend Contin Educ Pract Vet 1987; 9: 1022–1029.
5 Dennis R. Imaging features of orbital myxosarcoma in dogs. Vet Radiol Ultrasound 2008; 49: 256–263.
6 Mason DR, Lamb CR, McLellan GJ. Ultrasonographic findings in 50 dogs with retrobulbar disease. J Am Anim Hosp Assoc 2001; 37: 557–562.
